E
arth is covered and immersed in water and air,
which are both fluids. Fluids are liquids or gases,
substances that are able to flow and take the shape
of their containers. They are a big part of your life.
Your body depends on fluids like blood, water, and air.
The fluids in your circulatory, respiratory, and digestive
systems have many things in common. Properties of
fluids help explain what happens in these systems.
The properties of liquids can also help explain how
objects rise and fall in fluids and how some fluid-filled
devices work, like your heart and the brakes in a car.
First Nations peoples designed canoes and kayaks based
on their ideas about floating and sinking. These designs
continue to be the basis of innovative new technologies.
In this unit, you will discover many different
properties of fluids. You will investigate the relationships
among some of these properties and have a chance to
apply what you learn to building devices that use fluids
in a variety of ways.
1.0
All fluids demonstrate the property of viscosity, or the internal friction
that causes a fluid to resist flowing.
2.0
Density is another important property of fluids.
3.0
Fluids exert a buoyant force on objects that causes some objects to float.
4.0Understanding the properties of fluids helps in the design and
construction of useful devices.
B I G
I D E A S
3
U n i t
Forces, Fluids,
and Density
I n v i t a t i o n t o E x p l o r e
I n v i t a t i o n t o E x p l o r e
I n v i t a t i o n t o E x p l o r e
I n v i t a t i o n t o E x p l o r e
In Malawi, students from Canada’s Engineers Without Borders collaborate with community members on projects to provide clean water and improve sanitation.
E N G I N E E R S W I T H O U T B O R D E R S
In Canada, we are used to turning on our taps and having clean, clear water flow into our equally clean and clear glass. Imagine having to walk over 500 m to a water source, and then using an unwashed bucket to carry untreated water back to your family. This is exactly the daily experience of many people who live in countries of the Global South, such as Malawi in Africa. The Global South refers to the countries of Africa, Central and Latin America, and most of Asia. As populations have grown, many bodies of water have become polluted or have dried up.
Many people do not think about the pumping and pipe systems that bring water into homes, or the systems that remove sewage and make communities safe from disease. Engineers Without Borders (EWB) is a Canadawide group of professional engineers and students who do worry about these systems. Part of their work is to help communities in Africa create sustainable systems that can ensure clean drinking water, and remove open sewage from the environment.
Saskatchewan Abroad in EWB
Every summer, EWB Saskatchewan volunteers travel overseas to learn from families and communities. Their work involves using technology to create pumping systems that ensure clean water supplies from ground water, and constructing latrines to help improve sanitation. Volunteers work to make sure that communities take ownership of these projects. That way, the technologies are accepted and the projects are sustainable.
One example of a Canadian technology being used in Malawi is the Waterloo pump. In 1978, the Canadian government asked Alan Plumtree and Alfred Rudin from the University of Waterloo to design a pump that could be used overseas. It had to be durable, cheap, simple, and easily manufactured in developing countries. The pumps that were designed were based on Mennonite hand pumps that have been used on Canadian farms for generations.
The hand pump was created out of polyvinylchloride (PVC), a strong, lightweight plastic. Since 1978, improvements have been made. For instance, the hyenas of Malawi chewed the bone-white plastic to bits, so in Malawi the hand pumps are made of black metal. This pump technology has brought clean groundwater to thousands of people throughout Africa.
Engineers call water and sewage systems “fluid systems.”
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Engineers Without Borders (EWB) works to improve water and sanitation systems using technology and community education. Interestingly, you do not have to be an engineering student to join.
• Research EWB to find out who can be a part of this organization and what the members do overseas and in Saskatchewan to fight the causes of poverty.
Malawi
1000 km
Malawi is a landlocked country in Southeast Africa.
Is It a Fluid?
IN V E S T I G A T O R
The Question
What is a fluid?
Procedure
1 Pour about 2 cm of water into one
plastic glass. Spread out a sheet of waxed paper onto the table. Half fill your second glass with cornstarch.
2 Add enough cornstarch to your water
so that you can still mix it, but it is difficult to stir. Oobleck is 1 part water to 1.5–2 parts cornstarch. What appears to happen when you try to stir the mixture? Record your observations.
3 Pour this mixture into the middle of
the waxed paper. Record your observations.
4 Push the end of a spoon in a straight
line slowly through the middle of the mixture. Record your observations.
5 Push the end of a spoon in a straight
line quickly through the middle of the mixture. Do you think this is a fluid? Explain your reasoning.
Materials & Equipment
• 2 clear plastic glasses • plastic spoon
• cornstarch • water • waxed paper
6 Slap the mixture quickly with an open
hand. Record your observations.
7 Place your hand slowly onto the top of
the mixture. Record your observations.
8 Do you think the substance in this
activity is a fluid? Explain your reasoning.
Keeping Records
9 List the reasons you gave when
identifying the mixture as a fluid or not.
Use a checkmark () to indicate those
characteristics that you feel are always present in a fluid, a question mark (?) to indicate those characteristics that you feel are sometimes present in a fluid, and an “X” to indicate those characteristics that you know are never present in a fluid.
Is this mixture a fluid or not?
Focus Your Thoughts
1 Using your definition of a fluid, determine whether each of
the following is a fluid or not.
Analyzing and Interpreting
10 Create a table similar to the one on
the right to organize the results of your explorations and discussions about the characteristics of the mixture you explored.
Forming Conclusions
11 Using the characteristics you listed
as Always present, create a definition of a fluid. Always present Sometimes present Never present
2 What are other examples of fluids? List five fluids that
you have been in contact with today.
3 Suggest ways your class might investigate the characteristics
of fluids in more detail.
4 What are the benefits of the Waterloo pump to African
communities?
F
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D E A S1.0
B I G I D E A
All fluids demonstrate the property of
viscosity, or the internal friction that
causes a fluid to resist flowing.
Fluids can be found everywhere. We drink them, we breathe them, and we use them. But how much do you know about fluids? In this Big Idea, you will have an opportunity to investigate an important property of fluids called viscosity.
1.1
Investigating Viscosity
The Particle Theory of Matter attempts to explain matter in a number of ways. It describes:
• a model of matter
• the different states of matter
• what happens when matter changes from one state to another This model helps you understand and explain observations you will make when investigating matter.
Solids A flowing liquid Burning gases
The Particle Theory describes how particles behave in each of the three states of matter. When matter is a solid, the particles are close together and fixed in place. In a liquid, they are close together, but can slide around and over each other. In a gas, the particles are far apart and move around rapidly. A brief review of the Particle Theory is on the right.
