1. P2 – Forces and Motion
Lesson 4: Forces 1

2. To know the difference between mass and weight.
Lesson Objectives
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.

3. To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Mass and Weight
What is the difference between the terms mass and weight?
What are some of the important effects of gravity?

4. To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Mass and Weight
Mass – the amount of material. Measured in Kilograms (kg). This is the same anywhere in the universe.
Weight – the force which attracts a body. Measured in Newtons by a Newton meter (N). This is caused by the pull of gravity.
Therefore, 1kg bag of sugar has the same mass on Earth and the moon, but a different weight (10N on Earth, 1N on the moon) as the gravity on the moon is less.

5. Gravity Gravity has three important effects:
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Gravity
Gravity has three important effects:
On the surface of a planet, it makes all things accelerate towards the ground (on Earth it is about 10m/s2)
It gives everything a weight.
It keeps planets, moons and satellites in their orbit.

6. Very important formula
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Very important formula
Remember, mass is measured in kilograms (kg) and weight is measured in Newtons (N).
So, you can work out the weight of an object using:
W = m x g (W = weight, m = mass, g = gravity strength)

7. Very important formula
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Very important formula
W = m x g (W = weight, m = mass, g = gravity strength)
Q: What is the weight, in newtons, of a 5kg mass, both on Earth and on the moon?
(‘g’ on moon is 1.6N/kg, ‘g’ on Earth is 10N/kg)
A: W = m x g.
Earth = 5 x 10 = 50N, Moon = 5 x 1.6 = 8N.

8. The Three Laws of Motion
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
The Three Laws of Motion
Sir Isaac Newton

9. First Law: Balanced Forces mean no change in velocity.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
First Law: Balanced Forces mean no change in velocity.
Remember: Velocity is the speed in a certain direction.
So: As long as there is no change in the forces on an object, it will either:
Stay stationary
Travel at a constant speed.

10. First Law: Balanced Forces mean no change in velocity.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
First Law: Balanced Forces mean no change in velocity.
Why?
Team 1 Team 2
6N N
The resultant force is 0 (both forces are balanced).

11. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
This means that if there is a resultant force (i.e. the forces are unbalanced) there will be movement.

12. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Why?
Team 1 Team 2
6N N
The resultant force is
2N towards team 1

13. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Any resultant force will produce acceleration, and is calculated by:
F = ma (F = Force, m = mass, a = acceleration)

14. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Remember:
The bigger the force, the greater the acceleration or deceleration.
The bigger the mass, the smaller the acceleration.
To get a big mass to accelerate as fast as a small mass it needs a bigger force.

15. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Task:
1. A car of mass 1750kg has an engine which provides a driving force of 5200N. At 70mph the drag force in 5150N. Find its acceleration:
When first setting off from rest (assume that there is no friction at rest)
When at 70mph

16. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
2. A car of mass 1750kg has an engine which provides a driving force of 5200N. At 70mph the drag force in 5150N. Find its acceleration:
When first setting off from rest
Resultant Force = 5200N
a = F/m = 5200/1750 = 3.0m/s2
5200N

17. Second Law: A Resultant Force means acceleration.
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
2. A car of mass 1750kg has an engine which provides a driving force of 5200N. At 70mph the drag force in 5150N. Find its acceleration:
When at 70mph
Resultant Force = 5200 – 5150 = 50N
a = F/m = 50/1750 = 0.03 m/s2
5150N
5200N

18. Third Law: Reaction Forces
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
Think about this one! This law says “if an object exerts a force on to another object, then that object exerts an equal and opposite force back”.

19. Third Law: Reaction Forces
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
So: If you push pram, the pram will push back against you just as hard. When you stop pushing, so does the pram.

20. Third Law: Reaction Forces
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
Q: If this is happening, then how do we ever go anywhere?

21. Third Law: Reaction Forces
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
Think about: Skater ‘A’ pushes Skater ‘B’ (the action force). Skater ‘A’ feels an equal and opposite force from Skater ‘B’s hands (the reaction force)
Skater A
Skater B

22. Third Law: Reaction Forces
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
So: Both Skaters feel the same sized force, in opposite directions. Therefore, they accelerate in opposite directions away from each other.
Skater A
Skater B

23. Third Law: Reaction Forces
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
Another thought: Think about swimming. You push against the water with your arms and legs. The water pushes you in the opposite direction, with the same force.

24. Task: How does mass affect acceleration?
To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Task: How does mass affect acceleration?

25. Are you now able to…. To know the difference between mass and weight.
To be able to calculate mass and weight.
To know the three laws of motion.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
I still feel unsure. I need some more help to understand.
I feel ok. I need to do some more work to check my understanding.
I am happy and feel I understand and can explain the main points.