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Newton's Second Law - Pulling a Card out from Under a Stack of Glasses

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You've probably all seen the parlor trick where a tablecloth is pulled from under dishes. We're going to try that here except we're going to do it on a smaller scale. For the dishes we're just going to have a glass of water, and for the tablecloth we're going to have this paper tag. I'll place the dish on top of the tag. Now because you've seen his before, you know that in order to make this work I have to pull the tag very quickly. So just keep your eye on this. This is often explained as a demonstration of the principle of inertia. However, there's much more to it than that. If I pull slowly, the glass moves right along with the tag and I can pull it right off the edge of the table. Even when I pull the tag quickly, the glass will move some. Watch closely. So the tag shifted toward the table, or rather, the glass shifted toward the table when the tag was pulled out. Now let's review the physics of this. I pulled on the tag with a fairly large force. I was pulling on a small mass, because the tag has very little mass. As a result, the acceleration on the tag was quite high, because the acceleration is equal to the force divided by the mass. Force is large; mass is small. Gives you a large acceleration. On the other hand, the glass was being acted by a relatively small force of friction, but it has much more mass than the tag does, so in that case we had a small force on a large mass, and that gave us small acceleration. So as a result of the smaller acceleration of the glass compared to the tag, in the same amount of time the glass didn't move nearly as far as the tag did. And as long as I'm pulling quickly, I can make that happen. Let's go a little bit further with this and see what happens if we have a second glass on top of the first. Now if this was strictly a demonstration of inertia, one might say, well more mass means more resistance to a change in motion, and this would be easier, but there's something else going on here. As you know, because the glass moves, and this is a tower, and it's an unstable tower - it could easily topple. So if I'm going to be successful in pulling the tag out, I have to pull VERY quickly. So I'm going to get ready by pulling the tag right to the edge, and then I want to pull very quickly and be sure to pull down rather than up because the slightest upward motion will topple that tower of wine glasses. Here we go. Now, we were successful with that so why not go with one more glass?


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