Can the Particle Theory be used to describe fluids and how they behave? And what exactly are fluids? Use the Particle Theory and your observations from the Investigator: Is it a fluid? to improve your definition of a fluid.
Most people think of liquids when they hear the word “fluid.” But gases are also fluids. In fact, we can think of a fluid as any matter that has no fixed shape but that takes the shape of its container. For example, the air in a bicycle tire takes the shape of the tire. The air in a beach ball takes the shape of the ball. In the same way, water in a bottle takes the shape of the bottle, but in a bowl, it takes the shape of the bowl. In this unit, you will investigate the properties of fluids. In many of these activities, you will use the Particle Theory to help explain your observations.
Working Safely with Fluids
Before proceeding with the investigation of viscosity you need to think about safety. There are many fluids that are safe to handle — we eat them, put them on our skin, and swim in them. There are, however, some fluids that are unsafe. There are two different labelling systems used in Canada to help us know which fluids are safe and which fluids need to be handled cautiously. At home, you may have seen Hazardous Household Product Symbols (HHPS) on cleaning or automobile fluids found in your garage. These symbols tell you the level and type of danger.
Particle Theory Points
• All matter is made of tiny particles.
• Different substances are made of different particles. • The particles of matter are
always moving and vibrating. • Particles move differently in
solids, liquids, and gases. • Adding heat makes particles
move more and vibrate faster. As a result, the state of matter may change.
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Hazardous Household Product Symbols alert you to the level and type of danger. The triangles indicate “caution” and the octagons indicate danger.
Remember, the more corners the symbol has, the greater the danger.
In all workplaces in Canada, including schools, you will see a second type of safety symbols. They are the Workplace Hazardous Materials Information System (WHMIS) safety symbols. You will see these symbols in your classroom, and it is important to know what they mean. Understanding WHMIS symbols can help you handle dangerous materials safely.
It is important to stay safe in the science lab. Check the label of all materials to determine whether they are potentially
dangerous. Refer to Toolkit 1 for more safety information before you begin labwork for this unit.
Viscosity
One property of fluids is how they move or flow. Think about the fluids you have used in the last couple of days. How would using them be different if they did not flow the way they usually do? For example, what if soda pop was like a thick syrup or ketchup was like water? In both these situations, the properties of the fluids are dramatically different.
Knowing WHMIS Classes and Hazard Symbols will help you stay safe when handling materials in the classroom.
CLASS A
compressed gas flammable and combustible material
oxidizing material
1. materials causing immediate and serious
toxic effects
2. materials causing other toxic effects
3. biohazardous infectious materials
corrosive material dangerously reactive material
CLASS B
CLASS D
CLASS E CLASS F
CLASS C
With your partner, look at the five fluids that you listed in the Focus Your Thoughts at the beginning of this unit. Describe what they would be like if they were thicker or if they were thinner. Here is an example:
How fluids flow is determined by a property called viscosity.
Viscosity is a liquid’s internal resistance or friction that keeps it
from flowing. Recall from the Particle Theory that the particles in a liquid slide around and roll over each other. In a gas, the particles move around more easily because they are far apart. In a fluid, the greater the friction or rubbing between the particles, the higher the viscosity. Fluids with a high viscosity do not flow as easily as fluids with a low viscosity.
Fluid Thicker Thinner
Shampoo • hard to get out of bottle • would take a lot to wash hair
There are several ways to determine the viscosity of fluids. Two common ways you will investigate are the bubble test and the ramp method.
• In the bubble test, you time how long it takes an air bubble to rise through a tube of fluid.
• In the ramp method, you time how long it takes a fluid to flow down a ramp.
The longer it takes for the bubble to travel through a fluid, or for a fluid to flow down a ramp, the higher its viscosity.
When you are performing a scientific investigation, such as the bubble test or ramp method, it is important to control the variables, or things that can be changed. You will be performing a fair test when you ensure that only one part of your experiment, or one variable, is being changed at a time. That way, you will know that the changes you observe are due to the one variable you are changing.
The Bubble Test
IN V E S T I G A T O R
Procedure
1 Make a mark on each tube at 2 cm
from the end.
2 Draw a data table similar to the one
shown below.
3 Place a stopper in the unmarked end
of a tube. Use the funnel to fill the tube up to the mark with one of the liquids.
Step 1: Mark each tube at 2 cm from the end.
Before You Start
Look at the labels of the fluids you are working with. Are there safety concerns with any of them?
After observing the liquids in the containers, predict how the viscosity of the liquids tested will compare to each other. List them in order, from most viscous to least viscous.
The Question
How can you compare the viscosity of different liquids?
Materials & Equipment
• 5 clear plastic tubes (cut into different lengths from 5–30 cm) • 10 stoppers or plugs (medicine dropper
bulbs, binder clips, etc.) • stopwatch
• funnel
• 5 liquids: water, liquid furniture or floor polish, shampoo, pancake or table syrup, vegetable oil
Time for Bubble Test
Fluid Trial 1 Trial 2 Trial 3 Average Time
Analyzing and Interpreting
9 Look at the three measurements you
took for the first liquid. Is one of these measurements very different from the other two? If so, circle this one and do not include it when you calculate the average time. Do the same for the readings for all the other liquids. There could be an error in these circled measurements. You will look at them later in question 13, part f).
10 Calculate the average time that each
bubble took to move up its tube of fluid.
11 Based on your results, list the fluids
in order from most viscous to least viscous.
12 Compare your results to your prediction.
Forming Conclusions
13 Write a summary for your
investigation using the following questions as a guide.
a) What question were you trying to answer?
b) What did you do?
c) What variables did you try to control?
d) What were your results?
e) How did your results compare to your prediction?
f) What sources of error or problems did you encounter? (If you had an error in measurement, describe why and how you think it happened.) g) What safety precautions were
needed with these fluids?
4 Seal the marked end of the tube with
a second stopper.
5 Hold the tube vertical, then turn it
upside down. At the moment that you turn the tube upside down, start your stopwatch. Make sure to hold the tube vertical once again.
6 In your table, record the time it takes
for the bubble to reach the top of the tube.
7 Repeat steps 5 and 6 two more times
with the same tube and record the times in a table of results.
8 Repeat steps 3 through 7 for each liquid.
Viscosity Is Important
Think back again to the five fluids you identified in the Focus Your Thoughts section at the beginning of this unit. How might you change the viscosity of these fluids? Why would you want to? For example, when you are cooking, you can change the viscosity of gravy. If the gravy’s viscosity is too low, you need to add more flour. If its viscosity is too high, you need to add water.
