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Action and reaction forces

Whenever two objects interact with each other, each object exerts a force of equal strength on the other. However, each of these forces, while equal in strength, are in opposite directions.  Created by Sal Khan.

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Video transcript

- [Lecturer] You've probably heard the phrase that for every force, there's an equal and opposite reaction force. And this is also known as Newton's third law of motion, but it's also one of the most misunderstood laws of physics. So that's why we're going to dig into it a little bit in this video. So I have two examples here where Newton's third law or this notion of an action and a reaction force is happening. So over here, you have this plane flying and the plane is able to move forward by pushing air particles through these jet engines. So these air particles are pushed outward at a very, very high velocity out the back of the engines. If you are to enlarge one of those air particles, let's say, this is this purple dot right over here. There is a force that is being exerted on it by the jet engine. And that force is going in that direction. So what is the equal and opposite reaction force? Well, the equal and opposite reaction force is not also are occurring on that molecule. It's what the molecule is doing to the plane. The equal and opposite reaction force is that the molecule is going to be pushing on the jet engine with an equal but an opposite force. So it's going to go in the opposite direction. And that's how the jet is able to accelerate forward by pushing on these particles and accelerating them backward by exerting a force on them. The equal and opposite force is the force that the particles, those molecules of air are exerting on the jet and moving it forward. The same thing here is going on with this rocket. You have some rocket fuel in there. It gets ignited. It explodes. And as it explodes, there's a force that exerts on those little molecules and that force is going in this direction. But as it does that, there's an equal and opposite force that the molecules are exerting on the rocket. The rocket is having a force acted on it, once again, equal and opposite. So it's important to realize that the reaction force is not on the same object. It's on the other object. If one object is putting an action force on another, then the second object is putting a reaction force on the first. The forces do not cancel out. It's also important to realize that both forces are generated in pairs and happen at the exact same time. There's no delay. We can look at other examples of this. This is a scenario that I would never want to be caught in being drifting through space. Now this astronaut here has some type of a rocket pack that might help him move around. But let's say that your rocket pack ran out of fuel and you're just drifting through space. How can you get back to your spaceship? Well, if you have a wrench or something on you that you can throw, if you can take that wrench, and if you can push that wrench in that direction, and let's say your spaceship is over here to the left. Well, the equal and opposite force is the force that the wrench is going to exert on you, the astronaut, and then it will push you in that direction and accelerate you in that direction. So that's a useful thing, if you're ever get caught drifting through space. But you could do an experiment right now. Press on the table in front of you. When you press on that table, you're clearly putting a force onto that table. If your table is soft, you will see it get compressed. But notice your finger itself is also getting compressed. And the whole reason why you can even feel that is because your finger is getting compressed. And that is the equal and opposite force that the table is putting on your finger. And this can happen at very, very large distances as well. The whole reason why the Moon is in orbit around the Earth is because there's a gravitational force of Earth's mass acting on the Moon. But there's an equal and opposite force of the Moon acting on Earth. And it's actually not that the Moon is rotating around the Earth. It's actually, they're both rotating around the center of mass of their combination, that just happens to be so much closer to Earth. It's actually within Earth's volume that it looks like the Moon is rotating around the Earth. And this isn't just celestial bodies. I weigh 165 pounds. That is the force that Earth is acting on me due to gravity, but it turns out that there's an equal and opposite force of 165 pounds that I am pulling on Earth with. So I will leave you there. Look around the world. This is happening everywhere. For every force, there's an equal and opposite reaction force. But they don't cancel out because they're acting on different bodies.