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LeBron Asks: How does shooting a basketball illustrate Newton's 3rd Law?

Video transcript

LEBRON JAMES: Newton's third law tells us that to every action, there's always an equal and opposite reaction. When I'm shooting the ball, I am clearly applying a force to it. Where's the equal and opposite reaction? SALMAN KHAN: That's an excellent question, LeBron. So as you mentioned, if someone's shooting a ball, they're clearly applying a force to that ball. And let me see if I can draw that for us. So let's say that this is the force that your hand is applying on the ball. And this force is what accelerates the ball upwards and towards the basket. So force is force applied by hand, applied by hand on the ball. So where is this equal and opposite force that Newton's third law tells us about? Well, that's going to be the force that the ball is applying to your hand. And so I'll draw the force right here. I could draw it anywhere on the hand. But I'll draw it here, just so you see it's going in the exact opposite direction. And it has an equal magnitude. So it's an equal but opposite force. Opposite is the direction. So just like that. So that is the force applied by the ball on your hand. But then you might say, wait, this doesn't make a lot of sense. The ball is clearly getting accelerated in that direction. But my hand isn't accelerating backwards in kind of that opposite direction. And the reason why your hand isn't accelerating backwards, isn't just flopping backwards because of this force of the ball applying to the hand, the reason is because your hand or your fingers have other forces acting on it that counteract this force. So for example, the reason why your fingers and your hand don't flop backwards, don't accelerate backwards because of this force, is because you have muscles in your forearm. You have muscles in your forearm, roughly right around there, that are pulling on your fingers and on your hand and are counteracting this force. So they are counteracting that force. So you say, OK, fair enough. I see why my fingers aren't flopping back and why my hand isn't flopping back. It's because of these muscles right over here. But why isn't my entire forearm being pushed or accelerated back by this force applied by the ball on my hand? And the reason is that you have other muscles in the back of your upper arm, in particular your triceps, that are keeping-- that are tugging on your forearm. It's contracting. They're contracting-- that more than offset any net force on your forearm that would want to make it go that way. It's pulling on your forearm. And in fact, that's what's contracting and allowing your forearm to go in actually that direction. And we could keep going. I mean, the fun, or the complicated thing, about Newton's third law is if every force has an equal and opposite force, will every one of these we have have an equal and opposite force? And we can keep going on as, OK, there's an equal and opposite force of the tricep tugging on the forearm. Well, that's the forearm tugging on the tricep. But why doesn't the tricep move? Well, the tricep is attached to other parts of your body. And we could keep going on and on and on. But just to focus on the question, your hand is definitely applying a force to the ball, but the equal and opposite force is the force that the ball is applying to the hand. But we actually-- even though we said that your hand isn't flopping back because it's being balanced-- you actually can sense this force. And the reason why you do sense this force is you actually do have the ball compressing against the palm of your hand. If it didn't do that, you wouldn't even feel the ball. So that gives you at least a sensory clue of Newton's third law. If Newton's third law wasn't true, you wouldn't even feel the ball. You wouldn't even have the ball compress against the skin of your palm.