My daughter missed a high school physics problem that seemed obvious to me until I understood her reasoning. The question was does playing the radio in a car re

My daughter missed a high school physics problem that seemed obvious to me until I understood her reasoning. The question was does playing the radio in a car reduce fuel efficiency, true or false? She didn't realize that the electricity to run the radio came from a rechargeable battery plus generator system. She had seen me replace a car battery and assumed that it was like an ordinary flashlight battery that provided electricity until used up. The generator in a car doesn't run for free. It is powered by the engine and it's operation while charging is felt as a braking effect on the main engine. This additional drag is produced by a phenomena known as back electromagnetic force (back emf) that causes the generator to resist turning while generating current to power electrical devices in the car or recharge batteries. Another way to realize that using electricity in a car isn't free is to consider why it wouldn't work to add a number of these generators and then use them to power electrical motors to propel the car. There is no free lunch. On the other hand (and not really related to the points I'm making above), stopping a moving vehicle reduces the overall energy of the system because the kinetic energy is proportional to the square of the vehicle's velocity. Due to conservation of energy this energy has to go somewhere. Conventionally, this stopping energy is dissipated through heat (and to some extent breaking down a bit of the the brake pads). Some vehicles, like a Tesla, are able to capture a portion of this kinetic energy during braking, but its complicated, see [1]. [1] https://www.tesla.com/blog/magic-tesla-roadster-regenerative-braking https://www.tesla.com/blog/magic-tesla-roadster-regenerative...