Clark Adams, age 13, of Santa Cruz, California, for his question:

HOW DOES A CAR'S DIFFERENTIAL WORK?

A differential is a system of gears monted between the rear axles of a motor vehicle. These gears make it possible for one rear wheel to turn faster than the other when the vehicle goes around a corner.

The differential gears of a car are assembled inside a metal housing and are turned by the drive shaft from the engine. An axle extends from each side of the housing to a rear wheel. These axles are connected only by the gears of the differential.

When the vehicle moves straight ahead, the differential gears divide the driving force equally between the two axles. This keeps each of the rear wheels spinning at the same speed. But when the car turns a corner, the gears permit one axle to turn faster than the other. This makes it possible for the outer wheel, which has farther to go, to spin faster than the innner wheel. Therefore, the car does not skid.

In the same way, when a car gets stuck in snow or mud, the differential gears allow one rear wheel to spin while the other does not move.