NA - Mass Weight Density - E-Learning

2. LESSON OBJECTIVES
At the end of the lesson, you should be able to:
a) State that mass is a measure of the amount of substance in a body
b) State that a gravitational field is a region in which a mass
experiences a force due to gravitational attraction
c) Define gravitational field strength, g, as gravitational force per unit
mass
d) Recall and apply the relationship weight = mass x gravitational field
strength to new situation or to solve related problems
e) Distinguish between mass and weight
f) State that the mass of a body resists a change in the state of rest or
motion of the body (inertia)

3. HOW DO YOU MEASURE YOUR WEIGHT?
• Bathroom scale
• kilograms (kg)

4. MASS
• Mass is a measure of the amount of substance in a body.
• SI Unit: kilogram (kg)
• Scalar
• Represented by m
• Mass does not change wherever the body is.

5. WEIGHT
• The weight of an object is the gravitational force, or
gravity acting on it.
• SI Unit: newton (N)
• Vector
• Represented by W

6. WEIGHING MACHINE
• Weighing machine measures weight but the units is kg.
So what does the weighing machine measure?
• Weighing machine measuresWEIGHT.
But it is calibrated to display the mass of the body.
• How do you calibrate the weighing machine?

7. FORMULA
Weight (in 𝐍) = mass in 𝐤𝐠 ×
gravitational
field strength
(in 𝐍/𝐤𝐠)
𝑾 = 𝒎𝒈
W
m g
What is
gravitational
field strength?

8. GRAVITATIONAL FIELD & ITS STRENGTH
• A gravitational field is a region in which a mass
experiences force due to gravitational attraction.
• In a gravitational field, a mass experiences a force.
• Weight is also known as gravitational force.
Earth’s gravitational force acts
towards the centre of Earth.

9. GRAVITATIONAL FIELD & ITS STRENGTH
• The strength of this field is strongest at the surface of
Earth and gets weaker farther away.
say, 50 N >50 N

10. GRAVITATIONAL FIELD & ITS STRENGTH
• Definition:
Gravitational field strength, g, is the gravitational force
acting per unit mass.
• Gravitational – an effect due to gravity
Field – a region where force is experience
Strength – the magnitude of the force
• Actual gravitational field strength, g of Earth= 9.80 N/kg.
In calculations however, we use 10 N/kg to make it simple.

11. ACTIVITY!
Find Mr.Wong’s weight on these places!
My mass = 100 kg
Planet
Gravitational
field strength, g
Weight
Earth 10 N/kg
Moon 1.6 N/kg
Jupiter 24.9 N/kg
Pluto 0.6 N/kg
Conclusion
Weight is dependent on the gravitational field strength of
the location. (direct proportion)

12. FUN FACT!
Planet
Gravitational
field strength, g
Weight
Earth 10 N/kg
Moon 1.6 N/kg
Jupiter 24.9 N/kg
Pluto 0.6 N/kg
The more massive the
object, the greater the
gravitational field
strength!!!
Mass of these
planets
5.97 × 1024 kg
7.25 × 1022 kg
1.90 × 1027 kg
1.25 × 1022 kg

13. FUN FACT 2!!
• Do you know that every object including yourself also
have some gravitational field strength?
• For a person of 60 kg,
his gravitational field strength at 1 metres away
≈ 4 × 10-9 N/kg
• Maybe that’s how you attract someone!

14. Weight is measured by using spring balances.
DO YOU REMEMBER HOW IS WEIGHT
MEASURED?
Extension Spring Balance Compression Spring Balance

15. HOW TO MEASURE MASS?
Beam balance with standard masses can be used to measure mass.
Since the standard mass experiences the same g, this method can be
used on any planet.
Beam Balance Electronic Balance
Nowadays, the electronic beam is more commonly used to measure
mass as it is more accurate and easier to use.

16. DIFFERENCE BETWEEN MASS & WEIGHT
Differences Mass Weight
The amount of substance in a
body
The gravitational force acting
of the body
Scalar Vector
kilograms (kg) newtons (N)
Independent of g Dependent on g
Measured with beam balance
Measured with spring
balance
Definition
Scalar or
Vector?
SI Unit
Dependence
on g
Measuring
tool

17. INERTIA
• Mass is a measure of inertia
• The inertia of an object refers to the reluctance of the
object to change its state of rest or motion, due to its
mass.
From at rest to moving; or
Moving to rest.
Changing direction
• The greater the mass, the more inertia an object has.

18. INERTIA IN ACTION
• A MRT train – large mass
Time is required to start moving a train.
Large braking force is required to stop a moving train.
• Wild boar
A wild boar is unable to change its direction quickly
due to its mass
Wild Boar Drill: Zig-zag running pattern allows us to
run away from it.

19. INERTIA IN ACTION
• Oldest trick in the world!
http://youtu.be/-cM9S2AzU28
What is the reason the bottles did not move?
The bottles have inertia which resists the change of
motion.
Now watch the video again – Is it real or fake?
How do you think they manage to get it done?

20. INERTIA IN ACTION
Answer: