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Newton's third law of motion

Newton's third law states that for every action there is an equal and opposite reaction. The "action" and "reaction" refer to forces; if Object A exerts a force on Object B, then Object B exerts an equal amount of force on Object A in the opposite direction. Examples include pushing an object, stepping on the ground, and rockets. Created by Sal Khan.

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  • blobby green style avatar for user tom dwane
    If there is an equal and opposite reaction for every action (force), what exactly is an unbalanced force? Are they just two separate ideas?
    (20 votes)
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    • leafers ultimate style avatar for user Massimo Boscherini
      This is a common misconception when the idea of action/reaction pairs is introduced. The point is that there is an equal and opposite reaction to every action, but these two forces are acting on different objects! So, for instance, if I kick a ball, I apply an unbalanced force to the ball, and the ball will accelerate in the direction of the applied force. The same force, in the opposite direction, will be applied by the ball on my foot. What will happen to my foot will depend on how firm is my standing on the football field ;-)
      (45 votes)
  • blobby green style avatar for user Maham
    I am confused about the last part of the video? why can not we move in space?? suppose if we try to push ourselves in some direction, why can not we??? i am very confused :( i mean where does my force go that i use in the attempt to move?
    (11 votes)
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    • starky ultimate style avatar for user Soul Seeker
      There is no friction or air resistance that will support you to move. Whereas ,on earth we have friction while walking, air to sail a boat etc which is not there in space. Therefore, we are not able to push our self in space. Its just emptiness in space, Nothing is there to hold onto.
      Hope it helps.
      (21 votes)
  • spunky sam blue style avatar for user Pillutla Murali
    the horse pulls the cart forward and the cart pulls the horse with an equal and opposite force both forces cancel each other neither the cart nor the horse should move but why does the cart move
    (14 votes)
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  • leaf grey style avatar for user Atomic A
    Sal pretends that he isn't good at art, but if he took his time and drew something, it would be a masterpeice.
    (10 votes)
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  • piceratops tree style avatar for user Rahul R
    Let there be a rock on a grassy surface. You applied a force on it and it started moving. Now, when you push the rock, the rock will also apply an equal and opposite force on your hand. This will result in a zero net force. Now, if there is zero net force on the body, how can it move? Please clear my doubt. Thanks
    (4 votes)
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    • hopper cool style avatar for user obiwan kenobi
      Actually, guptaakshat0505, I believe you are incorrect. According to Newton's third law, the magnitude of the force that you exert on the rock is always exactly equal to the magnitude of the force of the force that the rock exerts on you. The rock moves because there is a net force acting on it. Zero net force occurs when two forces act in equal magnitudes in opposite directions on a single object. Here there are two objects each of which experiences the same magnitude force. Therefore, the rock will move. Does this help?
      (7 votes)
  • aqualine sapling style avatar for user ging freeccs
    ok, so why does this equal and opposite reaction happen? you explained only how it happens.
    (5 votes)
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    • male robot hal style avatar for user Andrew M
      We can't really answer why that happens. In our universe, one of the laws of physics is that momentum is conserved (which implies equal and opposite forces). Asking why this happens is like asking why masses attract other masses, or why positive charges attract negative, or why energy is conserved. Why are the laws of physics what they are? We can't say.
      (6 votes)
  • leafers seedling style avatar for user MathematicBlack
    So in space, why does it have to be a massive object that you throw to be propelled back to where you want to be? Won't any object do? Because regardless of whether you exert 100N of force on a 10kg object or on a 100kg object, it will still exert 100N of force back on you right? So why does it have to be the most massive object on you?
    (4 votes)
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    • male robot hal style avatar for user Andrew M
      Because a small object will very quickly accelerate out of reach of your hand, so it won't be able to exert force on you for very long, and therefore it won't accelerate you very much in the opposite direction.

      Try it for yourself. Hold a small ball in your hand, jump in the air, and throw it. You didn't go backwards very much. Now stand next to a building, jump in the air, and push the building away from you. What happened to you?
      (5 votes)
  • blobby green style avatar for user Andy Shen Si Eht Tseb
    If you are pushing the brick, the brick is exerting an equal and opposite force on you. That means you are being pushed back. How can you still push the brick?
    (4 votes)
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  • blobby green style avatar for user Sulayman Jaiteh
    Does the 3 laws of motion work in the space.
    (2 votes)
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  • leaf green style avatar for user Gilmand
    When Sal said to find something massive in your space suit and throw it when you are in space Sal said you will move because of Newton"s third law, but what would happen if you just moved your arm, wouldn't you still move since you are putting force to something?
    (5 votes)
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Video transcript

