Why is sliding a box across the floor usually hardest at the start? The answer to that question is that the surfaces of the box and floor settle into one another while they're at rest, so they're particularly resistant to the start of sliding. Friction originates in the microscopic interactions between two surfaces, and those interactions depend on several things. On the type of surfaces involved, on how hard those surfaces are pressed against one another. And on whether or not the surfaces are actively sliding across each other. For the purposes of this story, I'm going to talk about a box on the floor, and, they're two types of forces to, to separate, distinguish between the box and floor. First, is old news, support forces. When I push the box against the floor, and try to make those two surfaces occupy the same space at the same time, they respond by developing support forces. And like all support forces, Those forces, [NOISE], act perpendicular to the surfaces involved. So, the floors support force on the box is straight up. The box's support force on the floor is straight down. So far, so good. The frictional forces are all horizontal. All like this, these directions. And they're caused, they basically originate in the tiny structures that make up the floor's top surface, and the tiny structures that make up the box's bottom surface. When I put these two objects on top, you know, against one another, picture two mountain ranges, one projecting up from the floor and the other projecting down from the box. When you set the box on the floor, and try to slide the box, well, the two mountain ranges, because they, they're, interdigitated like that, they press against each other with local support forces, and that makes it difficult to start the box sliding across the floor. Once you do get the box moving across the floor, those mountains are crashing into each other, passing through, one's going through a mountain pass and the other. There are all kinds of collisions going on. And even some of the mountains are being broken off. So, this leads to most of the effects that we're familiar with as friction. Now, there's an entire field known as tribology, that deals with the interactions of two surfaces that are in relative motion. And friction, and the, and the lubrication effects that can come in play, if you put liquids in there or even solids that have structure that allows for sliding in special ways. But, I'm going to leave all that aside and just focus on the simple part of friction. Which derives from these mountain range projections, basically what are known as contact points often between surfaces. That gives us enough to really anticipate many of the effects that you normally see. For example, the nature of the surfaces involved matters. The rougher the surfaces are, in some respects, the bigger the frictional effects become. Well, if you, choose those mountain ranges, so that they're pointy and tall, they dig deep into each other and they collide particularly hard against one another when you try to slide, one surface across the other. Thus, rough surfaces, sandpaper on sandpaper, experience pretty big frictional forces. On the other hand, if you smooth off all those mountain ranges and make them casual domes, as smooth and broad as you possibly can make them. This is at a very, you know, tiny microscopic scale. Well, they can kind of slide across one another pretty easily, and it turns out that polishing surfaces, getting rid of all the, the, smoothing out the hills and valleys to the extent that you possibly can, leads to relatively lower frictional effects. You can't get rid of friction entirely because, issues arise even at the atomic level of, of, of surfaces sliding across another, but you can get the friction down significantly, and certainly the wear and tear that, that comes up by smoothing out the surface at the most microscopic possible levels. So, choosing your surface affects friction. Second thing that affects friction is how tightly you press those surfaces together. In other words, the bigger the support forces, acting between these two surfaces, the bigger the frictional forces acting between those two surfaces. They're not independent, even though they point in very different directions. Remember, support forces point perpendicular surfaces. Frictional forces are parallel to surfaces. Well, to, to understand this pressure effect, think about the box sitting on the floor. It, the box bottom is not perfectly smooth and neither is the floor's top. So when I set the box on the floor, the contact area is not the entire bottom of the box. No. It's a lot of little points, contact points. Where the bottom of the box is truly touching, the top of the floor. And, given the current situation, with the box sitting, pressed down only by its own weight, there might be, just to pick a number, a 1.000 contact points between the bottom of the box and the top of the table. If I push harder on the box, I lean on it and add my own weight to, to what is only pushing the box against the table, the support force is increased. The, the table is now pushing up extra hard on the box to support not only the box, but me too, and it's doing that by way of more contact points. It turns out that the number of contact points between the box and the table is approximately proportional, to the support forces these two objects are exerting on one another, that leads to two remarkable results. First, because those contact points are themselves responsible for frictional forces, it turns out, that the frictional forces between floor and box are proportional to this support forces between floor and box. In other words, the harder you press those two surfaces together, the larger the frictional forces are or can