So, what causes some fluids to have higher viscosity than others? Think of how glue feels on your fingertips. It feels sticky because the glue particles are attracted to your skin. Similarly, certain types of particles attract each other more than others. It is hard for these particles to flow or move easily by each other. Even though all fluids flow, the greater the attraction between particles, the higher the viscosity. The attraction between fluid particles is related to their size and shape. Small particles can move past each other more easily, resulting in a lower viscosity. As well, some particle shapes flow past each other more easily.
Imagine a container full of marbles or buttons and a container full of yarn strands. If you dump both jars out, the marbles or buttons easily move around each other and flow through a space, while the yarn strands get caught on each other. This result is similar to the effect of particle shape on viscosity.
1 Write a short paragraph to describe viscosity. Include at
least two examples of fluids and use the words flow,
fluid, particles, and viscosity in your description.
2 Which of the following materials is the most viscous?
Which is the least viscous? How could you determine this? a) syrup
b) house paint c) water
3 Describe two substances that are useful because of their
viscosity. What criteria would you use to determine if a brand name of that substance is high quality or not?
4 How can you ensure a fair test if you are trying to compare
the viscosities of two different substances?
The shape of the particles in a fluid helps to determine its viscosity.
Careers and Profiles
Dr. Len Proctor:
Honey Farmer
Imagine spending every summer for the past 35 years gathering and extracting honey. That is exactly what Dr. Len Proctor, a beekeeper operating out of Langham, Saskatchewan, chooses to do. Many of his days are long if the weather co-operates. When asked if he ever takes holidays, he declares, “I am on holidays right now!” Fieldwork is very different from his full-time position as a professor at the University of Saskatchewan. “I grew up on a farm. As a beekeeper, I can be outside without having the year-long commitment of other types of farming. By spring, I can’t wait to get outside. I have to admit that by the end of our honey season, I am anxious to be back in my office.”
Proctor’s knowledge about bees and honey comes from trial and error, reading journal articles, and talking to other beekeepers. “I remember the first hive a friend and I ever set up. We pulled the boxes out of his parent’s storage shed, painted them, purchased some bees, and set them out. We did everything wrong that summer, but still managed to get 180 pounds of honey to share with family and friends.” Proctor started Cornucopia Honey in 1975. It is a family-run business, with Len’s two sons, Jorden and Brendan, helping to oversee a crew of young workers. A look of pride is on his face as he says, “You have to have a special touch to raise queen bees, and Jorden has that gift. Brendan, he’s the foreman in the plant. If something
goes wrong with the machinery, he is the one who knows what to do.”
Honey and Viscosity
Lifting a dripping frame out of a hive, you can feel how viscous honey is. “Bees do lots of work for us,” Proctor says. “They use their wings to dry out honey in the honeycombs before they cap it with wax.” It is important to keep the honeycomb as intact as possible, so bees do not have to completely rebuild it every time honey is harvested. When honeycomb is brought into the extraction plant, the honey’s viscosity needs to be maintained. “Pure honey
crystallizes naturally. In fact, canola honey can sometimes crystallize even before we have a chance to extract it.” The viscosity of honey is affected by temperature and moisture in the air. Viscosity can be maintained by using heaters to keep the air warm and using dehumidifiers to keep the air dry. Knowing about the science of fluids helps honey farmers extract the most honey from
the frames their bees work so hard to fill. It is important to control
1.2
The Effect of Temperature
on Viscosity
Earlier in this Big Idea, you thought about different fluids and what would happen if their viscosity changed. What might cause a fluid’s viscosity to change? Think back to the last activity, The Bubble Test. As you held the tube in your hand, the liquid may have heated up a little. Was there a change in the time it took for the bubble to rise between the first, second, and third trials?
Temperature is one factor that can have a big effect on viscosity. Look at the photos below. What will happen to the viscosity of these liquids?
Chocolate-dipped ice-cream cones are a great treat on a summer day.
• What is added to chocolate to change its viscosity to allow it to be thin enough to dip into, but viscous enough to stick to the ice cream?
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The Ramp Method
The ramp method of testing viscosity involves pouring a measured volume, say 15 mL, of a fluid down a ramp and timing how long it takes to get to the bottom. By pouring the same volume of another fluid and timing it, you can compare the viscosities of different fluids. You can also investigate the effect of temperature on viscosity by testing the same fluid several times.
A car owner will use oil with a different viscosity, depending on the season. Oil is used as a lubricant for vehicle engines. Its viscosity is important, as it must be thin enough to flow through the engine, and thick enough to coat engine parts. The Society of Automotive Engineers (SAE) assigns all motor oils a viscosity number between 5 and 50. In the next activity, you will use the ramp method to design your own test to find out what the numbers on a motor oil mean, and which motor-oil grade is best to use in each season.
Table syrup poured on hot pancakes
Flowing lava reaching the ocean Olive oil put into a refrigerator Room-temperature engine oil put into a hot engine
Understanding Viscosity and Temperature
Your Problem Solver activity may have shown how a change in temperature can affect the viscosity of a fluid. The Particle Theory can help to explain the effect of temperature on fluid viscosity.
What Oil Should You Use?
PR O B L E M SO L V E R
In this activity, you will use the ramp method to determine what the grades given to motor oil mean, and the effect of temperature on the viscosity of motor oil.
• You will investigate four different grades of motor oil.
• Design a fair test that will allow you to collect evidence to answer the questions:
1 What do the numbers on a sample of
motor oil tell us about its viscosity?
2 How does temperature affect the
viscosity of different motor oil grades?
3 What type of motor oil would be useful
in the summer in Saskatchewan? in the winter?
• Look at the labels for the oils you are investigating. What safety information do you need to be aware of?
• Use a temperature probe or thermometer to measure the temperature of the oils being investigated. Be sure to think about both cold and warm temperatures. • Have your teacher approve your
procedure.
• Carry out your test. Create a written, visual, or digital summary explaining your results, including:
– the safety precautions you needed to take during this activity;
– the type of motor oil you would recommend for summer and winter driving, and what criteria you used to determine this; – what the numbers on a label
describing the grade of motor oil mean.
10W30 is a common multi-grade oil used in Canada.
CAUTION!
As temperature increases, the viscosity of a liquid decreases. Particles at higher temperatures move with more energy. Particles are able to move out of the way and make room for other particles to pass, allowing the liquid to flow more easily. The opposite is also true. As temperature decreases, the viscosity of a liquid increases. Particles at lower temperatures move with less energy. The spaces between particles get smaller, making less room for other particles to pass. This causes the liquid to flow less easily.
