Couple two spring balances A and B as shown in the figure. When we pull the balance B both the balances show the same reading indicating that both the action and reaction forces are equal and opposite. In this case, the pull of either of the two spring balances can be regarded as action and that of the other balance as the reaction.
Let us now discuss some of the situations in daily life to illustrate Newton's third law of motion.
• When we walk on the ground, then our foot pushes the ground backward (action force) and the ground in turn exerts a force on the foot (reaction force) pushing the foot forward
• When a man jumps from a diving board he pushes the board (action force) and the board in turn pushes the man forward in the opposite direction (reaction force)
• The birds, while flying, push the air downwards with the help of their wings (action force) and the air in turn exerts a force on the bird in the upward direction (reaction force)
• A swimmer pushes the water in the backward direction (action force) and the water exerts a force on the swimmer (reaction force) which pushes him forward
Action and Reaction Forces Acting on a Swimmer
Even though the action and reaction forces are equal and opposite their resultant is not equal to zero as the action and reaction forces are acting on two different bodies
• The third law holds good when the interacting bodies are at rest or in motion
• Newton's third law describes the relationship between the force of interaction between two objects
• This law does not give us the magnitude of force
