Lesson 2 The Buoyant Force

(1)

Forces and Fluids 25

Student Labs and Activities Page

Launch Lab 26

Content Vocabulary 27

Lesson Outline ²⁸

MiniLab 30

Content Practice A 31

Content Practice B 32

School to Home 33

Key Concept Builders 34

Enrichment 38

Challenge 39

Skill Practice 40

C365_025_025_CRF_L2TOC_892513.in25 25

C365_025_025_CRF_L2TOC_892513.in25 25 2/27/10 2:00:02 PM2/27/10 2:00:02 PM

(2)

26 Forces and Fluids

Launch Lab LESSON 2: 10 minutes

How can objects denser than water float on water?

Many ships are made of aluminum. Aluminum is denser than water. Why does it float?

Procedure

Data and Observations

Think About This

Key Concept

What do you think caused each object to float or sink?

1.

Read and complete a lab safety form.

2.

Use scissors to cut three 10- × 10-cm squares of aluminum foil.

3.

Form a boat shape from one square of foil. Squeeze another square into a tight ball. Fold the third square several

times into a 2- × 2-cm square. Flatten it completely.

4.

Predict whether each object will sink or float. Then, gently place each in a tub of water. Record your observations in the Data and Observations section below.

C365_026_032_CRF_L2_892513.indd 26

C365_026_032_CRF_L2_892513.indd 26 2/27/10 2:02:58 PM2/27/10 2:02:58 PM

(3)

The Buoyant Force

Directions: Use Archimedes’ principle to rank the three objects below in order, from the one with the greatest buoyant force to the one with the smallest buoyant force. Explain your answer on the lines provided. Use––and underline––each of the terms below at least once in your explanation.

Archimedes’ principle buoyant force displace

fluid volume weight

Content Vocabulary LESSON 2

A B C

C365_026_032_CRF_L2_892513.indd 27

C365_026_032_CRF_L2_892513.indd 27 2/27/10 2:03:03 PM2/27/10 2:03:03 PM

(4)

The Buoyant Force

A.

What is a buoyant force?

1.

A(n) is an upward force applied by a fluid on an object in the fluid.

a.

The upward buoyant force on an object is balanced by the force of .

b.

When the buoyant force on an object in a fluid is equal to the total of the object, the object

will .

2.

A buoyant force acts on any object in a(n) , such as water, and in a(n) , such as air.

3.

Forces from water pressure on an object within a fluid are in

and directions.

a.

forces on the object are equal on all sides of the object, but the forces are not equal.

b.

The difference between the upward and downward forces from pressure on an object in a fluid is the .

c.

The buoyant force on an object is always a(n) direction.

4.

The buoyant force on an object in a fluid stays the same when the object goes in the fluid.

B.

Archimedes’ Principle

1.

states that the weight of the fluid that an object displaces is equal to the buoyant force on the object.

2.

As more water is displaced by an object that is pushed under water, the buoyant force acting on the object .

Lesson Outline LESSON 2

C365_026_032_CRF_L2_892513.indd 28

C365_026_032_CRF_L2_892513.indd 28 2/27/10 2:03:04 PM2/27/10 2:03:04 PM

(5)

C.

Sinking and Floating

1.

When an object weighs more than the weight of the water it displaces, the object .

2.

When the weight of an object is equal to the weight of the water it displaces, the object .

3.

If an object has greater density than the fluid in which it is placed, the object .

a.

A metal boat because the overall density of the metal and the in the boat is less than the density of water.

b.

If a metal boat fills with water, then the density of the metal and the water within the boat becomes greater than the density of water outside of the boat, and the boat .

c.

A helium balloon in air because the density of the helium within the balloon is than the density of air outside of the balloon.

d.

As the volume of helium in the balloon , the balloon’s density and the balloon falls to the ground.

Lesson Outline continued

C365_026_032_CRF_L2_892513.indd 29

C365_026_032_CRF_L2_892513.indd 29 2/27/10 2:03:05 PM2/27/10 2:03:05 PM

(6)

MiniLab LESSON 2: 10 minutes

Can you overcome the buoyant force?

An air-filled balloon floats in water because the buoyant force pushing up on it balances the weight pushing down. Can you overcome the buoyant force and make the balloon sink?

Procedure

1.

2.

Inflate a balloon until it is 2 cm in diameter.

3.

Place the balloon in a tub of water.

