Measuring a Car’s speed

A car’s speed is indicated by the speedometer – the dial fitted in the front panel of the driver. The speedometer is made of a cable that is connected to a set of gears in the car’s transmission, a core shaft, and a magnet housed inside a metal drum called speed cup. As the vehicle moves, the gears turn the core shaft, which in turn triggers the magnet to revolve. This exerts a turning force on the speed cup causing the needle in the speedometer to rise. A control free movement is however, prevented with the help of a hairspring that helps bring the needle to “zero” when the car stops.