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Forces and Motion

What is Newton's Third Law of Motion?

Newton's third law of motion says that
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.

One example of Newton's third law is when you
are sitting still on a chair. Your weight is a force pushing
down on the chair. It is not obvious that there is an
equal opposing force from the chair, pushing upwards
you. Newton's First Law says that there
must be an equal opposing force 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 in the direction of the resultant force.

A second example would be staying still while
pushing against a wall. If the wall did not provide
an equal force pushing back against you, then
the wall would accelerate away from your hand.

Other examples of Newton's third law of motion
are a rocket flying and an explosion.

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 see in an exam.

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