We're now ready for Newton's third law of motion. And something, once again, you've probably heard, people talk about. But in this video, I want to make sure we really understand what Newton is talking about when he says-- this is a translation of the Latin version of it-- to every action, there's always-- and just to be clear-- Newton was English, but he wrote it in Latin because at that point in time, people wrote things in Latin because it was viewed as a more serious language. But anyway-- to every action, there is always an equal and opposite reaction, or the forces of two bodies on each other are always equal and are directed in opposite directions. So what Newton is saying is that you can't just have a force acting on some object without that object also having an opposite force acting on the thing that's trying to act on it. And just to make it clear, let's say that we have a-- and we'll talk about these examples in the second. Let's say that I have some type of block right over here. And that I move, and I press on the block and I try to push it forward. So this is my hand. This is my hand trying to press on the block and exert a force, a net force in that direction. So that the block moves to the right. Maybe this block is sitting on some type of ice so that it can move. So let's say that I have some-- that doesn't look like ice-- I'll give it a more ice-like color. So the block is sitting on, maybe, some ice like that. So Newton's third law is saying, look, I can press on this block, and sure, I'll exert a net force on this block and that net force will accelerate the block assuming that I can overcome friction, and if it's on ice I can do that. But that block is going to exert an equal and opposite force on me. And for direct evidence-- this is something, even though it might not be so intuitive, when it's said-- this equal and opposite force. But direct evidence that it's exerting an equal and opposite force-- is that my hand will get compressed. I could actually feel the block exerting pressure on me. Take your hand right now and push it against your desk or whatever you have nearby and you are clearly exerting a force on the desk. So let me draw-- so let's say I have a desk right here. And if I try to push on the desk-- so once again that's my hand right here, pushing on the desk. If I push on the desk, and I'm actually doing it right now while I record this video. You'll see. So I'm clearly exerting a force on the desk, if I do it hard enough, I might even get the desk to shake or tilt a little bit. And I'm actually doing that right now. But at the same time, you'll see that your hand is getting compressed, the palm of your hand is being pressed down. And that's because the desk is exerting an equal and opposite force on you. If it wasn't, you actually wouldn't even feel it, because you wouldn't even feel the pressure. Your hand would be completely uncompressed. Another example of that-- say you're walking in the beach, and you have some sand right here. If you were to step on the sand. So let's say that this is your shoe. I'll do my best attempt to draw a shoe. So this is the shoe. If you were to step on the sand, clearly you are exerting a force on the sand. The force that you're exerting on the sand is the force of your weight. The gravitational attraction between you and the Earth. You are exerting that on the sand. The sand is also-- and another evidence of that is that the sand is going to be displaced. You're going to create a footprint. The sand is going to move out of the way because it's being pushed down so hard. So clearly you are exerting a force on the sand. But the sand is also exerting an equal and opposite force on you. And what's the evidence of that? Well, if you believe, Newton's second law-- if you have this gravitational force on you, you should be accelerating downwards unless there is some other force that balances it out. And the force that balances it out is the force that the beach, or the sand is exerting on you upwards. And so when you net them out there is a zero net force on you. And that's why you get to stay there. Why you don't start accelerating down towards the center of the Earth. Other examples of this- this is maybe the most famous example of Newton's third law-- is just how rockets work. When you're in a rocket, either trying to escape the atmosphere, or maybe you're in space, there's nothing to push off of, nothing to push off, that lets you accelerate. So what you do is you keep stuff to push off in your fuel tanks, and when you allow the proper chemical reactions or the proper combustion to take place, what it does is it expels gases at ultra high velocities out the back of your rockets. And each of those particles you're exerting a force on them. Enough force even though they're super small mass for each of them, they're going at super high velocity. So they're being accelerated tremendously. So there's an equal and opposite force on the rocket, the thing that is actually expelling the gas. And so that's what allows a rocket to accelerate even when there's nothing in this direct vicinity to push off of. It just expels a bunch of things or accelerates a bunch of things at a super fast rate. It exerts a force on all these particles, and that allows an equal and opposite force to accelerate the rocket ahead. And another example of this is, if you ever find yourself drifting in space. And this is an actually useful example, so that you don't end up drifting in space forever. Let's say, we don't ever want this to happen. This astronaut, by some chance he loses his connection to this little tool arm right here in the space shuttle, and he starts drifting away. What can that astronaut do to change the direction of his motion so that he drifts back to the space shuttle? Well you look around, there's nothing to push off of. He doesn't have any wall to push off of. Let's just assume he doesn't have any rocket jets or anything like that. What could he do? Well the one thing you could do-- and this is the situation if you're ever drifting in space-- is you should find the heaviest or I should say the most massive thing on you-- and we'll explain the difference between mass and weight in a future video-- but you should find the most massive thing that you can carry, that you can take off of you, that you could throw. And you should throw it in a direction opposite yourself. So let me put it this way. If I throw-- let's say I'm in space and I'm floating. I'll make it look like the glove of a-- so let's say that this the glove of the astronaut. There you go. That's his hand, that's the astronaut's hand right over here. And let's say he find some piece of equipment on his, or she finds some piece of equipment on them that they can throw. They can take off of their tool set. And they could find the most massive object that they could throw. So what's going to happen is-- for some period of time, while they push this object away, they will be exerting a force on that object for some period of time while they have contact with the object. And that entire time, that object, while it is accelerating-- while the astronaut is exerting a force on it-- will be exerting an equal and opposite force on the hand of the astronaut, or on the astronaut itself. So the object will accelerate in that direction, and while the astronaut is pushing, the astronaut will accelerate in this direction. So what you do is you throw in the opposite direction and that'll allow the astronaut to accelerate towards the space shuttle and hopefully grab on to something.