200 Words or Less: Newton's Third Law

Don’t worry too much when that schoolyard bully shoves you — according to Newton’s third law, you’ve already shoved back.

The law — for every reaction there is an equal and opposite reaction — really means that when one thing pushes on another, the second one pushes back just as hard in exactly the opposite direction.

It's easy to see what the third law is all about. Grab a friend and head to a skating rink. Once on the ice, stand next to your friend and gently push. She’ll go in the direction you pushed her, but you’ll go in the opposite direction. That's because when you pushed your friend, she pushed on you, too.

Now think about this: when you jump up in the air, you push on the ground, just like you pushed your friend. That means two things: it's the earth pushing on you that tosses you in the air, and you're pushing on the earth, making it move, if only a little bit.

Now, if the earth isn't immovable, can it be the center of the universe? The third law says "no," and along with other insights, it began to permanently undermine the idea of an earth-centered universe.

Bonus experiment! It doesn't fit in 200 words or less, but you can also feel Newton's third law. Grab your friend again, get a basketball, and pass it between you. Pay attention to what you feel on your hands — pressure. That's the force of the ball pushing on you when you pass it.