If you drive a manual transmission car, you may be surprised to find out that it has more than one clutch. And it turns out that folks with automatic transmission cars have clutches, too. In fact, there are clutches in many things you probably see or use every day: Many cordless drills have a clutch, chain saws have a centrifugal clutch and even some yo-yos have a clutch.

In this article, you'll learn why you need a clutch, how the clutch in your car works and find out some interesting, and perhaps surprising, places where clutches can be found.

Clutches are useful in devices that have two rotating shafts. In these devices, one of the shafts is typically driven by a motor or pulley, and the other shaft drives another device. In a drill, for instance, one shaft is driven by a motor and the other drives a drill chuck. The clutch connects the two shafts so that they can either be locked together and spin at the same speed, or be decoupled and spin at different speeds.

In a car, you need a clutch because the engine spins all the time, but the car's wheels do not. In order for­ a car to stop without killing the engine, the wheels need to be disconnected from the engine somehow. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the slippage between them.

To understand how a clutch works, it helps to know a little bit about friction, which is a measure of how hard it is to slide one object over another. Friction is caused by the peaks and valleys that are part of every surface -- even very smooth surfaces still have microscopic peaks and valleys. The larger these peaks and valleys are, the harder it is to slide the object. You can learn more about friction in How Brakes Work.

A clutch works because of friction between a clutch plate and a flywheel. We'll look at how these parts work together in the next section.