[MUSIC] How to measure friction. Here's a simple experiment you can do at home and that you might include in your second experimental investigation. I put an object on an inclined plane and slowly increase the angle, theta, Until it slides. What does that tell me about friction? Let's do this together. You start. Cool, you did that! Well [LAUGH] done! Yes, the tan of the critical angle gives the coefficient of static friction. So, for the rubber on this book, we can get the coefficient of static friction from the critical angle. Now we can easily answer a few questions. Does Mu s depend on the area of contact? I put it on its side and raise the book to the critical angle. Now I put it on its edge, raise the book to the critical angle. Well, for this measurement and these two surfaces, there isn't much of a difference. It looks as though Mu s doesn't depend strongly on the area of contact. Yeah, that's right. For many pairs of surfaces, the coefficient of friction is approximately constant, independent of the area of contact. But the coefficients of friction for different pairs of materials vary [LAUGH] quite a bit. In particular, they also depend on the cleanliness of the surface. A layer of oil or a grease spot can have quite a dramatic difference. So, in such experiments, repetition and care are particularly important. Now, here's another little puzzle for you. If I tilt this turntable slowly, we can work out the coefficient of static friction [SOUND]. I put the mass of the distance, r, from the center. What is the maximum rate I can spin the turntable before the mass slides off? Okay, how about we put that one in the quiz. But here's a related one. How fast can a truck go around a curve of a given radius [SOUND] without skidding? There's a subtlety here Here's a wheel rolling At the bottom of the turn, the wheel is instantaneously stationary so it doesn't slide. There's no relative motion between the tire and the road at the point of contact. It's static friction. I can feel it with my hand, no relative motion, static friction. Now, here's a wheel skidding [SOUND]. There is relative motion, so it's kinetic friction, and I can feel the relative motion. If the car goes around the curve [SOUND] with the wheels rolling, it's static friction. But if it goes too fast and skids sideways, [SOUND] [LAUGH] that's a kinetic friction, and it's a pretty important road safety problem. So let's put that one in the quiz, too, and we'll give you a link to a resource about rolling and skidding. But, before the quiz, let's answer the puzzle from the beginning of the lesson. These nails are sharp. I can easily push down on one with a force of 10 or 15 newtons, but, after that, it becomes painful. My weight is 700 newtons, so provided I spread it over a few a hundred nails, it ought to be pretty comfortable [LAUGH]. Well, don't try things like that at home, however, because you could hurt yourself if you make a mistake, and there's a danger of infection too. Now, on to the quiz and the test. We'll meet again next week to look at work, energy, and power, which is a completely new way to look at Newton's second law, and it's a new way of looking at the world as well. See you then. [MUSIC]