Newton's Third Law

A challenge quiz about Newton's Third Law

Isaac Newton

Isaac Newton revolutionized physics. He discovered how to predict the orbit of a planet or the trajectory of a golf ball, using the same simple equations. But, Newton's physics contradicts many of our "commonsense" beliefs.

How does your intuition about the physical world compare to Newton's physics? Take this challenge quiz about his Third Law.

How much does the Earth weigh?

I weigh 150 pounds. The English pound is a unit of force, so in physics parlance, the Earth exerts a 150-pound downward force on my body. What gravitational force does my body exert on the Earth?

a. None. My body is too small to create a gravitational force.

b. 13 trillion trillion pounds (convert the mass of the Earth in kg to pounds), upward

c. 150 pounds, upward

The correct response is c. This is Newton's Third Law, describing any kind of force between two agents, not just gravity. When two objects exert forces on each other, those forces are of equal strength and in opposite directions. Here's a way of demonstrating this.

An arm wrestling match

Challenge your friend to an arm wrestling match. Tell him to push as hard as he possibly can, and try your best to do the same. You'll both feel each other's pushing force on your hands. Now repeat the match. Tell him to push as hard as he possibly can, but completely relax your own arm and let it flop over. Your opponent will notice that he was not able to create a strong force on your hand. Newton is telling us that this is because you were not reciprocating with a strong force. The hands exert equal and opposite forces on each other.

But if this is true, then how can one of you win? We need to make the right comparison. To predict which way your hand will move, we need to compare the two forces acting ON your hand. Your opponent's hand pushes your hand in one direction and your arm muscles push your hand in the other direction. If your arm muscles produce the greater force, you win.

Another example: Think of two trains in face-to-face contact, each trying to push the other back. They exert equal and opposite forces on each other. The winner will be the one that pushes harder on the track.

A civil war cannon firing

We see the Third Law in action all the time. A civil war cannon recoils because there's a force that launches the cannon ball forward and an opposite force that pushes the cannon back. This is also how a rocket works. The explosive burning of fuel pushes out hot exhaust gas; the tremendous pressure of the hot gas pushes the rocket in the opposite direction. The Third Law limits what we can do; you can’t levitate by pulling up on your own bootstraps. When you pull up on the straps, they pull down on your arms with an equal force.

Newton's physics prevailed because it works

There's a reason why Newton's physics prevailed over the ideas of Aristotle and other ancient and medieval thinkers. It better describes the way the world works (at least the world of macroscopic objects). It also passes another test of a good scientific theory. We can use it to predict things quantitatively. We can calculate how high a golf ball flies and how long it takes to come back down. Or we can predict the orbit of a planet or comet. As with all revolutionary ideas, Newton's physics challenges us to rethink our intuitive notions. It's a big reinterpretation of our experience.