Attach a large paper clip to the balloon.

4.

Continue adding paper clips, one at a time. Record your observations in the Data and Observations section below.

Data and Observations

Analyze and Conclude

1. Analyze

What happened to the balloon as you added paper clips? What happened to the buoyant force on the balloon?

2. Key Concept

Were you able to make the balloon sink below the surface? Why or why not?

C365_026_032_CRF_L2_892513.indd 30

C365_026_032_CRF_L2_892513.indd 30 2/27/10 2:03:06 PM2/27/10 2:03:06 PM

(7)

The Buoyant Force

Directions: On the line before each statement, write T if the statement is true or F if the statement is false. If the statement is false, change the underlined word(s) to make it true. Write your changes on the lines provided.

Content Practice A LESSON 2

1.

Objects in liquid experience a buoyant force.

2.

Objects in a gas experience a buoyant force.

3.

A buoyant force is a downward force.

4.

The buoyant force increases with depth.

5.

The greater the volume of a fluid, the more it weighs.

6.

The buoyant force of an object depends on its mass.

7.

An object floats if the buoyant force acting on it is equal to its weight.

The object that has the greatest volume...

has the greatest weight (gravitational

force).

displaces the greatest amount

of water.

has the greatest buoyant, or upward, force.

The buoyant force on an object is equal to the weight of the fluid that the object displaces. (Archimedes’ principle)

C365_026_032_CRF_L2_892513.indd 31

C365_026_032_CRF_L2_892513.indd 31 2/27/10 2:03:10 PM2/27/10 2:03:10 PM

(8)

The Buoyant Force

Directions: Answer each question on the lines provided.

1.

What is a buoyant force?

2.

On what does a buoyant force act?

3.

From which direction(s) does a fluid apply pressure?

4.

Why is a buoyant force always an upward force?

5.

When does an object float?

6.

When does an object sink?

7.

Why does the buoyant force on an object not change with depth?

8.

What does Archimedes’ principle state?

Content Practice B LESSON 2

C365_026_032_CRF_L2_892513.indd 32

C365_026_032_CRF_L2_892513.indd 32 2/27/10 2:03:11 PM2/27/10 2:03:11 PM

(9)

The Buoyant Force

Directions: Use your textbook to respond to each statement.

For this activity, you will need a tank of water and several small household objects that can be placed in the tank and tested for buoyancy.

1.

Buoyant force describes the force that pushes upward on objects and causes some objects to float. With your learning partner, define buoyant force in your own words.

2.

Demonstrate for your learning partner the buoyant force of several objects, using the tank of water and objects.

3.

Describe the nature of the objects you demonstrated in step 2. Include the following terms in your response: density, buoyant force, weight, Archimedes’ principle.

4.

With your learning partner, adjust your definition of buoyant force based on your demonstration.

School to Home LESSON 2

C365_033_033_CRF_L2StoH_892513.i33 33

C365_033_033_CRF_L2StoH_892513.i33 33 2/27/10 2:07:21 PM2/27/10 2:07:21 PM

(10)

The Buoyant Force

Key Concept

How are pressure and the buoyant force related?

Directions: On each line, write the term from the word bank that correctly completes each sentence. Some terms will be used more than once.

buoyant depth directions equal fluid force gravitational force gravity

liquid perpendicular surface upward force water pressure weight

A buoyant force is a(n) (1.) applied by a(n)

(2.) on an object in the fluid. Other forces also act on any object.

The gravitational force on an object is the object’s (3.) .

(4.) is a pulling force. Atmospheric pressure also acts as a force on objects.

A buoyant force acts on any object in a(n) (5.) . The buoyant force also acts on objects in a gas. A buoyant force is acting on you right now as are

atmospheric pressure and (6.) . A fluid applies pressure (7.) to all sides of an object within the fluid, so an object in

the water has pressure from the sides, top, and the bottom acting on it. Horizontal forces are (8.) in all (9.) , so they cancel themselves out. Forces acting from the bottom and top are not

(10.) . Pressure increases with (11.) . Because the bottom of an object in water has more depth than the top of the object,

(12.) at the bottom of the object’s surface is greater than water pressure on the top (13.) of the object.

Because upward and downward forces on an object in water are not equal, there is a(n) (14.) force. Also, because the (15.) from pressure is always greater below an object than above the object, the

(16.) force is always an upward force.

