An engineer tests a new generator by attaching it to a motor-powered cart. The generator operates such that the cart's batteries charge when the cart's wheels turn. Assume the following true observations.
- Starting from a location, the cart travels Y meters in Z seconds, where Y and Z are non-zero, non-negative numbers.
- The engineer may power the car using one single motor, which is remote controlled. No human beings are pushing the cart or otherwise applying a force to it.
- The motor's battery levels are higher after the Z seconds have passed. The readings are accurate.
- The engineer has not altered the laws of physics and is on planet Earth.
Has the engineer somehow successfully built a perpetual motion machine, or is there some other explanation?