become. That's an example of what's known as Amonton's First Law of Friction, which observes that for stiff objects like the box and floor, ones that are not liquid-like, they can't just flow into each other. The forces of friction are proportional to the support forces, and, for most situations, that's a pretty good approximation. Press things harder together they are harder to slide. The other observation that comes to this contact point idea is that, the surface that you see touching the floor, isn't really the surface that is touching the floor. So, the forces of friction between this box and the floor doesn't depend on the apparent contact area. It's not the whole box! So, that's actually Amonton's second law, which says that the apparent contact area between the two objects, in that those surfaces doesn't affect friction. For this box and floor, as for any pair of surfaces, there is what's known as a coefficient of friction, which relates the support forces between the 2 surfaces to the frictional forces that are available, when you try to slide those surfaces across one another. Those coefficients of friction are typically something like .5, maybe as much as one, maybe a little above one for certain metals on metals. You can look these up. And those values are actually fairly useful. In this case, the coefficient of friction, I, I measured it secretly, is about 0.3 or 0.4. Meaning, that for every [SOUND] newton of force I exert, to press these, these surfaces together, that support force, I get about 0.3 or 0.4 newtons of frictional force. That's what's available to me. Since this is the episode on wheels, [SOUND] I hope you won't mind if I jump ahead a little bit to talk about a rather important coefficient of friction. That between tire rubber and pavement. The value of that coefficient of friction is typically in the range between about .6 and one, depending on the exact circumstances. What that means is, that for every one Newton of support force, pushing the tires down against the pavement, there is available about 0.6 to one Newton of frictional force. Well, when you're sliding a wheel-less wagon across the pavement, you want to keep friction low, because friction's a nuisance. But when you're rolling the tires of a vehicle across the pavement, you actually want friction between the two. Well, we'll come to that shortly in other videos, but, but those frictional forces between the pavement and your tires, are what allow you to speed up and slow down and turn. You need those horizontal forces to accelerate, and if they're not there, there's no friction, you become inertial. This is why driving on a snowy day, or sometimes on a rain-slicked highway is treacherous. You don't have very much frictional force available to you. And you tend to go straight at a steady pace. Even when the road doesn't. So, friction then is a good thing. And you want a lot of it. That means you want tires that grip the road well. They have a high coefficient of friction. And also you want them pressed tightly against the road, so that there are large support forces between the road and the tire. That brings me to a question. The heaviest component of a typical car is its engine, and that engine is usually in front of the car, above the front wheels. That said, which of the car's two pairs of wheels, front and back, can obtain the largest frictional forces from the pavement. It is therefore most responsible for the car's horizontal accelerations, including starting and stopping. Because the car's front wheels bear most of the car's weight, the support forces between the front tires and the pavement are very large, and consequently, the frictional grip that those front wheels have on the pavement is also large. This is good for when you're stopping, those front wheels get most of that frictional force that stops the car. But it's also good when you're starting forward from a red light. One of the reasons why most modern cars are front wheel drive, is because the excellent grip between those front tires, pressed hard against the pavement, allows them, not only to stop the car quickly, but to pull the car forward quickly, when you start out at a, at a green light. So, putting most of the weight on top of those front wheels means that those front wheels are responsible for most of the cars acceleration. So far, we know that frictional forces between two surfaces depend on the nature of those surfaces, and on how hard they're pressed against each other. But there's one more crucial issue. Whether or not those surfaces are actively sliding across each other. It turns out there are two main types of friction distinguished by whether or not there's active relative motion. If there's no relative motion, if the two surfaces are gripping each other but they're not sliding across each other, that's known as static friction. On the other hand, if the surfaces are cruising across each other, that's known as dynamic or sliding friction. I prefer the latter because it tells you what's happening. You've got sliding going on. Let's start with static frictional forces. The forces between two surfaces that are not yet moving across each other. This box is sitting at rest on the floor, so it can experience static friction. I'm going to enhance that static friction by pressing the box tightly against the floor with some weights, and now I'll show you the frictional force. Not directly, because I can't, I mean, I can't get really in between the floor and the box. But what I can do, is pull the box to your right with a spring scale, and we'll see static friction from the floor fight me, try to keep that box from moving. So right now I'm pulling with, well, two newtons, three newtons, four newtons of force, let me pause here. I'm pulling