1. Forces and their Effects
KS3 Physics
Forces and
their Effects

2. 7K Forces and their Effects
Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities

3. What is a force?
A force is a push, pull or twist. A force cannot be seen but you can see how a force affects an object.

4. Forces activity

5. When forces are balanced
balanced forces
10 N
10 N
If you link two forcemeters and pull equally hard from both ends, the forces recorded on both will be the same.
The forces acting on the central hooks cancel each other out – they are equal in magnitude and opposite in direction.
Because the forces are balanced the hooks do not move.

6. When forces are unbalanced
unbalanced forces
11 N
10 N
movement
What happens if the pull on one end is greater than the pull on the other end?
The forces acting on the hooks are no longer balanced.
Both hooks will start to move to the left – their speed will change. This is called acceleration.
Unbalanced forces lead to a change in speed or direction.

7. Balanced or unbalanced forces?

8. 7K Forces and their Effects
Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities

9. What is friction?
If you rub your hands together they get warm. There is resistance to the rubbing motion.
What is the name of this resistive force called?
It is called friction.
What causes this force?
Your hands might look smooth, but on a microscopic level they have rough surfaces. So when you rub your hands together you feel the resistive force of friction.

10. More about friction
Friction always tries to slow moving objects down. We say it opposes motion.
Friction is created whenever two touching objects or surfaces move past each other.
Friction also occurs when things move through air. This is called air resistance or drag.
There is a maximum value for the frictional force which depends on:
the force pushing the two surfaces together;
the state of the surfaces in contact.

11. What are the sources of friction?
Label all sources of friction that can act on this bike.
brake pad and rim
pedal bearing
wheel bearing
wheel bearing
One more? Probably the most important…
air resistance or “drag”
links in chain
tyre and road

12. Effect of frictional forces

13. 7K Forces and their Effects
Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities

14. upthrust thrust friction weight
Forces in water
What is thrust?
The force that pushes an object forward.
The force that pushes an object upwards in a liquid or a gas, also called buoyancy.
What is upthrust?
upthrust
thrust
friction
weight

15. Ferry forces

16. Changing forces
The ferry arrives at Calais and is loaded with travellers.
What happens to the weight of the ship?
What happens to the upthrust on the ship?
Which is the largest force - weight or upthrust?
increases
increases
both equal

17. 7K Forces and their Effects
Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities

18. What are mass and weight?
Mass and weight are not the same!
Mass is the amount of matter in an object and is measured in kilograms. Mass is not a force.
Mass will have the same value anywhere in the Universe, including outer space.
Weight is a force and is caused by the pull of gravity acting on a mass. Like other forces, weight is measured in newtons and has both magnitude and direction.
Weight has different values depending on where you are in the Universe.

19. Weight and mass on the Moon
The force of gravity is less on the Moon than it is on Earth. This is because the Moon has a smaller mass than the Earth.
Any object will weigh less on the Moon than it does on Earth.
An astronaut could jump 20 feet into the air on the Moon because gravity is less.
However, the astronaut still has the same body and the same mass: he just weighs less because gravity is weaker on the Moon.

20. Talking about weight and mass
A scientist should never say:
“She weighs 50 kilograms,”
but should always say:
“She has a mass of 50 kilograms”.
Alternatively, the scientist could say:
“The gravitational force acting on her mass is about 500 newtons”.
This is the same as saying:
“Her weight is about 500 newtons”.

21. Weight and mass on different planets

22. Weight or mass?

23. 7K Forces and their Effects
Contents
7K Forces and their Effects
Feel the force
Friction
Forces in water
Weight and mass
Summary activities

24. Glossary
air resistance – A frictional force that acts against an object moving through air.
force – A push or a pull, measured in newtons (N).
friction – A force that occurs between two surfaces rubbing against each other.
gravity – The force of attraction between two objects.
mass – The amount of matter that an object is made of, measured in kilograms (kg).
thrust – A force that pushes an object forwards.
upthrust – A force that pushes an object upwards.
weight – The force caused by gravity acting on an object, measured in newtons (N).

25. Anagrams

26. Missing words

27. Multiple-choice quiz