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# Newton's third law of motion

Newton's third law of motion states that forces always occur in equal and opposite pairs.

A common way of stating Newton's third law of motion is that for every action there is an equal and opposite reaction. 'Action' and 'reaction' are paired, opposing forces (not a chain of events).

Newton's third law means that if an object A exerts a force on an object B, then object B exerts a force of equal magnitude and opposite direction on object A.

The gravitational force of the Earth pulls the moon. But the moon also pulls the Earth.

The forces of action and reaction act on different bodies and therefore do not automatically "cancel" each other out although they are of the same magnitude and opposite direction.

When a horse pulls a cart, the cart also exerts a force on the horse. To move forward, the horse must exert a forward force greater than the reverse force exerted on the horse by the cart.

Newton's third law of motion: The horse is exerting a force on the Earth, pushing it down. The Earth is exerting a force on the horse pushing it up.