Soft-Serve Ice Cream
Soft ice cream is something we take for granted, but it has not always existed! A soft-serve ice-cream formula was invented in 1938. The first Dairy Queen in Canada opened in Melville, Saskatchewan, in 1953.
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1 All matter is made of tiny particles. Different substances are made of different particles.
2 The tiny particles of matter are always moving and vibrating.
3 Adding heat to matter makes the particles move around faster and vibrate faster.
Temperature changes can greatly affect manufactured devices. Look back at the Problem Solver: What Oil Should You Use? In colder weather, motor vehicle engines cannot function well if an oil becomes too viscous. Therefore, when the weather is cold, an oil with a low viscosity is used. As the temperature drops, the oil’s viscosity remains low enough to allow it to flow through the engine. During warm weather, a higher viscosity oil is used. As the temperature increases, the oil’s viscosity remains high enough to allow it to still coat the engine properly. Thin oil is less effective as a lubricant. Manufacturers add chemicals called viscosity modifiers to motor oil to change the oil’s characteristics. Some of these modifiers help to improve fuel economy or oil performance in extreme temperatures.
1 Look at other students’ summaries of the results of their
motor oil ramp tests.
a) Do your results agree with the others?
b) Are there differences? Why do you think differences might occur?
2 In a fair test, you have to keep most variables the same
so that you can see what the effect of one variable is. a) What things did you keep the same for each test? b) What did you change during the tests?
3 How does temperature affect the viscosity of a liquid?
4 You are given three samples of the same shampoo at three
different temperatures: 35°C, 50°C, and 75°C.
a) Which sample would have the highest viscosity? Which sample would have the lowest viscosity?
b) Support your answer with ideas from your Problem Solver activity.
5 What safety precautions are needed when you work with
liquid materials in a science lab?
1.3
Check Your Progress
1 What is a fluid? Make a list of all the words related to fluids
in this Big Idea. Use those words to create a concept map illustrating your understanding of fluids. Save your concept map for future use, as you will add to it after you complete each Big Idea.
2 Use the Particle Theory of Matter to explain what viscosity
is and why temperature affects viscosity.
3 Describe two methods used to test viscosity.
4 When you pour cereal into a bowl for breakfast, the cereal takes
the shape of the bowl. Is cereal a fluid? Explain your answer.
5 Give some examples of how an understanding of viscosity
benefits people.
6 a) How are the Hazardous Household Product Symbols and
WHMIS symbols the same and different? Use a table similar to the one below to help you organize your thinking. b) What is the importance of having both systems?
Hazardous Household Product Symbols
WHMIS Symbols
Different fluids not only have different viscosities, they also show a difference in their densities. Density is the amount of matter or number of particles in a given volume. The Particle Theory can help you understand density.
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B I G I D E A
Density is another important property of fluids.
If you had the same volume of gold and concrete, which would have the greater mass? Which would be heavier (have the greater weight)? How could you find out without using a scale?
2.1
Weight, Mass, and Volume
We often use the terms “weight” and “mass” to mean the same thing. People may say “my weight is” to mean the same as “my mass is.” When you are standing on Earth, weight and mass seem very similar. So, why is it important to know how they are different?
Weight is a measure of the force of gravity on an object, while
the mass of an object measures the amount of matter that is in the object. If you were to go to the Moon, you would find that the force of gravity is one-sixth of that on Earth. The mass of an object on the Moon is the same as on Earth. So, what would happen to an astronaut’s weight when she travels to the Moon?
The volume of an object is a measure of how much space it takes up. Volume and mass are both important measurements when determining the density of an object. Density can be measured by finding the mass of a substance in a given volume.
Comparing Measurements
What do you already know about the relationship among mass, density, and volume? How have you used the word “dense” in the past? In this activity, you are going to be looking more closely at how these three measurements are related.
A Base Unit
Mass is often measured in grams (g), but scientists have defined the base unit for mass to be the kilogram (kg), which is equal to 1000 grams. A kilogram is equal to the mass of a cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures in Paris.
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1 Assume that two objects have identical particles.
a) Both objects have exactly the same number of particles but have different volumes. Visualize and sketch representations of these objects and describe the difference in their density. b) Both objects have the same volume, but have a different
number of particles. Visualize and sketch representations of these objects and describe the difference in their density.
2 a) What is the relationship between mass and weight?
b) If you were to travel to Mars where the force of gravity is about one-third that of Earth’s, would your body mass change or would your weight change? Explain your answer.
Exploring Mass, Density, and Volume
IN V E S T I G A T O R
The Question
What is the relationship that links mass, density, and volume?
Procedure
1 Measure out the same volume of rice
and crispy rice cereal into two identical beakers. Using the balance scale, find the mass of each substance and beaker.
2 Using the balance scale, measure out
25 g of crispy rice cereal and 25 g of rice into separate beakers. You will need to create a procedure for doing this. Check your idea with your teacher before you proceed.
Keeping Records
3 Record your data in tables similar to
the following:
Analyzing and Interpreting
4 Assume you have two substances with
the same volume. How do different masses affect density?
5 Assume you have two substances
with the same mass. How do different volumes affect density?
6 Which is more dense, rice or crispy
rice cereal?
Forming Conclusions
7 What is the relationship among mass,
density, and volume?
Materials & Equipment
• crispy rice cereal • rice
• two 250-mL beakers • balance scale
Volume Mass of rice Mass of rice cereal
Which is more dense, rice or crispy rice cereal?
Mass Volume of rice Volume of rice cereal
Which is more dense, rice or crispy rice cereal?
Think About It
• Which would weigh more: the same volume of gold on Earth or on the Moon? • Which would have the greater
mass: the same volume of gold on Earth or on the Moon?
2.2
Water and Other Fluids
Water is a special fluid for many reasons. First Nations and Métis peoples have observed the similarities between our bodies and Mother Earth. Imagine viewing your body as a system of rivers, tributaries, and channels. As you learned in earlier grades, water is so good at dissolving substances that it is called the universal solvent. Your body is a watery environment, and so, if you pour anything toxic into your body’s system, your health can become affected in profound ways. The same is true for lakes and rivers. Adding toxic substances to waterways can affect the quality of the water available. Careful stewardship is needed.
From the scientific viewpoint, water is special in its properties. All substances do not have the same density. Scientists defined a basic unit of mass, the gram, so that the density of water is exactly equal to 1 g per mL. Other substances have densities greater or less than water. In this section, you will investigate the density of several substances and compare them to the density of water.
