Question

A second inventor was driving down the highway in her Prius one day with her hand out the window. She happened to be driving through the middle of a wind farm when an idea snapped into her head. She thought to herself, what if she mounted a small windmill generator on the roof of her Prius and wired it into the battery? She thought, with a little head wind to get her started she could just drive off under wind power! The faster she drove, the more power she would make, and the faster she could go… What’s wrong with her idea?

Answer 1

first fact you need to learn is that no machine is 100 percent efficient. that means if you work 100 joule on rotating windmill blades you will able to convert only a part of this energy into another energy like you can say 65 joule in battery. so assume you mounted the windmill to the roof and driving.

1. first thing -

Wind hit the blades of windmill and work 100 joule on blades and you store 65 joule in battery but simultaneously your car have to work 100 (approx) joule more than previous work to drive due to friction increased by windmill

2. second-

when you drive with this battery energy only a certain part of that energy change into mechanical energy. Say only 45 joule of mechanical energy you got. A great part of car's mechanical energy waste against friction force.

So as you can see our system is highly inefficient and energy deficient, so this plan is not gonna work