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Anderson Video - Breaking a Window Rock or Clay

Professor Anderson
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 >> Hello class, professor Anderson here. Let's ask you a question. Let's say you're trying to break a window, alright? We all know how to break windows, we've all been kids, right? We've probably broke a lot of windows in our day. Let's say you were trying to break a window. Are you going to throw a rock at the window or are you going to throw a wad of clay? What do you guys think? Rock, obviously. Everybody knows that, right? Let's say that the masses are the same, same mass. And let's say that you can throw them at the same velocity. And let's say that they both basically come to rest, which means that vf is equal to zero. So they both have the same initial velocity, they both have a final velocity of zero. What is the difference between throwing a rock and throwing a wad of clay? Well, you all know the answer to this. What's the difference between a rock and a wad of clay? Anybody have a thought? What's your name in front? Mohammed? Can you grab the mic from if [inaudible]? So, Mohammed, what's the difference between a rock and a wad of clay? >> I don't know what's a wad of clay. >> Oh, okay. So it's Play-Doh. It's squishy material, right. Like you might mold. It's something that can bend. So what's the difference between a rock and this wad of clay? >> I think the rock weighs more than the wad of clay. >> Okay, but let's say they weigh the same. Let's say they are the exact same mass, alright? If they are the exact same mass you might think you can throw either at the window. But you know from experience that you don't want to throw that clay at the window, you want to throw the rock. What's the difference between the two. >> The rock doesn't bend like >> Exactly, the rock does not bend. The rock is a rigid material that doesn't flex, very much. Everything flexes a little bit, Everything flexes a little bit, but it doesn't flex very much compared to the wad of clay which is very deformable. And so we need to think about those properties of the material and what it means for the breaking of the window. And what it means is the following. If they both have the same mass and they both have the same initial velocity and they both have the same final velocity, they in fact have the same impulse. They both have the exact same impulse. But J is not only delta P, it's also this, the integral of fdt. And so when I think about the clay heading towards the window, if I plot out the force as a function of time, what I get is something like this. There is a long delta T that it is in contact with the window because the clay bends, right? It comes in and it starts to hit the window and then it bends into a different position and that increases this delta T. If it increases the delta T that means the force is relatively low that whole time. But if these have the same impulse, that means that the area under the curve is the same. And here I have an area that looks like that and now when I come along with my rock, what you said is the rock is rigid, it doesn't bend very much. Hasn't hit the window yet, hasn't hit the window yet, but then all of a sudden it hits the window and it goes up to some very big force and then it comes down. If it's going to have the same area as the curve above it, if it goes higher it has to do that in a shorter amount of time delta T. And so what in fact breaks the window is "force". Force breaks the window. You have to apply some force to something to break it. And in a window there's lots of atoms that are glued together in a particular arrangement and when I push on them I can get to a point where those atoms and molecules can't hold together anymore and when I get to that point they break apart and it requires force to do that. And so if your breakage threshold is right here, this is your threshold for breaking, the clay never got above that breaking threshold and so the window doesn't break. But the rock got above that threshold and so it breaks. And so you kind of know some physics just from growing up on the earth and throwing rocks at stuff. You knew that the rock was the one that was going to break the window. Here's why, because it's in contact with window for a shorter amount of time the force that it exerts is much higher. That's why. Alright. Any questions about that? Your assignment is to go out and break some windows.