Newton's Third Law
Newton's Third Law
Identifying Action and Reaction
Force Pairs
A force is a push or a pull that acts upon an
object as a results of its interaction with
another object. Forces result from
interactions! As discussed in Lesson 2 , some
forces result from contact interactions
(normal, frictional, tensional, and applied
forces are examples of contact forces) and
other forces are the result of action-at-a-
distance interactions (gravitational,
electrical, and magnetic forces). According
to Newton, whenever objects A and B
interact with each other, they exert forces
upon each other. When you sit in your chair,
your body exerts a downward force on the
chair and the chair exerts an upward force
on your body. There are two forces resulting
from this interaction - a force on the chair
and a force on your body. These two forces
are called action and reaction forces and are
the subject of Newton's third law of motion.
Formally stated, Newton's third law is:
For every action, there is an
equal and opposite reaction.
The statement means that
in every interaction, there
is a pair of forces acting
on the two interacting
objects. The size of the
forces on the first object
equals the size of the
force on the second object. The direction of
the force on the first object is opposite to
the direction of the force on the second
object. Forces always come in pairs - equal
and opposite action-reaction force pairs.