英語(yǔ)時(shí)差：了解能量守恒定律（下）

Unlike the eight ball situation, the energy seems to have vanished! In fact, we now know that the energy merely changed forms. When the driver hit the brakes, what else happened? For one thing, there was a loud screeching noise. That’s kinetic energy being converted into sound waves. The brakes and the skidding tires became hot. That’s kinetic energy being converted into heat. Pebbles were knocked out of the way. That’s a mini-version of the billiard balls — kinetic energy being passed from one object to another. All these little outlets add up to exactly as much kinetic energy as the car had in the first place. It works out just the way the law of the conservation of energy says: energy can change forms, but can’t ever disappear.