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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.