Newton's Third Law of Motion
The 3^{rd} law tells us that all forces come in pairs; for every force that one
object experiences, there is another object that is subject to a force of equal strength
that is in exactly the opposite direction. For example, consider the forces between a
pair of objects that are touching each other. Whatever force the first one exerts on the
second, the second exerts a force of the same size in the opposite direction! Otherwise,
Newton's 2^{nd} law would tell us that the pair of objects, taken together, would
be accelerating in some fashion due to their contact.
As another example, consider the downward force of gravity that the Earth exerts on you
(also called your weight). In turn, you exert an exactly equal upward force on the Earth.
Together, those two forces form an actionreaction force pair.
As an aside, if the actionreaction force pair is the Earth's gravity pulling on you and
your gravity pulling on the Earth, why don't we all fall into the center of the Earth
(and simultaneously the Earth falls up towards us)? If there were no other forces acting
on us, we would do exactly that. But, there is an upward electric repulsion between the
atoms at the bottom of our feet (and shoes) and the ground we are standing on. This force
must exactly balance the downward force of gravity to keep us from sinking into the ground.
After all, according to the second law we need to have opposing, balancing forces acting
on us to keep us from accelerating. The "reaction" force of your gravity pulling
on the Earth does just that, it acts on the Earth and not on you.
What practical consequences does the third law have for us?
Remember that the 2^{nd} law can be applied to groups of objects, considered
together. When we do that, all the individual forces acting on all the objects get
added together (with their directions taken into account). The third law tells us
that forces acting between objects within the group always come in actionreaction
pairs, i.e. there are two forces of equal size opposing each other and canceling out.
So, the net force acting on a grouping of objects is always due to forces that act on
the group from outside the group. It is known as the net, external force.
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