How Everything Works
How Everything Works How Everything Works

Question 66

What's going on with the wheels when a car accelerates?
As a car heads forward, its freely turning wheels begin to rotate. The torque that starts them rotating comes from static friction with the ground. The ground pushes backward on the bottoms of the wheels to keep them from sliding and this backward frictional force exerts a torque on the wheels. They begin to rotate so that their bottom surfaces head backward and their top surfaces head forward.

The car's powered wheels turn for a different reason: they are driven by a torque from the car's engine. As you step on the accelerator, the engine exerts a torque on the wheels and they begin to turn. They would skid backward across the ground where it not for static friction between the wheels and the ground. This static friction opposes the skidding by exerting a forward force on the bottom surface of the wheels. This static frictional force produces a torque on the wheels and that torque partly balances the torque from the engine. The wheels don't skid and they're angular velocities increase relatively slowly. However, the forward frictional force on the wheel's bottom surface isn't balanced elsewhere in the car and the car experiences a forward net force. The car accelerates forward.


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