Newton's Third Law of Motion

Newton in his third law of motion stated a relation between action and reaction. According to this law, to every action there is an equal and opposite reaction.

The action and reaction act on two different bodies if action and reaction are on same body they will constitute a balanced force and body will not move.

Examples

When you jump from a boat to the river bank, the boat moves in the backward direction. While jumping out of the boat, your foot exerts a backward force on the boat. This force is called action. At the same time a force is exerted by the boat on your foot, which makes you move forward. This force is known as reaction. Two bodies and two forces are involved in this problem. You pushed the boat backward and the boat pushes you forward. These two forces are equal in magnitude but opposite in direction.

When a rubber balloon filled with air is released, the balloon moves opposite to the direction of the air coming out of it. In this case the air coming out of the balloon (action) exerts a force of reaction on the balloon and this force pushes the balloon backwards (reaction).

Action and reaction forces enable us to walk on the surface of the earth. While walking on the ground we push the ground with our foot in the backward direction. This is the force of action. In return the ground exerts an equal force of reaction on our foot in the forward direction. The force that actually makes us walk in the forward direction is this reaction force.

Similarly, during swimming we push the water in the backward direction, with our hands and feet, to move in forward direction. It is the reaction to this force that pushes us forward.

Rockets and jet-planes also work on the principle of action and reaction. In each of these, when the fuel burns, hot burning gases are ejected from the tail. The hot gases come out in the backward direction and the rocket or the jet plane moves in the forward direction.

Features

There are three significant features of third law of motion:

  1. We cannot say which force out of the two forces is the force of action and which one is the force of reaction. They are interchangeable.
  2. Action and reaction always act on two different bodies.
  3. The force of reaction appears so long as the force of action acts. Therefore, these two forces are simultaneous.

It is not necessary that the two bodies, among which the forces of action and reaction act are in contact. They may be quite far from each other. For example, attraction or repulsion between two magnets can take place even without being in contact.