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Physics GCSE
P4: Explaining Motion

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Table of Contents
A: Forces in all directions


Contact forces

B: How things start moving

C: Friction


The limit

D: Reaction of surfaces


A reaction of a surface

E: Adding forces

Resultant force

F: How fast are you going?

Average speed

Instantaneous speed

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G: Picturing motion

Distance-time graphs

Speed-time graphs


Velocity-time graphs

H: Force, interaction and momentum


Change of momentum

I: Car safety

J: Laws of motion

Driving forces


K: Work and energy


Gravitational potential energy

Kinetic energy

Conservation of energy

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A: Forces in all directions
Forces are needed to change the motion of an object, and are the pushes and pulls

experienced by an object when it interacts with another.

Forces can be represented by an arrow.

Forces arise from interactions between two objects.

Forces always come in pairs, called…

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B: How things start moving
Rockets use the force from burning hot gases to move.

Jet engines work similarly to rockets, but with air instead of burning hot gases.

Cars need to grip the surface to exert a force on it, thus making an equal force forwards.

When you walk,…

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C: Friction
Friction is the force exerted on an object due to the interaction between it and

another object that is sliding over it that is caused by the roughness of both surfaces.

Friction adjusts its size in response to the situation up to a limit.

The limit depends on…

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D: Reaction of surfaces
Gravity causes objects to fall towards the centre of the Earth however this can be

cancelled out by a surface exerting an opposite force on the object.

A reaction of a surface is when a hard surface exerts an upward force when

something presses it.


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E: Adding forces
If there is a force on an object, but the object is not moving, there must be another force

cancelling it out.

If forces balance each other, they are said to add to make zero.

Resultant force is the sum of all the forces acting on an…

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F: How fast are you going?
Average speed can be calculated using the equation:

o Speed = distance

Average speed is not always useful, because in reality, during most journeys the speed


Instantaneous speed is the speed at a particular moment in time, and is

calculated by working…

Page 10

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G: Picturing motion
Distance-time graphs show how far an object is from its starting point at every

moment of the journey.

On a distant-time graph:

o Horizontal lines represent no movement

o Straight lines represent a steady speed

o Curved lines represent a change in speed; sloping up curves represent…


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