Newton's third law is about forces acting in
pairs, we call one of these forces the action and the other the
reaction.
If you hit a wall with your fist then you will feel a force on
your fist as well as there being a force on the wall. When a bullet leaves a gun
there is a force on the bullet but there is also a force on the gun and this
makes it recoil.
In both the
two examples, the two forces are equal and opposite although their effects on
the objects may well be very different. In the example of a fist hitting the
wall the effect on your fist is very different from the effect of your fist on
the wall.
Even a person sitting on a stool is subject to this law, the
action of them on the stool (their weight) is just balanced by the upward thrust
of the stool on them. Think of the result if these two forces were not
equal.
You can also use the example of the apple on a stalk (Figure 1)
(see Newton's First Law). The force on the apple due to the stalk is the same as
the force on the apple due to gravity and so the apple stays where it
is.
Newton's third law can be expressed as:
Notice that the two forces, the action and
reaction, act on different bodies. If they acted on the same body it would be
impossible to move anything.
Another demonstration of the law can be
shown with the spring-loaded trolleys (Figure 2). If they are placed back to
back and then the spring in one released they will both move off because there
is a force acting on them both.
