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Characteristics of Forces - Normal, Tension, and Weight

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We're going to be looking at the forces that you typically encounter and 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 with 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 farther 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 a wire, and it's always the same amount at each part of the medium. 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 farther apart, that is a sign that more force is being exerted to pull bungee man back up again. For strings there are tension forces, its not so obvious that the string is stretching, but if you pull hard enough you can feel it stretching, and if you have ever messed with fishing line or thread, you can definitely feel those stretching. A third force is friction. We are going to leave that one as a separate topic for another video. So that leaves us with the fourth force, the normal force. This is the name of the force given to the force exerted by solid surfaces, such as this table. When I put the brick on the table, we say the table pushes up on the brick and supports it. We call that our normal force. The reason we call it normal, is we are using normal in a different sense then one usually encounters, that is being perpendicular. Normal is a mathematical term for perpendicular. So the normal force on the brick acts perpendicular to the brick. That means if we put the brick on a slanted surface, the normal force will no longer be upward. On this incline plane, the force on the brick is off to an angle, 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. If the brick were here and the plane here, there would be nothing pushing the brick this way

Normal force can be a difficult concept to understand. Let's talk about it from two different points of view. First from a theoretical point of view, we know that an object that is at rest has no net force acting on it. So the brick is at rest here, so the sum of all the forces acting on it is equal to zero. Well one of those forces is the weight, that points down. Well if all the forces acting on it add up to zero, there must be a force pointing up. That is what we call the normal force. Now, if you are not satisfied with a theoretical description, we will try to do it in a different way to explain what is going on the surface, because the table appears to be doing nothing, and how can something that is doing nothing exert force. Well, one way to see this is put a pad right there and put the object on the pad. Now, I don't know if you can tell, but the brick pushes down on the pad and compresses it. If you have enough weight, you can see the compression on the pad. The more weight, the more compression. Its sort of like a spring, where you stretch more on a spring, you have more force required to do that. Let's use a lead brick, its covered in tape. Now its very heavy, and you can tell that it presses quite a bit. So the fact that the pad is pressed so much means the pad is exerting quite a bit of force. The scales that you step on, like a bathroom scale, work the same way. The heavier you are, the more you compress a spring inside, and the larger the reading is. Now what you need to think of is this table is acting like a spring, or a cushion in the same way. Well not quite the same, it's a quite stiffer cushion. Think of it, there are lots of atoms in there, and those atoms, think of them as holding, or having strong bonds between them. While those bonds are strong, they can be stretched, and if you put a heavy enough weight, if you put any weight on them, there will be a small bit of displacement. You might think of it as a fireman's net that is stretched so tight that you can barely see it move when something lands on it. That is the same case here, except the table is stretched even more. Now, if we put enough weight on the table, we could see the amount of compression on the table. For example, 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 actually see the table deform and move in response to that. That's what we will do next, we will 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 are going to magnify the motions of this concrete supporting column. This column actually goes down four stories into the basement, so its very sturdy and very difficult to move. I have an aluminum bar attached to the column with this piece of clay. And the bar rests, on the other end, on a steel block. Now between the bar and the block I am going to put this pin, and glued to the pin is this mirror. Just a small piece of a mirror. And that slips right in between the bar and the block. And because the needle is cylindrical, it rotates back and forth. Now, what is going to happen is that either the supporting beam moves, or the table moves, those motions will be transferred to the needle, which is going to move back and forth, so the needle will move back and forth. Now they are going to be very tiny, in order to do that we will have to magnify them, and in order to do that, we are going to use a device called an optical lever. That just requires a beam of light that reflects off the mirror and onto a distance screen. For a beam of light, we will use this laser beam, so it reflects off the mirror surface, and reflects back this way and onto this screen, about 5 meters away, where we can see the beam.

Here I am just going to hit the table real sharply, and you can see that this cause the table to vibrate, cause the beam bounces up and down on the screen. Actually, you can see its bouncing a little bit all the time, because there is always vibration in the building. Now if I will just push down on it, you can see that the beam goes down a long distance. If you look at the table, you can tell its not moving, but it obviously is moving. So it is responding to the force I am applying on it, it is exerting a normal force back up.

Now I am pushing right on the column, you can see that the beam is moving a small distance, just not as much as before when I was pushing on the table, which has a lot more give to it. But it is certainly moving more than just its normal vibration.

Now we are on the opposite side of the wall then we were before, the apparatus is on the other side of the column, which is right here. And, so now I am going to push on it from the opposite side, I am just going to put my shoulder into it.

So we have seen some examples of several different forces that you will encounter during physics problems. One of them is the force of weight; another is the tension force, which we see on springs, wires, strings. And a third one is the normal force, the force that surfaces exert perpendicular to objects. The fourth one is friction, which will be the subject of a different video clip.



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