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Newton's third law concerns how objects interact with each other, how they can transmit forces from one to the other. In Newton's words, the law is "to every action there is always opposed an equal reaction, or the mutual actions of two bodies upon each other are always equal and directed to contrary parts." Now more simply stated it is for every force there is always an equal and opposite reaction or some people say for every action there is always an equal and opposite reaction.
When an object pushes another object, it gets pushed back in the opposite direction an equal amount. So, let's go ahead and take a look at how this works. Here we have a simple pendulum, which just rocks back and forth. Now, when I add in an object for it to hit, what's going to happen is that pendulum object is going to transmit its momentum as a force onto this object. But this object will also push back on the pendulum and decelerate.
So, what is this force doing? This force is actually pushing against it, so it's going in this left to right direction, so it's going to push back in the opposite direction and decelerate it. Now, this force is going to push against this stationary object and accelerate it. So, let's see how this works. So basically, it accelerates the ball that's sitting there and stops the pendulum, because it's decelerating or accelerating it in the opposite direction.
Now another way to look at this is by something as simple as pushing a ball. So, we have this hand and as this hand contacts the ball, they push against each other. Now, if the ball is really heavy, the hand is going to kind of resist and squash there. Now, if we can bring the hand back and give it more force, we can overcome friction and get the ball moving. Now, when we put Newton's laws together, we have a very simple case of the bouncing ball.
Now, we have the force of gravity, which acts equally on every objec, and what it does is it pulls objects down. So, this red arrow is the force of gravity. Now, when this object hits the floor, we have the opposite reaction. In other words, the floor or the ground pushes against the ball in the opposite direction, which causes the ball to go skyward again. But as we can tell, the force of gravity is still pushing down on the ball so it will eventually slow to a stop and start coming down again.
So let's go ahead and see how that works. So, the ball has accelerated down and then decelerated to a stop and accelerated down again and when it hits the floor, the equal and opposite reaction of the floor bounces the ball. So, that's a very quick physics lesson in Newton's Laws of Motion. So, as we go through the rest of the chapter, we will see how these Laws of Motion apply to character animation.
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