Forces always occur in equal, opposite pairs.
Examples:
-- When the soccer guy uses his leg to hit the ball with some force, the ball hits back at his leg with equal force. If he hits the ball with too great a force, the ball can hit back hard enough to break his leg.
-- The gravitational forces between two objects always pull on both objects equally, in opposite directions. Your weight on Earth is equal to the Earth's weight on you. And when you fall out of a tree and accelerate toward the ground, the Earth accelerates toward you.
