Newton’s third law of motion tells us that forces always occur in pairs of equal and opposite magnitude. Explain how the choice of the “system of interest” affects whether one such pair of forces cancels.
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whenever the one body experience force on the second body,The first body experience a force that is in equal magnitude and opposite direction to the force that it exerts.
Consider a swimmer pushing off from the side of a pool, as swimmer swimmer pushes against the pool wall with feet and accelerates in the direction opposite to their push. The wall has exerted an equal and opposite force back to the swimmer. But they do not cancel because they act on different systems.If we consider swimmer to be the system of interest, then force exerted by the wall on the feet is an external force on this system and affects its motion. The swimmer moves in the direction of force exerted by the wall on the feet. But The force exerted by the feet on the wall acts on wall not our system of interest. Hence The force exerted by the feet on the wall does not directly affect the motion of the system and does not cancel force exerted by the wall on the feet.