Explain Newton’s third law of motion with the help of two examples.
Newton’s Third Law of Motion
If an object ‘A’ exerts a force on object ‘B’, then object B must exert a force of equal magnitude and opposite direction back on object ‘A’.
This law represents a certain symmetry in nature: forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as action-reaction, where the force exerted is the action and the force experienced as a consequence is the reaction.
According to Newton’s third law, “To every action, there is always an equal and opposite reaction”.
It must be remembered that action and reaction always act on different objects. The third law of motion indicates that when one object exerts a force on another object, the second object instantaneously exerts a force back on the object. These two forces are always equal in magnitude, but opposite in direction.
These forces act on different objects and so they do not cancel each other. Thus Newton’s third law of motion describes the relationship between the forces of interaction between two objects.
For example, when we placed a wooden block on the ground, this block exerts a force equal to its weight, W = mg acting downwards to the ground. This is the action force. The ground exerts an equal and opposite force N = mg on the block in an upward direction. This is the reaction force.
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Illustrations of Newton's Third Law
Some of the examples of Newton's third law of motion are given below:
1. A gun recoils when a bullet is fired from it: When a bullet is fired from a gun, the gun exerts a force on the bullet in the forward direction. This is the action force. The bullet also exerts an equal force on the gun in the backward direction. This is the reaction force. Due to the large mass of the gun it moves only a
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little backward by giving a jerk at the shoulder of the gun man. The backward movement of the gun is called the recoil of the gun.
2. Walking in order to walk, we press the ground in backward direction with our feet(action). In turns, the ground gives an equal and opposite reaction R, (a). The reaction R can be resolved into components, one along the horizontal and other along the vertical. The component $$H = R \cos \theta$$ along the horizontal, help us to move forword, while the vertical component, $$V = R \sin \theta$$ opposes our weight, (b).
