Newton's Third Law of Motion

In these revision notes for Newton's Third Law of Motion, we cover the following key points:

• What are action and reaction?
• What does Newton's Third Law of Motion says?
• Why two forces that are involved in an action-reaction process do not cancel each other?
• What is the condition that two forces obey the action-reaction principle?
• What are some examples of action-reaction principle?
• Why not all forces do appear in a force diagram?

Newton's Third Law of Motion Revision Notes

When an action occurs from an object A to another object B, there is a reaction from the object B to the object A as well. This phenomenon is on the base of the Newton's Third Law of Motion, which says:

"For every action, there is an equal size but opposite reaction."

The above statement represents the simplified definition of Newton's Third Law of Motion. A more scientific version of this law is:

"When one body (object) exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body."

The Newton's Third Law of Motion is also known as the "Action-Reaction Principle."

Mathematically, we can write:

Action and reaction forces do not act on the same object and therefore, they cannot be balanced. As a result, there is no equilibrium although the action and reaction forces are equal in magnitude and opposite in direction. Despite action and reaction forces are equal in magnitude, they cause different effects in the objects they act (more precisely, in their acceleration) because these objects have different masses.

Not all forces that are equal in magnitude and opposite in direction can qualify to be part of action-reaction principle. The condition is that they must be of the same nature (type).

Only half of all forces involved in a system is shown in a force diagram. This is because we are interested only in the active forces as they cause motion.

There are many examples of the application of action-reaction principle in daily life. Some of them include:

1. Gravitational force of the Earth on objects near it - action; gravitational force of objects on the Earth - reaction
2. Frictional force caused by the surface of the ground to a sliding object - action; frictional force caused by the surface of the sliding object to the ground - reaction
3. The force of an object pushing down the ground - action; the force exerted by the ground when pushing up on the object - reaction, and so on.