 You've probably seen the demonstration of a tablecloth pulled out from underneath dishes. If you pull the tablecloth quickly enough, the dishes stay there. This demonstration is similar to that, but instead of dishes, we have a stack of wooden discs, and instead of a tablecloth, we have a stick. And the goal is to remove the discs from the stack, there are eight of them, and to remove them from the stack one at a time from the bottom up without making the stack topple over. Here's how to go about doing it. To get the bottom disc out, just hit it very quickly with the stick. Now I could hit it slowly, but if I do that, the whole stack moves, or a little bit faster and the stack may topple over. What's happening here is that when I hit the stick, I'm hitting it with a fairly large force, and that force is acting on that disc on the bottom. So I have a large force acting on a fairly small mass. Now you know that acceleration is equal to force over mass, and so we have large force over small mass equal large acceleration. When I have the whole stack there, let's talk about the force and acceleration of the stack that is above the bottom disc. Why does that stack move at all? If I move slowly, you can see it does move. It moves because the bottom disc is exerting a force of friction on the stack. Now that's a relatively small force compared to the force that I exert on the stick. That small force is accelerating a fairly large mass. So in that case, we have acceleration equal small force over large mass, so we get a small acceleration. So as a result, when I hit quickly, the bottom disc requires a large acceleration, while the stack of disc above it requires a small acceleration. The acceleration, the forces act for the same period of time for both. As a result, the bottom disc speeds up much more than the top does, and we're able to get the bottom disc out. And we can just keep going back and forth and get rid of the whole stack that way.