to your right with four newtons of force. And, the box isn't moving, which means, it's experiencing a net force of zero, no acceleration. So, a static frictional force from the floor is pushing the box towards your left. Four newtons, five newtons, six newtons, seven, oop. When I got to seven newtons, the box began to move. See right there, seven newtons and oh, there it goes. So, what we see then is that, that static friction is adjustable. Right now it is exerting zero force on the box. The box stays at rest because of inertia alone. And I'm not pulling. But as I begin to pull to your right, static friction begins to pull to your left with just the right amount, to prevent the box from beginning to move. It's that maximum static frictional force of about seven, seven and a half newtons in this case, that obeys Amontons first law of friction. It is proportional to the support forces between these two surfaces. So the harder i press these two surfaces together, the more peak static frictional force I can obtain . And, I can show you that by adding more weight so I'll, I'll roughly double the weight inside this, box, and now instead of reaching a maximum static frictional force of seven, I ought to get somewhere up towards 14. Here we go. We're at 11, 12, 13, 14, oh there we go. See, I pretty much doubled the static frictional force. That brings us to sliding friction. Once static friction gives up, it allows the box to begin sliding across the floor, the frictional force acting on the box, is now the force of sliding friction exerted by the floor on the box. And at, that sliding frictional force is different from the static frictional force. Let me show you. I'm going to start with the box experiencing static friction, as it is right now, and I'm going to pull on it until it begins to move. And once it moves, watch the force I have to exert on the box to keep the box moving at constant velocity. I think you'll be surprised at how small that force is. So here we go. I'm going to start the, the, the box moving by pulling harder, harder. Still not moving yet. Ready. We're at 12 Newtons. I'll get to 13, 14. Oh, there it goes, Oh, look! Much smaller force down there in the 8, 9, 10 range. What do we get from that? Well, once the box began to slide across the floor, the floor exerted a frictional force on it, a sliding frictional force that was relatively small. Only eight, nine, maybe 10 Newtons, whereas it took, when I first got the box started, I had to overcome a static frictional force that could get as large as 14, 15 Newtons. So, the force of sliding friction, first off, is not adjustable. It's a specific value that depends on essentially nothing other than, well, it depends on how hard these surfaces are pressed each other and their character, but I can't otherwise affect it. In particular, how fast I pull the box across the floor doesn't matter, and that's an observation known as Coulomb's Third Law. I don't know why everyone gets credit here, there and everywhere, but that's Coulomb's Third Law of Friction, that, once you've got sliding friction going on, the speed, with which the surfaces are sliding over each other, doesn't matter. Here we go, I'll, I'll go fast this time, 10, nine, 10 it doesn't matter, whether I go fast or slow, I'm getting about eight, nine, 10 of, of, of sliding friction. See there's slow, it doesn't really matter. Anyway, the force of static friction's adjustable up to a maximum. And that maximum is quite large. The force of, sliding friction is not adjustable, it's a specific value, it always is in the direction that opposes relative motion to surfaces, and it's less general than the maximum of static friction. In other words, static friction is a stronger force. Better grip than sliding friction. And you can understand this fairly simply in terms of the contact point, and, sort of, the interdigitation of the peaks that are, that are act acting to support the two surfaces against each other. When you let the box sit, and static friction can kick in, those, those, mountain peaks up and mountain peaks down, dig into each other, grip tightly. And they're hard to get, to let go. So static friction is, is strong up to a point, when, eventually, it does give up. Once it gives up, and sliding friction becomes the, the, the da, the, the frictional force, it's relatively weak. Because the peaks, and mountain ranges basically, are moving past each other. They're already, they're already going, they don't have time or, or inclination to dig into each other, settle in and grip tightly. So in general, static friction is stronger, than sliding friction. We're now ready for the question I asked you to think about, in the introduction to this episode. Will your bicycle or car accelerate forward fastest, when you twist the wheels so hard that they begin to skid, or, if you twist them somewhat less hard so they barely avoid skidding. If you can avoid skidding the wheels, the forward frictional force that the pavement will exert on the tires, will be a static frictional force, and that, in general, will be stronger at its peak, than any sliding frictional force you can get from those wheels. If you do skid the wheels, well, then the frictional force from the pavement will be a sliding frictional force, and it won't be as strong in the forward direction, and you won't accelerate as quickly. So the strength of the frictional force is dependent on the natures of the two surfaces, in how hard they're pressed together, and in whether or not they are actively sliding across one another. That is, whether it is static friction or sliding friction. But there is another important distinction between static and sliding friction. Sliding friction wastes energy. And that's the topic for the next video.
