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LESSON NOTES

NEWTON’S THIRD LAW OF MOTION

You probably know that when you throw a ball against a wall, the ball exerts a force on
the wall. Likewise, the wall puts force on the ball as a result of which the ball bounces
off the wall. Similarly, earth pulls you down with gravitational force. What you may not
realize is you are also exerting an equal amount of force on the earth. This remarkable
fact is a consequence of Newton’s third law.
Newton’s 3rd Law: If an object A exerts a force on object B, then object B must
exert a force of equal magnitude and opposite direction back on object A.
This law signifies a particular symmetry in nature: forces always occur in pairs, and one
body cannot exert a force on another without experiencing a force itself.
Force is a push or pull acting on an object resulting in its interaction with another object.
Force is a result of an interaction. Force can be classified into two categories: contact
force such as frictional force and non-contact force such as gravitational force.
According to Newton, when two bodies interact, they exerted force on each other and
these forces are known as action and reaction pair which is explained in Newton’s third
law of motion.

Newton’s third law of motion states that
When one body exerts a force on the other body, the first body experiences a force
which is equal in magnitude in the opposite direction of the force which is exerted.
The above statement means that in every interaction, there is a pair of forces acting on
the interacting objects. The magnitude of the forces are equal and the direction of the
force on the first object is opposite to the direction of the force on the second object.

IS STATES THAT TO EVERY ACTION THERE IS AN EQUAL AND OPPOSITE
REACTION.

Solved Example

Aswini and Deepa are standing on identical skateboards. Aswini and Deepa push off of each
other and travel in opposite directions. If Aswini (M) and Deepa (m) have identical masses,
who travels farther?

Answer : Neither. Both will travel the same distance because the force applied to each will be
the same.
Ma = ma

a = a

Acceleration controls how far each of them will travel. Since both have the same acceleration,
they travel equal distance.