Why can't the efficiency of a heat engine be 100%?

Apiano tuner uses a tuner to create a tone of 5.00 x 10² hz. when a key on the piano is struck he hears 5 beats per second. what is a possible frequency of vibration for the piano string? 2500 hz 1300 hz 605 hz 495 hz

What is the single most important equation in all of physics?

As the distance from a magnet increases, what happens to the magnet force?