Volume and Mass
of Water
Scientists designed the metric system so that there is a very simple relationship between the volume and mass of water at 4°C. • 1 mL of volume occupies the space of 1 cm3. • 1 g is defined to be the mass of exactly 1 mL of water. Therefore, 1 cm3of water 1 mL, which has a mass of 1 g.
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Dense and Denser
PR O B L E M SO L V E R
You have five identical containers full of different materials. The list tells you what is in each container. If a substance has been changed, your teacher will tell you.
Contents of Containers
• water • canola oil
• corn syrup • rubbing alcohol
• glycerin
• Without touching the containers, predict the density ranking of the substances from highest to lowest Which substances do you think will have a lower density than water? Which substances do you think will have a higher density than water?
• Determine the ranking of the substances, using any method you like. However, you cannot open the containers.
• Compare your actual ranking with your prediction. Be prepared to explain your reasons for your ranking.
• Later you will be testing some of these substances. You will be able to compare your ranking with your test results.
D E VE L O
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Recall that the Particle Theory of Matter states that different substances are made up of different particles. So, the particles in each fluid are different from the particles in every other fluid tested. The result is each substance has a different density.
Refer to your ranking of the five fluids you investigated earlier in this lesson. This ranking will help you to perform the following activity.
Liquid Layers
IN V E S T I G A T O R
The Question
How do layers form when fluids are used?
Procedure
1 Measure 20 mL of each fluid into
separate containers.
2 Mix one colour with the corn syrup, and
the other colour with the water.
3 Refer to your density ranking. Using the
funnel, pour the most dense fluid into the bottom of the graduated cylinder, trying not to pour it down the sides. Wash the funnel with soap and water.
4 Pour the next most dense fluid into the
graduated cylinder. Use the funnel to pour the fluid down the side of the cylinder to prevent the fluids mixing.
5 Continue until all five liquids are in the
graduated cylinder, washing the funnel before adding each new fluid.
Materials & Equipment
• 20 mL of each: corn syrup, water, glycerin, canola oil, rubbing alcohol
• 2 types of food colouring • 100-mL graduated cylinder • 5 identical containers • funnel
Step 4: With careful handling, you can show that fluids that do not mix will form layers.
6 Keep your liquid layers in a safe spot
for use in Big Idea 3.0.
Keeping Records
7 Sketch the layers of fluid that you
created in the graduated cylinder.
Analyzing and Interpreting
8 Why did the layers of fluid form?
9 How is density related to the position
of each fluid layer?
10 Would the same layers have been
created if you poured the fluids in a different order? If there is time, try this to find out.
Forming Conclusions
11 Use the Particle Theory to describe
Canadian Oil-Spill Experts
Large oil spills make headline news in Canada and around the world, but there are small spills that occur daily. In Canada, there are approximately 12 spills of over 4000 L every day in our water systems. Some of these are accidents, while others are caused by purposeful dumping of waste oil.
Oil has a lower density than water. This allows it to float on top of water. This difference can both help and hurt the environment. Floating oil coats sea birds and animals swimming near the surface of the ocean. Oil affects birds’ feathers, reducing their insulating properties. Birds trying to clean themselves may be poisoned by the oil, or die of hypothermia.
Floating oil is easier to clean up than if it sinks to the bottom of the ocean. It is only a few millimetres thick and can be captured or treated by oil-spill experts. Because shorelines are vulnerable, Canadians have developed the Shoreline Clean-Up Assessment Technique (SCAT). This method allows scientists to evaluate shoreline damage and determine the amount of oil washed ashore. Canadian oil-spill experts have been called to assist many other countries when oil spills threaten their shores.
The Shoreline Clean-Up Assessment Technique (SCAT) is a method developed by Canadians. • How does SCAT work? • How often are Canadian
scientists asked for help with oil spill clean-up around the world?
• What countries ask Canadian scientists for help with oil spill clean-up around the world?
r e
SEARCH
Oil Tankers — What Is the Risk?
DE C I S I O N MA K E R
The Issue
Oil tankers are built specifically to
transport oil. Over time there have been a number of oil spills that have resulted in long-term environmental damage. You will decide whether using oil tankers is worth the risk to the environment.
Background Information
Oil is deposited in various places on Earth. Canada has oil reserves but many other countries do not. These tankers transport much-needed oil from areas that produce it to areas that require it. Many industries depend on oil shipped from far away. If oil were not available, important goods could not be produced unless another source of energy was found.
Using the Internet and other resources, research the following questions.
• How much oil is transported yearly? • How much oil is spilled worldwide? • What other options exist for
transporting oil across the ocean?
In Your Opinion
1 State three positive facts and three
negative facts about the use of oil tankers.
2 Are tankers worth the environmental
risk they pose?
3 Suggest options for connecting sources
of oil to areas where oil is in short supply.
1 In the Investigator: Liquid Layers, which of the four fluids has the greatest mass in the volume used, and which has the smallest? How do you know?
2 The four liquids in Liquid Layers do not have identical
particles. How might you sketch their differences in density?
3 What practical applications are there for knowing whether
the density of a substance is greater than or less than water?
2.3
Finding the Density of Substances
In the Investigator: Liquid Layers, some of the liquid layers were positioned above water and some of them were below water. This result occurred because of the difference in density of the substances, that is, the difference in the amount of matter or number of particles in a given volume. Think about the different densities of fluids as you continue to work with these liquids.
Determining Density
Now that you have ranked your materials and have an understanding of mass, you will be able to calculate the density of substances.
A Calculation of Density
The density of a substance is the ratio of its mass to its volume.
This can be written as an equation: D
For example:
• If 242 mL of a substance has a mass of 424 g, what is the density of the substance?
• the mass-to-volume ratio
Calculating Mass/Volume Ratio
IN V E S T I G A T O R
Step 3: Measure the mass of the cylinder and fluid.
3 Place the graduated cylinder containing
the substance on the balance and measure its mass. Record the mass in your table.
4 Repeat steps 2 and 3 with volumes of
40, 60, 80, and 100 mL of the same substance.
5 Repeat this procedure for each
substance. Wash the cylinder before measuring each new substance.
6 Clean and return your equipment to
the proper location.
The Question
How can you calculate the density of a variety of fluids?
Procedure
1 Make a table similar to the one shown
below to record your data. Measure the mass of the graduated cylinder and record it in your table.
2 Measure 20 mL of the
first substance into the graduated cylinder and record the volume in the table.
