Mass, Weight &
Density
At the end of the lesson, you should be able to….
• State that mass is a measure of the amount of substance in a body.
• State the mass of a body resists a change in the state of rest or motion of the body (inertia).
• State that the gravitational field is the region in which a mass experience a force due to gravitational attraction.
• Define gravitational field strength, g, as gravitational force per unit mass.
At the end of the lesson, you should be able to….
• Recall and apply the relationship weight = mass x gravitational field strength to new situations or to solve related problems.
• Distinguish between mass and weight.
• Recall and apply the relationship density = mass / volume to new situations or to solve
QUESTION:
Mass
• Mass measures the amount of matter in a body.
Beam balance Electronic balance
MEASURING Mass
• To measure mass, we use a beam balance or an electronic balance.
VIDEO
INERTIA
• Inertia is the reluctance of an object to change its state of motion.
• It is dependent on
mass only.
• The greater the mass, the greater is the object’s
Driver not wearing the seat belt.
Driver wearing the seat belt.
The reason why we need to :
- wear seat belts
- hold onto the railings on the MRT or
bus.
Weight ?
• Weight measures the gravitational pull on an object. It varies with the mass and the distance between two bodies.
Mathematically,
Weight = mass x acceleration due to gravity
W = mg
•To measure weight, we use a spring balance or compression balance.
Spring balance Compression balance
Weight
Near Earth surface
g = 9.81 ms-2
g < 9.81 ms-2 g = 0 ms-2
If a book has a mass of 1.2 kg, what is it’s weight on (a) Earth, (b) Moon?
Example 1
(a) W = m g
= 1.2 x 10 = 12 N
Weight of book on
(b) W = m g
= 1.2 x 10/6 = 2 N
Gravitational Field Strength
•The force is greater near the surface and gets weaker as we go further away.
•The gravitational field strength near the surface of the Earth is 10 Nkg-1.
• This means that near Earth’s surface, an object
with mass of 1 kg experiences a pull of 10 N
towards the Earth.
Gravitational field strength is defined as the gravitational force acting on a unit mass in the region of gravitational field.
Gravitational Field Strength
Summary on differences between mass and weight
Mass Weight
Definition The amount of
matter in an object.
The gravitational pull on an object
SI unit kilogram (kg) Newton (N)
Measured by Beam balance,
electronic balance
Spring balance,
VIDEO
Density
volume mass
Density
• Density is defined as its mass per unit volume.
• Mathematically,
V
m
SI Unit: kgTable: Densities of common
A metal box has a mass of 30g and is placed in a measuring cylinder containing water. The water level then rises from 25 cm3 to 30 cm3. Find the density of the metal in
(a) gcm-3
(b) kgm-3
(a) = m / V = 30 / 5 = 6 gcm-3
(b) gcm-3 ---> kgm-3
3 3 m kg cm g
x 10-3
x 10-6
1 cm3 = 1 cm x 1 cm x 1 cm
(b) = m / V
= (30 x 10-3) / (5 x 10-6) = 6000 kgm-3
1 gcm
-3= 1000 kgm
-3or
Answer:
The cruise ship also has a large volume not entirely made up of steel.
The rooms and cabins contain a considerable amount of air.
Floating and Sinking
• To find out whether an object will float or sink in a liquid, we need to compare their densities.
• If density of object is more than liquid, object sinks.
Example 3 Liquid P Liquid Q Cube A Cube B Cube C
Density > or < Density
Cube A Liquid Q Cube B Liquid P Cube C Liquid Q Liquid P Liquid Q Cube A Cube C
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>
>
Coke vs Diet Coke
• Sugar is denser than water
• CO2 is less dense compared to water
• Classic coke contains more sugar than diet coke.
• Diet coke contains more CO2 than classic coke.
classic coke denser than water (sinks)
What about 100plus ??
• Salt is denser than water
• 100 plus has a lot of salt!!!
100 plus denser than water (sinks)
Implications?
Dead Sea
- Salt lake between the West Bank
- Israel to the west, and Jordan to the east.
Dead Sea
- 330 m (1,083 ft) deep, the Dead Sea is the deepest hypersaline lake in the world.
- It is also the world's second saltiest body of water.
