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.

Newton’s Third Law of Motion states that whenever a body exerts a force on another body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts.

Take the example of a swimmer pushing off from the side of a pool when the swimmer pushes against the pool wall with her feet and accelerates in the direction opposite to the push because the wall has exerted an equal and opposite force back to the swimmer. But the two forces balance each other because they act on different systems.

Take the swimmer as the “system of interest”, then external force acts on the system, which is exerted by the wall on the feet, thereby affecting its motion. The swimmer moves in the direction of the force exerted by the wall on the feet. But the force exerted by the feet on the wall acts on the wall but not on the “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 the force exerted by the wall on the feet.