Key Concept Builder LESSON 2

C365_034_037_CRF_L2KeyCo_892513.34 34

C365_034_037_CRF_L2KeyCo_892513.34 34 2/27/10 2:11:10 PM2/27/10 2:11:10 PM

(11)

The Buoyant Force

Key Concept

How does Archimedes’ principle describe the buoyant force?

Directions: On the line before each statement, write T if the statement is true or F if the statement is false. If the statement is false, change the underlined word(s) to make it true. Write your changes on the lines provided. Use the diagram to answer numbers 1 through 5.

Key Concept Builder LESSON 2

1. Archimedes’ principle states that a buoyant force on an object is equal to the volume of the fluid that the object displaces.

2. As the diagram shows, the weight of the water displaced by the balloon is greatest among the three different balls.

3. The buoyant force does not depend on an object’s depth in a fluid.

4. The diagram shows that the balloon has greater mass than the tennis ball or billiard ball.

5. According to Archimedes’ principle, the balloon has the greatest buoyant force because it displaces the most water.

6. As the diagram shows, the buoyant force depends on an object’s weight.

7. Concerning buoyant forces, it is important to remember that the

greater the volume of an object is, the more it weighs.

8. The buoyant force of an object does not change as the object moves deeper into a fluid.

Balloon Tennis ball Billiard ball

C365_034_037_CRF_L2KeyCo_892513.35 35

C365_034_037_CRF_L2KeyCo_892513.35 35 2/27/10 2:11:15 PM2/27/10 2:11:15 PM

(12)

The Buoyant Force

Key Concept

What makes an object sink or float in a fluid?

Directions: Answer each question or respond to each statement in the space provided.

1. A billiard ball sinks to the bottom of a fish tank filled with water. Explain why this happens.

2. When a rubber duck is placed in a fish tank filled with water, it floats. Explain why this happens.

3. Two metal boats of equal weight are placed in water. One boat is filled with air. The other boat is filled with water. Which boat will float? Explain.

4. A balloon filled with helium air floats. Explain why the buoyant force pushes the balloon upward.

5. A young child blows up a balloon hoping it will float, but instead it falls to the ground. Explain.

Key Concept Builder LESSON 2

C365_034_037_CRF_L2KeyCo_892513.36 36

C365_034_037_CRF_L2KeyCo_892513.36 36 2/27/10 2:11:21 PM2/27/10 2:11:21 PM

(13)

The Buoyant Force

Key Concept

What makes an object sink or float in a fluid?

Directions: Put a check mark in the column to show if the object described will sink or float.

Object Sink Float

1. a ball that weighs the same as the water it displaces when it is placed in the water

2. an anchor whose density is greater than the density of water

3. a ship that has greater mass per volume than water

4. a toy boat that has a buoyant force that is less than the toy boat’s weight

5. a helium balloon whose density is less than the density of the air around it

6. a bath toy filled with air and placed in a bathtub filled with water and floating soap bubbles

7. a boat filled with sand that has a weight equal to the weight of the water it displaces

8. a balloon whose volume decreases to a point that its density is greater than the air around it

9. a parade balloon that has a buoyant force acting on it that is equal to the balloon’s weight

10. a boat whose weight and the buoyant force acting on it are in balance

Key Concept Builder LESSON 2

C365_034_037_CRF_L2KeyCo_892513.37 37

C365_034_037_CRF_L2KeyCo_892513.37 37 2/27/10 2:11:25 PM2/27/10 2:11:25 PM

(14)

Who was Archimedes?

Enrichment LESSON 2

Archimedes’ principle relates the buoyant force on an object to the weight of the liquid that the object displaces. Born in 287 B.C., Archimedes was a brilliant mathematician who had a keen

understanding of the physical world. His work in mathematical physics predated the ideas of great thinkers such as Isaac Newton by nearly 2,000 years.

What did he do?

Archimedes discovered the laws of levers and pulleys. He once bragged “Give me a place to stand and I can move the Earth.”

Of course, there was no way to prove the claim, so he was challenged to move a large, fully loaded ship that usually required large groups of workers to move. Archimedes succeeded with a complex pulley system.

When the Roman fleet decided to attack Syracuse, where Archimedes’ lived, he designed catapults with adjustable ranges that could throw objects weighing more than 227 kg. Large grappling hooks attached to levers lifted the ships that survived out of the water and then dropped them. During another assault, Archimedes had the soldiers of Syracuse use specially shaped and polished shields to focus the sunlight onto the sails of the enemy ships to set them on fire.

