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Projectile Motion - The Ballistic Cart

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In this demonstration we'll see that the vertical and horizontal motions of a projectile do not influence each other. The projectile that we'll use is this plastic ball, and we'll project it with this cannon. This device has a spring in it, and I'll just hold it so you can see it, and I can force the spring down, and it will stay in place there. So the spring is compressed at this point. I'll push the cart along this level track. My table's not quite level so I put some cardboard here to make sure that it was level. You can see the cart doesn't move on the track. The other thing I need is to release the ball, and for that purpose I have an automatic release which is a photogate. A phot gate is this device on the side, which has a slot in it. There's an infrared beam passing across it. If I break that infrared beam with something, then that will release the ball. What I'll break it with is this piece of cardboard. So I'm going to mount this cardboard on this clamp right here at exactly the level of the photogate. I'll just show you how this works when I pass it through the photogate, it releases the ball.

So I'll get the flag in position, line it up, and we're set to go. Ready the cart, I'll give a push, and I'll stop pushing before the flag actually passes through the photogate. As you see the ball fell right back down into the cannon. Let's take another look at that. Ready? Right back down into the cannon before I actually caught it over here. So what that is showing us is that the ball continues to travel horizontally with the same speed that the cart has. The cart doesn't slow down, the ball doesn't slow down, and since they're both moving horizontally together, the ball comes right back down on the cart. And the fact that gravity was acting on the ball all the time and making it slow down on the way up and speed up on the way down has absolutely no effect on the horizontal motion.



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