With Java animations by Bill Casselman
5. The differential in your car
In an automobile, the differential connects the driveshaft to the axles. Its role is to balance the flow of power from the engine to the wheels. The equation a+b=2c means that the wheels share between them the rotation coming from the engine. When the car is traveling in a straight line, the wheels roll the same amount: a=b, and both are equal to c. When the car goes around a curve, the differential allows the outside wheel to travel farther, and the inside wheel to travel less far. Only the sum of their rotations is steady at 2c. If the engine is locked and the wheels are free to turn, turning one wheel will cause the other to rotate in the opposite directon (a+b=0). On the other hand if one wheel is free to turn (e.g. in snow or mud) and the engine is running, the other wheel will not turn at all: a=2c, b=0. This is frequently experienced.