In the first two laws of motion we learnt about what is force and how can we calculate its magnitude. In the third law of motion we will get to know one of the important characteristics of force. The law states that “To every action there is an equal and opposite reaction”. Suppose that a person is trying to push a wall. The person is applying a force ‘F’ on the wall, then according to Newton’s third law motion the wall also exerts a force ‘F’ on the person.The two forces shown in the figure are also known as Action Reaction pair. The action-reaction pair has certain properties:
(a) Both forces are equal in magnitude
(b) They act on different bodies i.e. in this case one force is acting on the wall while the other is acting on the person
Let’s consider some of the examples of the third law of motion. When a person jumps of a boat, it moves in the backward direction. This can be explained using third law of motion. When the person applies a force on the boat in backward direction, the boat also applies a force on the person which helps him to move forward. As the boat also experiences a force in the backward direction so it moves backward. In this case can we say that acceleration of boat is equal to that of the person? We know that the force acting on the boat and the person is equal but their masses are different. Hence their acceleration will also be different.
One of the similar example is the recoil of a gun. The gun exerts some force on the bullet, so the bullet also exerts some force on the gun and hence we feel the recoil.
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