 We're going to be looking at the forces that you typically encounter in solving force problems. There are four of them. Those forces are weight, tension, friction, and normal. Every object has weight. The weight is due in this particular case for this brick. The weight of the brick is due to the earth pulling on it. And the nature of that force is that it is directed vertically downward and directly toward the center of the earth. Another kind of force is tension. And tension force you can think of as a springy force. Here you can see the spring coils get further apart. It's obvious that a force is being exerted here. The nature of the tension force is that it always acts along the line of the spring or it could be a string or wire. And it's always the same amount in each part of the medium. A bungee man over here experiences a tension force. And that tension force is along the line of the spring that is supporting him. So as I pull, you can see the spring coils get further apart. That's a sign that more force is being exerted to pull a bungee man back up again. For strings, there are tension forces. It's not so obvious that the string is stretching. But if you stretch it hard enough, you can actually feel it stretch. And if you've ever messed with fishing line or thread, you can certainly feel those stretching if you pull them hard enough. A third kind of force is friction. We're going to leave that one as a separate topic to discuss in another video. So that leaves us with the fourth force, the normal force. This is a name that we give to forces that are exerted by solid surfaces such as this table. When I put the brick on the table, we say that the table pushes up on the brick and supports it. And we call that a normal force. The reason we call it normal is that we're using normal in a different sense than one usually encounters. And that is in the sense of being perpendicular. Normal is a mathematical term for perpendicular. So the normal force on the brick is exerted perpendicular to the brick. That means if we put the brick on a slanted surface, the force will no longer be directly upward. On this inclined plane, the force on the brick is off at an angle right here perpendicular to the plane. If I raise the plane all the way up to 90 degrees, at that point there would be no more normal force exerted on it. There would be nothing pushing. If the brick were here and the plane were here, there would be nothing pushing on the brick this way. Normal forces can be a difficult concept to understand. Let's talk about it from two different points of view. From a theoretical point of view, we know that an object which is at rest has no net force acting on it. So the brick is at rest here. We say that the sum of all the forces acting on it is zero. Well, one of those forces we've already said is the weight which points down. Well, if all the forces on it act up to zero, it means there must be a force acting up to balance the weight. And that is, in fact, what we call the normal force. If you're not satisfied with a theoretical description, we'll try to do it in a little bit different way to try to actually explain what's going on with the surface. Because the table looks like it's doing nothing. How can something that's doing nothing exert a force? Well, one way to see this is let's put a pad right there and put the object on the pad. I don't know if you can tell, but the brick pushes down a little bit on the pad and actually compresses it. If you have enough weight, you can see the compression in the pad. The more weight, the more compression. It's sort of like a spring. Where you stretch more on a spring, you have more force required to do that. Let's put a lead brick. This brick is covered in tape, but it's very heavy, and you can see that it presses quite a bit. So the fact that the pad is pressed so much means that the pad is exerting quite a bit of force. The scales that you step on in a bathroom scale, for example, work the same way. The heavier you are, the more you compress a spring inside and the larger the reading on the scale is. Now, what you need to think of is that this table is actually acting like a spring or a cushion in the same way. Well, not quite the same. It's a much stiffer cushion. Think of it, there are lots of little atoms in there, and those atoms think of them as holding hands and making very strong bonds between them. And those bonds, while they're very strong, they can be stretched. And if you put a heavy enough weight on them, well, actually if you put any weight on them whatsoever, there will be a very small amount of displacement. You might think of it as a fireman's net, but a stretch so tight that you can hardly see the stretch in the net when something lands on it. Same thing with the table here, except it's even stretched more tightly than that. Now, if we put enough weight on the table, we could see the amount of the compression in the table. You could probably imagine if we put an extremely heavy weight like a safe, we could make the table buckle under that weight. Now, with an object called an optical lever, we can see extremely small displacements, so we don't have to put a safe on the table. We can just put a small force on it, and we can see the table actually deform or move in response to that. And that's what we'll do next. We'll move to the classroom in order to see the demonstration of the optical lever. This is a setup to magnify very small motions in objects. For example, we're going to actually be looking at the motions of this concrete supporting column. This column actually goes down into the basement of the building four stories below us. So, it's very sturdy and very difficult to move. I have an aluminum bar here, which is attached to the wall with this piece of clay. And the bar rests at the other end on just a steel block. Now, between the block and the bar, I'm going to put this pin. Now, to the pin is glued a mirror, just a small piece of a mirror. And that slips right in between the bar and the block. And because it's a, the needle is cylindrical, it rotates easily underneath it. What's going to happen is that as either the table moves or the supporting beam moves, that the needle is, those motions are going to be transmitted to the needle, which is going to move back and forth, and so the needle will move back and forth. Now, they're going to be very tiny in order to detect them. We need to amplify them. And to do that, we're going to use a device called an optical lever. That just requires a beam of light that reflects off of the mirror and goes to a distant screen. For a beam of light, we're going to use this laser beam. So, it reflects off the mirror surface right here. And then it reflects back this way. And about five meters on the other side of the room, there is a screen where we can see the beam. So, I'll just, here I'm just going to hit the table real sharply like that. And you can see that this causes the table to actually vibrate because the beam bounces up and down on the screen. Actually, you can see it's bouncing a little bit all the time because there's always vibration in the building. Now, if I'll just push down on it, you can see that the beam goes down a long distance. So, although you can't, if you look at the table, you can't see it moving. It obviously must be moving. And so, it is responding to the force that I apply to it. It's exerting a normal force back up on me. Now, I'm pushing directly on the column. You can see that the beam is moving a small distance. It's not as much as before when I was pushing on the table, which has a lot more give to it. But it's certainly moving further than just its normal vibration. Now, we're on the opposite side of the wall as before. The apparatus is on the other side of the column, which is right here. And so, now I'm going to push on it from the opposite side. I'm just going to put my shoulder into it. So, we've seen some examples of several types of forces that you will encounter in physics problems. One of them is the force of weight. Another is the tension force, which we see on springs and wires and strings. And a third one is the normal force, the force that surfaces exert perpendicular to objects. The fourth kind of force you will encounter is friction. And that will be the subject of another video clip.