The King’s Crown

A famous story about Archimedes tells of a king of Syracuse who suspected that a golden crown that had been made for him wasn’t pure gold. The king asked Archimedes to find a way of telling whether the crown was pure gold without damaging it.

The crown weighed the correct amount, but that could have been accomplished by mixing silver and gold. As the story goes, Archimedes stepped into his bath and noticed that some of the water displaced by his body flowed over the edge of the tub.

He had a sudden flash of inspiration and was said to have run through the streets of Sicily screaming “Eureka!” which means,

“I found it!”

Archimedes realized that the crown should not only weigh the right amount, but should displace the same amount of water as the same mass of pure gold.

For example, the density of gold is 19.3 g/cm³. Therefore, a crown made of 1,000 g of gold should have a volume of about 52 cm³. If the goldsmith replaced some of the gold with silver, the volume of the crown would have to be greater because silver is less dense than gold. Therefore, if more water overflowed the container with the king’s crown than with an equal weight of gold, then the crown was a fake.

Applying Critical-Thinking Skills

Directions: Answer each question.

1. Analyze

Do you think that Archimedes could actually have moved Earth with a lever if he’d had a place to stand in space? Why or why not?

2. Calculate

The density of a solid is calculated by dividing its mass by its volume. You have a piece of yellow metal that a friend claims is pure gold. The metal has a mass of 2,625 g and a volume of 150 cm³. Is your friend right? Why or why not?

C365_038_039_CRF_L2EnCha_892513.38 38

C365_038_039_CRF_L2EnCha_892513.38 38 2/27/10 2:13:52 PM2/27/10 2:13:52 PM

(15)

Challenge

Float or Sink

Obtain an assortment of unopened soda cans of the same size. Be sure to include regular and diet sodas as well as different flavors and brands, including generic.

Predict whether the cans will float or sink when you place them in a bucket or other deep container of water. Write a description and a prediction for each can in the table.

Description What I Predict What happened?

1.

2.

3.

4.

5.

6.

7.

8.

Place the cans into a bucket or other deep container half-filled with tap water. Tip the cans to release any air bubbles that are trapped under the can. Observe and record what happens to each can.

Write a hypothesis about why some of the cans floated and some sank. Base your hypothesis on what you learned in this lesson about density and Archimedes’ principle.

LESSON 2

C365_038_039_CRF_L2EnCha_892513.39 39

C365_038_039_CRF_L2EnCha_892513.39 39 2/27/10 2:13:53 PM2/27/10 2:13:53 PM

(16)

Do heavy objects always sink and light objects always float?

You have seen many objects that sink in water and many that float. Is it possible to predict whether an object will sink or float if you know its mass? In this lab, you will measure the mass of various objects and then predict whether the objects sink or float.

Materials

tennis ball, pencil, triple-beam balance, craft stick

Also needed: pink pencil eraser, pumice rock, cork, rubber ball, plastic button, tub of water, pennies, crayon, rubber band

Learn It

When you predict the results of a scientific investigation, you tell what you think will happen. You should base your prediction on what you already know and on things you observe.

Try It

1.

2.

Use the data table below to record your data. Add more rows as needed.

3.

Measure the mass of a craft stick. Record it in your data table.

4.

Predict whether the craft stick will sink or float. Record your prediction.

5.

Place the craft stick in the tub of water. Does it sink, or does it float? Record your results.

Object Mass (g) Predict

Sink or Float?

Observe Sink or Float?

Predict

Skill Practice LESSON 2: 30 minutes

C365_040_041_CRF_L2Skill_892513.40 40

C365_040_041_CRF_L2Skill_892513.40 40 2/27/10 2:16:38 PM2/27/10 2:16:38 PM

(17)

Apply It

6.

Measure and record the mass of each of the other objects in your data table.

7.

Predict whether each object will sink or float. Record your predictions.

8.

Place each object in the water, and observe whether it sinks or floats. Record your results.

9. Key Concept

Do heavy objects always sink and light objects always float?

Explain your reasoning.

Skill Practice continued

C365_040_041_CRF_L2Skill_892513.41 41

C365_040_041_CRF_L2Skill_892513.41 41 2/27/10 2:16:41 PM2/27/10 2:16:41 PM