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Newton's Third Law of Motion

When a boatman wants to move away from the shore, he pushes the shore with his oar. This force is called the action force. The shore pushes the boat away with an equal and opposite force. This force is called the reaction force (Figure 3.3).


Newton’s first law defines force, whereas the second law helps us to measure force. Newton’s third law gives the relation between action and reaction forces. When a body exerts a force on another body, the other body also exerts a force on the first. We say that the two bodies interact with each other or an interaction takes place between the bodies. Newton’s third law is stated as
In any interaction between two bodies the force applied by the first body on the second is equal and opposite to the force applied by the second body’.

Such a pair of forces exerted by two bodies on one another is called an action–reaction pair. The alternate statement of the third law is, ‘To every action, there is an equal and opposite reaction’.
  • Action and reaction are equal in magnitude and opposite in direction.
  • Action and reaction act on different bodies. Action and reaction forces exist together as a pair.
  • Action–reaction pair of forces acting within a system are called internal forces.
  • Internal forces do not cause motion. Only an unbalanced external force can cause motion.


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