Newton’s Third Law of Motion creates a sense of balance in the universe. If you push on an wall, the wall also pushes you. The universe does not give one object preference over another. Another way to think about this is that interaction forces always come in pairs. Any time one object applies a force to another, there’s an equal and opposite force being applied back to it. This concept may seem simple, but it’s not always intuitive. We will look at a few examples of this law in action.

Moving around is one of the simplest things we can do, but it’s interesting to think deeply about the physics of it. Let’s start by thinking about walking. How is it that you are able to move forward when you walk? You push backwards against the ground, and the ground pushes you forward. This may seem strange at first. How does the ground push you forward? The underlying force is friction, and if you’ve ever tried to walk on a surface that has less friction (such as ice), then you’ve experienced what it’s like to not have the ground’s help when trying to move forward.

If we switched from walking to swimming, the same idea applies. The reason that you are able to swim forward is because you are pushing backwards on the water and the water is pushing you forward. But what if we pushed this idea further? What if we were trying to swim in space? Based on this logic, since there’s nothing for you to push backwards on, there’s no way that you will be able to move forward.

But then how is it that rockets can go forward in space? This is also by using Newton’s Third Law, but in a different way. For movement on earth and in water, we are relying on friction with the world around us to give us that forward push. Rockets work on a different principle. Rather than relying on objects in the environment to push off of, rockets bring those objects with it in the form of fuel. As the fuel gets pushed backwards, the rocket gets pushed forward.

This is a classic physics thought experiment: Does a truck full of birds weigh less if birds are flying?

One side of the argument is that since the birds are flying, they aren’t really attached to the truck, and so their weight won’t be measured and the truck is lighter. Think about jumping off a scale. When you’re in the air, the scale won’t read your weight. And the same works with the birds. At least, that’s how the argument goes.

This turns out to be wrong because of Newton’s Third Law. The comparison with jumping isn’t accurate because a jump is a type of falling. If all of the birds simultaneously stopped flapping and just fell to the floor, then there would be a temporary instant when then truck is lighter. But assuming that the birds are actually flying, we have to understand how it is that they’re flying. They fly because they are exerting a downward force on the air in the truck. That air is pushing down on the truck, and so the weight of the birds is transferred to the truck through the air.

This thought experiment has actually been tested with a real truck and real birds (Mythbusters, Season 5, Episode 9) as well as with drones (which are easier to work with than birds). They all verify that Newton’s Third Law and show that there is no change in the weight when the birds are in flight.