Materials & Equipment
• 100-mL beaker or graduated cylinder • triple beam or electronic balance
• water, corn syrup, glycerin, rubbing alcohol, and canola oil
• graph paper
Step 2: Pour 20 mL into a graduated cylinder.
CAUTION!
When you have finished the experiment, pour the fluids into large containers for your teacher to store or dispose of safely.
Analyzing and Interpreting
7 Find the mass of each substance by
subtracting the mass of the cylinder from the total mass of the cylinder and substance together. The example below shows this technique when a beaker is used.
8 For each of the five volumes of the
substance tested, find the ratio of the mass to the volume by dividing the mass of the substance by the volume. Report your answer using units of g/mL.
9 Calculate the average value of the five
ratios. This average value of the ratio is the density of each substance.
10 Set up a line graph with mass on
the vertical axis and volume on the horizontal axis. Plot your results for the first substance. Starting at 0.0, draw a straight line through or close to the points.
11 Plot your results for the
other substances in different colours on the same graph. Label each line.
12 Compare the incline of the lines away
from the horizontal axis (the slope of the line). Which incline is the steepest? Which is the most gradual?
13 Describe any relationship that you can
see between the average value of the ratio for each substance and the slope of each line on your graph.
14 Using your graph, predict the volume
of 5 g of corn syrup.
15 Using your graph, predict what the
mass of 125 mL of glycerin would be. HINT: You may need to lengthen your graph line.
16 Compare your calculated densities for
the substances with the ranking you made in the Problem Solver: Dense and
Denser in section 2.2. Was the ranking
of the densities you observed the same or was there a difference?
Forming Conclusions
17 Write a summary paragraph to explain
how you calculated the density of the substances used, and the usefulness of the graph you created. Your summary should include the words substance,
volume, mass, graph, ratio, and density.
Mass (
g)
Volume (mL) Title
CO
MM
U NI C A T E
Densities of Some Common Substances
You have investigated the relationship between mass and volume for different amounts of the same substance. You graphed your results to compare the density of each fluid investigated. You noted that a graph of mass vs. volume, showing a substance with a higher density, has a steeper slope than the graph of a substance with a lower density. It is important to note that all of these density observations occurred at the same temperature.
1 The table on the left shows mass and volume data for
baby oil. What happens to the mass of the baby oil as its volume changes?
2 a) Calculate the density of the baby oil from the data in the table.
b) What happens to the density as the mass and volume change?
3 Suppose you were to graph the baby oil data. Would the
slope of the line for the baby oil be steeper or more gradual than one for water? (The density of water is 1.0 g/mL.)
4 a) Use the graph above. What is the volume of 55 g of
gasoline?
b) What is the mass of 110 mL of isopropyl alcohol (rubbing alcohol)?
Density is the mass per unit of volume, which can be measured in mL or cm3. Density is calculated by dividing the mass of a substance by its volume. density ⫽
The units for the density of liquids and gases are usually given in g/mL or kg/L. mass
volume
Mass (g)
Volume (mL)
Careers and Profiles
Joanne Mueller:
Scuba Diving Instructor
Diving is like being Alice in Wonderland, gazing through the looking glass at a different world. Because water and the body have similar densities, Joanne Mueller is able to navigate this magical place.
Q: Have you always wanted to be a scuba diver?
A: Actually, no. I happened to be on holiday
in Australia when an acquaintance asked me if I wanted to learn to dive. As soon as I was submerged in the pool, I fell in love with the quiet magic and took all of the training from “open water” through “dive master” before I returned to Canada to complete my
instructor levels.
Q: Are there differences between diving in Saskatchewan and diving in Australia?
A: Obviously, it is colder here! Saskatchewan
divers tend to be more technical because of added challenges associated with temperature and altitude. Divers here will use extra thermal protection such as dry suits and gloves as well as different dive tables to help plan their dives.
Q: What is the most important part of professional scuba diving training?
A: When you become a dive master, then instructor,
you learn to be responsible for all divers in your diving group. You not only need to be proficient yourself, but help others stay safe. When we dive, we are in a space we are not meant to be in, and we need to respect that.
Q: What are your most memorable dives?
A: One of my favourite memories from outside
Canada is when I was hovering near my boat’s anchor chain while diving on Australia’s Great Barrier Reef. I was watching the manta rays at their cleaning station above the coral pinnacles. Sometimes, just hovering quietly allows you to see and hear magical things.
Joanne enjoys diving at Whiteswan Lake, north of Candle Lake, Saskatchewan. There is a dive bell submerged at 60 feet (~18 m) that you can actually sit in.
2.4
Experimenting with Density
In the last investigation, the volumes of all the fluids were held constant. But the amount of matter or mass of the fluid changed depending on the fluid used. What will happen if we keep the mass of the substance approximately the same but change the volume? Is it still possible to determine the density of the substance?
Eureka! — The Mind of Archimedes
Archimedes (287–212 BCE) was a Greek mathematician. He is
famous for many inventions and mathematical discoveries, but he is probably most remembered for his bath.
Legend has it that 2300 years ago, the king of Syracuse summoned this famous scientist and mathematician. The king suspected that his new crown was not made of pure gold as promised but had some silver in it. He wanted Archimedes to find out—but without damaging the crown. While thinking hard about this problem, Archimedes decided to have a hot, relaxing bath. As he lowered himself into the tub, he noticed water overflowing its sides. At that instant, he realized he had solved the king’s problem! The legend continues that, in his absent-mindedness, Archimedes jumped out of the tub and ran naked down the street, shouting, “Eureka! Eureka!” (Greek for “I've got it! I've got it!”).
What did Archimedes discover? When he saw the water overflowing from the bathtub, he realized that his body was displacing the water. The volume of water he displaced was equal to the volume of his body. He could now test the king’s crown in the same way to determine its volume.
It is said that Archimedes fashioned a piece of pure gold and a piece of pure silver with the same mass as the King’s new crown. He filled a large vessel to the brim with water, and dropped the piece of silver in. The volume of water that flowed over the rim was equal to the volume of the piece of silver. He repeated this with the piece of gold, and again with the King’s crown, resulting in three volumes of water. What did Archimedes discover? The volume of water displaced by the crown was more than the volume of water displaced by the pure gold, and less than the volume of water displaced by the pure silver. Not only did Archimedes discover that the King’s crown was not made of pure gold, he also discovered the idea of density.
Accurately measure butter using the displacement method.
Baking with Solid Fats
Measuring the volumes of fats such as lard, shortening, or butter is difficult when they are cold and hard.
The most accurate way to measure these is by displacement, using a measuring cup and enough water to cover the fat being measured.
