Newton's Third Law Of Motion: The Action Reaction Pair


Sir Isaac Newton gave his third law of motion in 17th century (1686) which came out as one of the most influential laws in the world.

Newton’s Third Law Of Motion

Statement: To every action, there is an equal and opposite reaction.

It means that if a body A exerts a force F1 on a body B, and the body B exerts a force F2 on body A, then:

Newton’s Third Law

F1=F2

That is, they are equal in magnitude but opposite in direction.

Whenever two bodies interact, they will exert equal and opposite forces on each other.

This law can be observed anywhere and everywhere in the surroundings. Some of the examples of action reaction pairs are mentioned below:

1) A swimmer pushes the water backwards by his/her hands and in return the water pushes the swimmer forwards, thus enabling him to go forward during swimming.

2) A man walking on ground: While walking, a person pushes the ground in the backward direction, and the ground in return pushes the person in the forward direction, thus making him/her to walk.

3) A bird while flying pushes the air downwards with the help of its wings. Consistent with Newton’s third law of motion, air pushes the bird upwards.

4) When a person is lying on a bed, his weight is opposed by a reaction force from the bed (assuming it must be there because of Newton’s third law of Motion). In turn, both forces cancel each other out, and the person enjoys the equilibrium position.

There are a million more examples of Newton’s third law of motion in our environment. If you observe closely, you can find it anywhere and everywhere.

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Practise This Question

A horse is harnessed to a cart. If the horse tries to pull the cart, the horse must exert a force on the cart. By Newton's third law the cart must then exert an equal and opposite force on the horse. Since the two forces are equal and opposite, they must add to zero, so Newton's second law tells us that the acceleration of the system must be zero and therefore no matter how hard the horse pulls, it can never move the cart.