Where Does a Go-Kart's Energy Go?

Data From Kart 7

Kenji Albright's aunt, Marisol, is a mechanic at an indoor go-kart track. Every kart at the track has an electric motor powered by a battery pack, a brake pedal and an accelerator pedal that sends more power from the battery to the motor. Each kart also carries a small data logger that records its speed many times per second.

On a quiet morning, Marisol let Kenji drive one practice lap in Kart 7 and then showed him the logger data. She split the lap into five parts. On the long straight after the start line, Kenji held the accelerator pedal down. In the coast zone he lifted his foot off both pedals. Before the tight hairpin turn he pressed the brake, and after the turn he pressed the accelerator again. On the last curve he held a steady speed.

Kart 7 slows down with ordinary disc brakes: pads squeeze a metal disc attached to the wheels. Kart 12 is a newer model. When its driver presses the brake, the motor works as a generator and sends charge back into the battery, and its disc brakes are used only for very hard stops. To compare the two, Marisol ran a brake test, driving both karts herself; the two karts have about the same mass. Each kart sped up to 12 m/s and stopped at the same line, ten times in a row. She measured each brake disc with an infrared thermometer before and after the test, and the logger recorded what happened to the battery during the stops only.

Marisol also wrote three notes about the morning. First, during a slow test, Kart 7 rolled into the foam barrier at the end of the pit lane at 2 m/s; the foam dented and the kart stopped. Second, both karts have the same size of tires. Third, the track is about 400 m long.