Newton's Third Law (Law of Reaction)

Newton's Third Law of Motion states that for every action there is an equal and opposite reaction: whenever one object (A) exerts a force on a second object (B), object B exerts a force of exactly the same size, in exactly the opposite direction, back on object A. These two forces always act on different objects, at the same instant, and neither can exist without the other — no force ever acts alone. When you walk, your foot pushes backward against the floor, and the floor pushes your foot forward with an equal force; it is this reaction force from the ground, not some force generated by the leg alone, that actually propels you forward.

Why the reaction force matters

Because action and reaction forces are always equal in size, the effect each one produces depends entirely on the mass of the object it acts on, through Newton's second law. This is why a light ball flies away fast when struck while the much heavier athlete striking it barely moves — the forces on both are equal, but the ball's small mass gives it a large acceleration while the athlete's large mass gives them only a tiny one. Understanding this is essential for analysing push-off, throwing and striking actions in sport.

Application in sports

• Diving: a diver pushes down and backward on the springboard; the board flexes and pushes back on the diver with an equal and opposite force, launching the diver up and forward into the air. A more forceful push down produces a correspondingly larger launching reaction.
• Jumping from a boat: jumping forward out of a small rowing boat pushes the boat backward with the same force used to move ahead — the lighter boat recoils noticeably, while a much heavier boat would barely move, even though the force on it is identical.
• Soccer: when you kick the ball you feel a kick-back force on your leg, equal to the force you applied to the ball. You feel it only slightly because your leg has far more mass than the ball, so the same force produces a much smaller effect on you.
• Jumping and sprint starts: your legs push down and backward on the ground, and the ground pushes back with an equal and opposite ground reaction force that propels you upward and forward — this is precisely the force a sprinter tries to maximise by driving hard against the blocks.
• Swimming: a swimmer pushes water backward with the hands and feet during the catch and pull phases; the water pushes the swimmer forward with an equal and opposite reaction, which is the actual source of forward propulsion through the water.
• Rowing: the oar pushes against the water, and the water's reaction against the oar blade drives the boat forward — stronger, cleaner strokes increase both the action and the resulting reaction.

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