Transcript:

Okay, consider a car. It's moving at a constant speed, let's say 20 miles per hour. Now, if I tell you that there are some parts of the car that are not moving at all—the velocity is zero. There are some parts of the car that are moving at 20 miles per hour. And there are some parts of the car that are moving at 40 miles per hour. Does it make sense? No? Okay, think of the wheel of the car. The axle of the wheel of the car, which is not spinning, is moving at 20 miles per hour, which is the speed of the car. The point of the wheel of the car, which is in contact with the ground, is, by definition, not moving because it is at the same velocity of the ground, which is zero. And now the top of the wheel of the car, diametrically opposite to the point that is touching the ground, is moving at twice the speed. So work it out—work it out. Draw the free-body diagram and work it out. It's amazing.