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Changing the Volume
Volume and Displacement
When a substance is placed in a fluid, a curious thing happens. The volume of the fluid rises. Try it. Put an object that will sink into a graduated cylinder filled with 100 mL of water. Note how much the water level rises. This rise in volume takes place because the substance moves aside, or displaces, an amount of water equal to its volume. This method of measuring volume is called the displacement method. A variation of this method is to completely fill a container with water and measure the volume of the overflow when an object is placed into the water. The overflow volume is the same as the volume of the object.
It is also possible to calculate the volume of regularly shaped objects. Recall what you know about the volume of rectangular prisms, cylindrical solids (right cylinders), and triangular prisms. You may have calculated these volumes in math class:
Unit Fact Reminder
Remember that cm3 mL for volume units.
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The following investigation will help you develop your understanding of volume and density. You have observed what happens to fluids when the volume is kept constant and the mass varies for different substances. In this activity you will change both the mass and volume of substances and determine the density of a variety of substances.
In the Technological Problem solver later in the section, you will have the opportunity to design and build your own hydrometer to determine the density of a variety of liquids.
1. Volume of a rectangular prism base area length
2. Volume of a cylindrical solid (right cylinder) radius2 height
3. Volume of a triangular prism height base length
How Dense?
IN V E S T I G A T O R
Step 3: Measure the volume of substances directly.
3 Measure the size of the cylindrical
block of wood, the rectangular eraser, and the plastic triangular prism using a centimetre ruler. Use these measurements to calculate the objects’ volumes. Refer to the formulas on the previous page.
4 Measure the volumes of these
substances using the graduated cylinder. Put 40 mL of water into the graduated cylinder.
The Question
What is the density of unknown substances?
Procedure
1 Make a prediction about the densities
of the substances you will be testing by listing them in order from highest to lowest density. Which substances have a greater density than water?
2 Measure out approximately 25 g each
of pennies, filings, and beads using a balance. Record the exact mass of each substance in a table similar to the one on page 207. Find and record the masses of the three regular solids.
Materials & Equipment
• triple beam or electronic balance • water
• cylindrical wood block, rectangular eraser, plastic triangular prism
• approximately 25 g each of copper pennies, iron filings or nails or metal nuts, and small beads or three other small solids • 100-mL graduated cylinder
• centimetre ruler • stirring rod
Step 2: Measure approximately 25 g of pennies, filings, and beads.
Analyzing and Interpreting
9 Calculate the volume displaced by
each substance (column 6) from the measurements you have recorded.
10 Compare the displaced volume
(column 6) with the original measured volume (column 3) of the cylindrical wood block, rectangular prism, and triangular prism. Are these numbers the same or different? Why do you think this is so? Which volume do you think is more accurate?
11 Calculate the density of each substance.
You do this by dividing the mass (column 2) by the displaced volume (column 6). Report the density in g/mL.
Forming Conclusions
12 Explain how to find the density of a
substance. Describe how your predicted order of densities compared with your calculated results.
5 Place the 25 g of the first substance
to be tested into the water in the graduated cylinder. If the substance floats, use the stirring rod to push it down so that it is just completely submerged. Stir well to get rid of any air bubbles. Record the new volume.
6 Remove the substance from the cylinder
and empty the water. Dry the graduated cylinder.
7 Repeat steps 4 to 6 for each remaining
substance, including the regularly shaped solids.
8 Clean and return your equipment to
the proper location.
1 Substance 2 Mass of Substance (g) 3 Measured Volume of Substance Used (mL or cm3) 4 Initial Volume of Water in Cylinder (mL) 5 Volume in Cylinder After Substance Added (mL) 6 Displaced Volume (mL) (column 5 minus column 4) 7 Density of Substance (g/mL) iron filings, nails, or metal nuts copper pennies small beads cylindrical wood block rectangular eraser triangular prism
CAUTION!
Build Your Own Hydrometer
TE C H N O L O G I C A L PR O B L E M SO L V E R
The Need
A hydrometer is an instrument that measures the density of liquids. In this activity, you will work in groups to design and build your own hydrometer.
Your teacher will provide a variety of standard solutions of pre-determined densities for the class to use to calibrate (determine the scale of) the hydrometers. Your group will then use your hydrometer in an investigation to determine the density of unknown liquids.
The Criteria
The hydrometer must be able to measure the density of unknown liquids.
Brainstorm
1 How does a hydrometer work?
Research how hydrometers function and how industry uses them.
Materials & Equipment
• 250-mL graduated cylinder • fresh water
• four different salt solutions • ruler
• funnel
• a variety of liquids of different densities • permanent marker
You may want to use some or all of the following when building your hydrometer. • clear plastic straws (10 to 15 cm long) • modelling clay
• sand or lead BBs
This commercial hydrometer can be used to measure the sugar content of syrup. Why do you think someone might need this information?
2 How might you build a hydrometer?
Research to find possible designs and determine other materials required.
Build
3 Sketch the design you have chosen for
your hydrometer.
4 Gather the materials you require and
build your hydrometer.
5 Calibrate your hydrometer with fresh
water, which has a density of 1.0 g/mL. Fill a 250-mL or larger graduated cylinder to the very top with water. Place your straw or other hydrometer in the cylinder. Be careful not to make the water overflow. Use a permanent marker to draw a line on the straw at the water level. This line will indicate a density of 1.0 g/mL.
6 Calculate the density of each solution
shown in the table on the next page. Now, calibrate your hydrometer to the density of one of these standard solutions.
1 Look at your results for both Investigator activities:
Calculating Mass/Volume Ratio and How Dense? Make
a table of all of the substances in order from most dense to least dense.
2 a) Calculate the density of 27.1 g of mercury that displaces
2.0 mL of water.
b) Calculate the density of 5.25 g of silver that displaces 0.5 mL of water.
c) Calculate the density of 28.5 g of lead that displaces 2.5 mL of water.
d) If you had 100 mL of each solid substance, which one would have the greatest mass?
3 Describe a situation where accurate measurement of a solid’s
volume would be important. Suggest a way to measure the volume of a solid with the highest level of accuracy.
Test and Evaluate
9 Your teacher will give you two
unknown samples. To determine the densities of the unknown samples, work with three other groups who each calibrated their hydrometers with a different standard solution. The four hydrometers will provide a range of known densities. Test your unknown solutions and record their densities.
10 Evaluate the designs of the
hydrometers you used. Were there some designs that appeared easier to build and use? Were there some designs that used simpler materials?
Communicate
11 Write a short report that describes what
you did, your results, and one new thing you learned in this activity.
