1. KS4 Physics
Forces

2. Forces Types of forces Mass and weight Forces and motion Friction
Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities

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

4. What type of force?

5. Different types of forces

6. Contact and non-contact forces
When two objects or materials need to be touching for a force to have an effect, it is a contact force.
Examples:
friction
air resistance
When two objects or materials do not need to be touching for a force to have an effect, it is a non-contact force.
Examples:
gravity
electrostatic
magnetic
Non-contact forces act over a distance.
Are these non-contact forces attractive, repulsive or both?

7. Forces Types of forces Mass and weight Forces and motion Friction
Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities

8. Gravitational attraction
Gravity is an attractive force that exists between all masses.
The larger the mass, the greater the gravitational attraction.
The greater the distance between masses, the smaller the gravitational attraction.
The Earth has a large mass and so produces a strong gravitational force.
The Moon is kept in orbit around the Earth by the pull
of the Earth’s gravity.

9. Gravity and weight
The pull of the Earth’s gravitational force on an object is called weight.
The Moon also has a gravitational force.
Why is the weight of an object
on the Moon less than the
weight of the same object on
the Earth?
The Moon is smaller than the
Earth and so the pull of the
Moon’s gravity is weaker than
the pull of the Earth’s gravity.
This means that the weight of
the object is less on the Moon.

10. Mass and weight 1 kg What is the difference between mass and weight?
mass = 1 kg
1 kg
weight = 10 N
Mass is the amount of matter that makes up
an object.
The mass of an object is always the same, wherever it is in the Universe.
Weight is a force due to the pull of gravity on an object.
The weight of an object will vary depending on where it is in the Universe.
The units of mass are kilograms (kg).
The units of weight are newtons (N).

11. Mass and weight – true or false?

12. Forces Types of forces Mass and weight Forces and motion Friction
Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities

13. Force diagrams air resistance weight
A force diagram uses arrows to show the forces acting on an object.
The direction of each arrow shows you the direction of each force.
The size of each arrow can be used to compare the sizes of the forces.
air resistance
What is the force diagram for this falling object when it first starts to fall?
weight

14. Forces on still objects
What forces are acting on Mel’s computer?
The computer is pulled downwards by the force
of gravity and causes
it to have weight.
The table exerts an
equal and opposite
force pushing upwards
on the computer. This is called the reaction force.
weight
reaction
force
These forces are balanced so the computer does not move.
What forces are acting on Mel as she works at her computer?

15. Forces on a swimming fish
What forces are acting on this fish as it swims?
weight
thrust
friction
upthrust
Upthrust is the upwards force on the fish caused by the water around the fish. Sometimes this is called buoyancy.
Thrust is the forwards force acting on the fish.

16. weight air resistance thrust friction upthrust
Forces on a moving boat
What forces are acting this moving boat?
weight
air
resistance
thrust
friction
upthrust

17. Changing forces on a ferry
An empty ferry arrives at port and is loaded with travellers.
What happens to the weight of the ship?
What happens to the upthrust on the ship?
Which force is now largest – weight or upthrust?
It increases.
It increases.
The increase in upthrust equals the increase in weight,
so both forces remain equal and balanced.

18. Forces and motion
If the forces on an object are balanced, the object will
continue to do what it is already doing without change.
If the object is stationary, it will remain stationary.
If the object is moving, it will continue to move
at the same speed and in the same direction.
If the forces on an object are unbalanced, two things
about the object can change:
The speed – the object may speed up or slow down.
The direction of motion.

19. Unbalanced forces and motion
If an object is stationary and unbalanced forces act on it, what will happen to the object?
The object will start to move – its speed and direction have changed.
If an object is moving and unbalanced forces act on it, what can happen to the object?
The speed of the object can change.
It might speed up or slow down.
The direction of the object can change.

20. Balanced and unbalanced forces
How many pairs of forces can you spot?
Are they balanced or unbalanced?
Teacher Notes
Items of note for the pairs of forces in this seaside scene include:
On the promenade, there are people walking, with one child jumping off the railings. There is also a family eating an ice cream and the woman’s ice cream is dripping and has just started falling to the ground.
There is a man selling helium balloons on the promenade.
There is a a roller coaster at the end of the promenade.
In the sea there are a couple of rowing boats – one being rowed and one anchored. Also, in the distance, there is a large ship.
In the sky there is a parachutist, a hot air balloon, seagulls and an aeroplane.

21. Balanced or unbalanced forces?

22. Forces Types of forces Mass and weight Forces and motion Friction
Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities

23. 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.

24. In which direction does friction act?
Friction always acts in the opposite direction to which an object is moving.
An object will only start to move if the forces applied to it are greater than any frictional forces.
What is the direction of friction acting on each moving ball?
friction
friction
friction
friction

25. Friction and movement

26. Effects of friction
Friction is a type of force that always acts in the opposite direction to which an object is moving and slows it down.
Whenever there is friction between two objects, heat is generated and their surfaces eventually wear away.
The effect of friction can be reduced by using a lubricant.
Oil is a common lubricant that is used in car engines and bike gears to reduce friction effects.

27. Friction – useful or a problem?

28. Forces Types of forces Mass and weight Forces and motion Friction
Contents
Forces
Types of forces
Mass and weight
Forces and motion
Friction
Summary activities

29. upthrust – An upwards force acting on an object in a liquid or a gas.
Glossary
air resistance – A frictional force that acts against an object moving through air.
contact force – A force that has an effect when two objects are touching.
friction – A contact force between an object and another material. This force opposes the motion of the object.
gravity – The force of attraction between any two objects that have mass.
mass – The amount of matter that makes up an object, measured in grams (g) or kilograms (kg).
unbalanced forces – Forces acting on an object that change its speed or direction.
upthrust – An upwards force acting on an object in a liquid or a gas.
weight – The force created by the gravitational attraction
on a mass, measured in newtons (N).

30. Anagrams

31. Multiple-choice quiz