Newton’s Third Law of Motion

Newton gave three remarkable laws for understanding the motion of bodies and relate it with force. newton’s third law of motion is also called action – reaction law.

According newton’s third law of motion:

To every action there is an equal and opposite reaction.

Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body.

The force exerted by the first body on the second body is known as “action” and the force exerted by second body on the first body is called “reaction”. Action and reaction are two different forces but they act on two different bodies simultaneously.

Examples of newton’s third law –

1. If a boy wearing roller skates stands facing a wall and pushes the wall with his hands, the boy finds himself moving backwards, away from the wall. Actually, when the boy exerts a force on the wall by pushing it with his hands, then the wall exerts an equal force on the boy in the opposite direction. Since, the boy is wearing roller skates, the opposite force exerted by the wall makes him move backwards.
2. If we take two similar spring balances A and B and join them hook to hook as shown in figure and the other end of the spring balance B is joined to the wall then on pulling the free end of the spring balance A we find that the reading of both the spring balances is same.

Applications of newton’s third law of motion –

1. When we walk on the ground, then our foot pushes the ground backwards and in return the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward. If, however, the ground is slippery or if there is all ice, it becomes very difficult to walk. This is because the friction is very less on slippery ground or ice.
2. Rocket and jet propulsion happen in accordance with newton’s third law of motion. The hot gases obtained by rapid burning of fuel rush out of a jet at the rear end of the aircraft at a great speed. The equal and opposite reaction of the backward going gases pushes the aircraft forward at a great speed.
3. When firemen are directing a powerful stream of water on fire from a hose pipe, they have to hold the hose pipe strongly because of its tendency to go backward. The backward movement of the hose pipe is due to the backward reaction of water rushing through it in the forward direction at a great speed.
4. It is a common experience that when a man jumps out of a boat to the bank of the river, the boat moves backwards, away from him. This is due to the fact that to step out of the boat, the man presses the boat with his foot in the backward direction. The push of the man on the boat is the action. The boat exerts an equal force on the man in the forward direction which enables him to move forward. This is the reaction force. since, the boat is floating on water and not fixed, it moves backwards.

Questions related to Newton’s Third Law of Motion from NCERT textbook – Page 126 and 127

Question 1 –

If action is always equal to the reaction, explain how a horse can pull a cart.

Solution –

To pull a cart, horse applies a force on the cart and in return the cart applies an equal and opposite force on the horse. If the force applied on the horse is greater than the force of friction between road and the cart, the horse is able to pull cart. The force applied on horse by cart is responsible for the movement of cart. Action is equal to reaction but the two forces act on two different bodies hence, they do not cancel the effect of each other. The force acting on horse determines whether the cart will be pulled by it or not.

Question 2 –

Explain why it is difficult for a fireman to hold a hose which ejects large amounts of water at a high velocity?

Solution –

When a fire hose pipe ejects large amounts of water in the forward direction at a high velocity then the forward going stream exerts a backward reaction due to which the hose pipe tends to go backwards and slips from the hand of fireman. This makes it difficult for the fireman to hold the pipe strongly.

NCERT textbook – Page 128 and 129

Question 12 –

According to the third law of motion when we push on an object, the object pushes back on us with am equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.

Solution –

The justification given by the student is wrong because action and reaction forces act on two different bodies hence, they cannot cancel each other. The truck does not move because the force applied is less than the friction force between the wheels of truck and road. The force applied is not able to overcome the force of friction hence, the truck does not move.

Question 17 –

Akhtar, Kiran and Rahul were riding in a motor car that was moving with a high velocity on an expressway when an insecthit the windshield and got stuck on the windscreen. Akhtarand Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum ascompared to the change in momentum of the motorcar (becausethe change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar wasmoving with a larger velocity, it exerted a larger force on the insect. And as a result, the insect died. Rahul while putting anentirely new explanation said that both the motorcar and theinsect experienced the same force and a change in theirmomentum. Comment on these suggestions. 

Solution –

• Kiran’s suggestion that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar is wrong.
• Akhtar’s suggestion that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect, is also wrong.
• Rahul’s explanation that both the motorcar and the insect experienced the same force and same change in their momentum is correct. Both experience the same force because the action and reaction are always equal and opposite. Moreover, the magnitude of change in their momenta is also the same.

Newton’s third law of motion focuses on action reaction pair of forces. According to Newton’s third law of motion: To every action there is an equal and opposite reaction. This law needs to be understood carefully because many times people confuse that the action reaction forces will cancel the effect of each other. But it is not true since action and reaction forces act on two different bodies simultaneously. Many explanations can be provided using Newton’s third law of motion for rocket propulsion, the backward movement of hose pipe, the movement of horse cart, backward movement of boat when we jump out of it.