7 Empty the water from the graduated
cylinder, and fill the cylinder almost to the top with the salt solution you are testing. Place your hydrometer in the cylinder.
8 Mark the new level on your hydrometer.
2.5
Density and the Particle Theory
of Matter
So far, in the activities in this Big Idea, you have been collecting evidence to describe density. You observed that different
substances had different densities when at the same temperature. It is time to investigate what happens to the density of a fluid when the temperature of the fluid increases or decreases.
Frozen Water
Most fluids become denser as they cool down to their freezing temperature and turn into a solid. Water is an exception. It reaches its greatest density at 4°C (1.0 g/mL). Below that temperature, water becomes less dense as its temperature drops from 4°C to 0°C.
In the fall, as the weather becomes colder, the water in a lake starts to cool. When the water on the surface cools down to 4°C, it sinks because it is denser than the warmer water below. As the winter air continues to cool, the temperature of the surface water drops below 4°C. That water rises on top of the denser water, and eventually this upper water freezes. That is one reason why a lake freezes from the top down!
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The density of ice is 0.92 g/mL. Can you explain why ice floats?
Density Changes with Temperature
The Particle Theory of Matter can help us to understand that if a given sample of a substance stays at a constant temperature, the energy of particles stays the same. Because the energy of the particles stays the same, they will occupy the same volume. Density does not change as long as the temperature stays the same.
According to the Particle Theory, the particles begin to vibrate when energy is added to a substance. As a solid changes to a liquid and eventually to a gas, the particles vibrate and move more rapidly. How does this affect the density of a substance?
What would happen if the fluid being heated were a gas? What would happen if energy were removed from a fluid by reducing the temperature? Consider the image on the next page while you think about these questions.
Heating substances changes their density.
The volume of a gas increases when it is heated and decreases when it is cooled. What happens to the density of the gas? In this activity, you will investigate the effect of temperature on the density of a fluid.
Changing Densities
PR O B L E M SO L V E R
Using an inflated balloon, you are going to compare the density of room-temperature air and cold air.
• Predict whether there will be a change in the balloon’s volume or mass when you decrease the temperature of the air. • Blow up a balloon and tie it. Measure
its mass at room temperature using a balance. Record your results.
• Cool the air in the balloon by placing the balloon in a refrigerator or, in winter, taking it outside. Compare the balloon’s volume and mass when cooled to its volume and mass at room
temperature. Explain your answer.
1 Was there a difference in the number
of particles inside the cool and warm balloon?
2 Was there a difference in the volume
of air in the cool and warm balloon? Explain your answer.
3 How do you think temperature affects
the density of a fluid such as air in a balloon?
4 What does this activity tell you about
how a hot-air balloon operates?
The mass of a balloon measures how many air particles are inside.
As particles move more rapidly, the space between them increases. This can cause two things to happen. First, the volume increases because the particles need more room to move. The volume
increases, but the number of particles does not, meaning that the density decreases. Unless water is colder than 4°C, warm water has a lower density than cold water. That is why warm water rises above cold water. If you swim in a lake, you may have noticed patches of warm water on the surface and cooler patches of water at lower depths.
1 What units are usually used for measuring the density of
solids? of liquids? Why would there be a difference?
2 Use the Particle Theory of Matter to describe what happens
to the density of a substance when it cools.
3 Most substances increase their density as they move from a
gas to a liquid to a solid. Water is an exception, as it actually becomes less dense as it freezes. Without measuring, how would you know that ice is less dense than water?
4 How do you think temperature affects the density of a fluid
such as air in a balloon?
5 What ideas do you have about how a hot air balloon might
operate?
Galileo’s Thermometer
The picture on the right shows a different kind of thermometer. It is Galileo Galilei’s thermometer (or thermoscope), first invented in the 1590s. The initial densities of the bulbs are the same. Weighted tags are then attached, indicating a temperature for each bulb. Those showing higher temperatures have lower densities and those showing lower temperatures have higher densities.
As the temperature of the water rises its density decreases. Bulbs of lower density rise and those of higher density sink. The bulb that has the same density as the water floats just below the surface and indicates the temperature of the water.
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2.6
Check Your Progress
1 Make a list of all the words related to density in this Big Idea.
To illustrate your understanding of density, add these words to the concept map you began at the end of Big Idea 1.0. Once again, save your concept map for future use.
2 How are viscosity and density the same? How are viscosity
and density different?
3 Define density and explain how you can measure it.
4 What happens to the density of a fluid as it warms up?
Explain your observation using the Particle Theory.
5 In the graph below, which fluid has the highest density and
which has the lowest? How do you know?
6 Which is denser—10 g of shampoo or 10 kg of the same
shampoo? Explain.
7 Which is denser—water at 15°C, water at 4°C, or water
at 60°C? Explain.
8 Using your understanding of the Particle Theory of Matter,
how do you think the densities of gases would compare with those of liquids and solids?
E X
P LO R E
D E VE L O
P
If you dropped a metal bar into the water, what would happen to it? Yet a ship made of metal can remain on top of the water. Buoyancy causes a ship to float on water. Buoyancy is another important property of fluids.
3.1
Buoyancy and Forces
You have had an opportunity to build a hydrometer and determine the density of a variety of fluids. What causes the hydrometer to float higher or lower? Some force must have been working against gravity to allow the hydrometer to float. A force is a push or a pull that tends to cause an object to move or change its motion.
Floating occurs when the upward force on an object is equal to
or greater than the force of gravity pulling the object down.
Balanced or Unbalanced Forces?
For something to be able to float, the downard force of gravity must be opposed by an upward force. Objects in fluids may rise or sink if these two forces are not balanced forces.
When an object is in a liquid, the force of gravity or
gravitational force on the object pulls the object down toward
Earth. The liquid, however, exerts an opposite force, called the
buoyant force, and tries to push the object up.
If the mass of the object is greater than the mass of the water the object displaces, then the
gravitational force is greater than the buoyant force and the object will sink or dive. If the mass of the object is less than the mass of the water that the object displaces then the gravitational force is less than the buoyant force and the object will rise. The unit that scientists use for gravitational force is called the newton (N). It is equal to the amount of force required to accelerate a mass of one kilogram at a rate of one metre per second per second.
B
I GI
D E A S3.0
B I G I D E A
Fluids exert a buoyant force on objects
that causes some objects to float.
Floating in student-made hydrometers to test the density of two different liquids
A
Object A: balanced forces
B
Object B: unbalanced forces. Upward force is greater; object moves up.
