GCSE SCIENCE PHYSICS HIGH SCHOOL - Force and Motion - Newton's Third Law - Opposite Forces

Forces and Motion

Newton's Third Law of Motion.
When two objects push or pull against each other,
the forces that they feel are equal and opposite.
These forces are called action and reaction forces.
The two forces together are called an interaction pair.

When you are sitting on a chair,
your weight is a force pushing down on the chair.
It is not obvious that there is an equal force from the chair
pushing upwards against you.
Newton's First Law says that there must be one
because the forces on a stationary object (yourself) must be balanced.

If the force of your weight on the chair
did not have an equal opposing force,
then the forces on you would be unbalanced
and you would accelerate downwards.

Another example would be staying still while pushing against a wall.
The wall must be pushing back against you with an equal force
otherwise the wall would accelerate away from your hand.

This all seems very strange at first and goes against common sense.
Once you get used to the idea
of balanced or unbalanced forces explaining how things move,
then you will see just how useful and simple
Newton's Laws of motion are.

The following pages show the type of examples of motion
which you might come across in the exam.

Headings Forces and Motion Search Questions

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