What happened to the purple car when it hit the blue car?

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It bounced back.

Newton realized that forces are not one-sided. Whenever one object exerts a force on a second object, the second object exerts a force back on the first object. The force exerted by the second object is equal in strength and in the opposite direction to the first force. In the animation above, the purple car exerted a force on the blue car, and the blue car exerted a force back on to the purple car causing it to bounce back.

This describes Newton's third law of motion, which is also called the law of action-reaction. The law states that forces act in equal and opposite pairs.

### Equal but Opposite

The picture below illustrates the third law of motion.

Person one is pushing person two with a force of 125 N. Even though person two does not move, he is pushing back with a force of 125 N. If he pushed with a force less than 125 N, he would fall backward.

Sources of images used for the interactive in this section:

• Bumper Car 5, GigiRocks, Clip Art Mojo
• Bumper Car 2, CaryGary, Clip Art Mojo