1.11-1.12 - Forces and their effects
Forces are how bodies affect one another. A force can change how an object moves, or it can change the object's shape. In this lesson, you will learn to describe those effects and to identify common force types such as gravitational and electrostatic forces.
Forces between bodies
A force is a push or pull that acts between bodies. In physics, a body just means an object or a part of an object.
Force
A force is a push or pull that can change the speed, direction, or shape of a body.
The phrase "between bodies" matters. If a racket hits a tennis ball, the racket exerts a force on the ball. If Earth pulls on an apple, Earth exerts a gravitational force on the apple. If a charged balloon attracts small pieces of paper, the balloon exerts an electrostatic force on the paper.
For Edexcel IGCSE, the first job is often to identify the effect of a force from the situation:
| Situation | Effect of the force |
|---|---|
| A trolley starts moving after being pushed | change in speed |
| A moving bicycle slows down when the brakes are applied | change in speed |
| A football changes path after being kicked | change in direction |
| A spring is stretched by a hanging load | change in shape |
A force may have more than one effect at the same time. A bat hitting a ball can change the ball's speed and direction. A hand squeezing a sponge can change its shape and may also make it move.
Changing speed
One effect of a force is a change in speed. This includes starting from rest, speeding up, slowing down, and stopping.
If a student pushes a stationary trolley, the push can make the trolley start moving. If the student keeps pushing in the direction the trolley is moving, the trolley can get faster. If the student pushes in the opposite direction, the trolley can slow down.
If a constant backward resultant force continues after an object stops, it starts moving backwards and then speeds up backwards. A stopping force such as ordinary sliding friction does not by itself make an object reverse: the continued pull in the test is essential.
The same idea applies when a brake pad rubs against a bicycle wheel. The bicycle was already moving, but the braking force makes its speed decrease. A good answer names the change in speed rather than just saying "there is a force".
Do not assume that a force must act in the direction an object is moving. A moving object can be slowed by a force acting against its motion. A force is linked to a change in motion, not simply to the existing direction of travel.
Changing direction
A force can change the direction in which a body moves, even when its speed changes little or not at all.
When a tennis racket strikes a moving ball, the ball may leave in a different direction. When a goalkeeper punches a football away from the goal, the force from the glove changes the direction of the ball. In both cases, the answer should focus on the body's path changing.
Non-contact forces can also change direction. A comet passing near the Sun is pulled by gravitational force, so its path curves. A charged plastic rod held near a thin stream of tap water pulls the stream sideways by electrostatic force, so the stream bends towards the rod without touching it.
In exam wording, distinguish direction from speed. "The ball turns to the left" or "the path curves" describes a change in direction. "The ball goes faster" or "the ball slows down" describes a change in speed.
Changing shape
A force can change the shape of a body. The change might be obvious, such as stretching a rubber band, or small, such as a table surface compressing slightly under a book.
Common shape changes include:
| Action | Shape change |
|---|---|
| pulling a spring | stretching |
| pushing both ends of a sponge | compression |
| twisting a cloth | twisting |
| pressing down on one end of a ruler held flat at the other end | bending |
At this stage, you only need to describe that the shape changes. You do not need a graph, calculation, or detailed material property to answer this specification point.
Be careful with wording. "The spring gets longer" is a shape change. "The spring moves down" is a change in position. If a force stretches a spring while also moving it, identify the effect asked for in the question.
Types of force
Edexcel expects you to identify different types of force. A useful first split is contact forces and non-contact forces.
Contact Force
A contact force acts when the bodies involved are touching.
Examples of contact forces include:
| Force type | How to recognise it |
|---|---|
| normal contact force / support force | a surface pushes on an object touching it |
| tension | a stretched string, rope, or cable pulls on an object |
| compression | a squashed object pushes back on another body |
| friction | two surfaces rub or tend to slide past each other |
| drag / air resistance | a fluid such as air or water pushes against motion through it |
| upthrust | a fluid pushes upward on an object in it |
A non-contact force can act without the bodies touching. The most important examples here are gravitational, electrostatic, and magnetic forces.
| Non-contact force | How to recognise it |
|---|---|
| gravitational force | attraction involving masses, such as Earth pulling objects downward; Earth's gravitational force on an object is called its weight |
| electrostatic force | attraction or repulsion involving charged objects, such as a rubbed balloon attracting paper |
| magnetic force | attraction or repulsion involving magnets or magnetic materials |
The name of the force depends on the interaction, not just the effect. A force that slows an object could be friction, drag, or a magnetic force, depending on the situation. A force that changes direction could be contact force from a wall, gravitational force from a planet, or electrostatic force from a charged object.
Quick recap
- Forces are pushes or pulls between bodies; they can change speed, direction or shape, sometimes together.
- A force opposite to motion can slow an object. A continued backward resultant can reverse it after it stops.
- Stretching, compression, bending and twisting are shape changes; moving to a different position need not change shape.
- Contact forces require touching bodies. Gravitational, magnetic and electrostatic forces can act without contact.