题目

Americans love sports—they love to play them, to watch them on television, and to talk about them. But this hobby sometimes has serious results—at least to the players. For example, when people play tennis, sometimes they hurt their elbow（肘部）and in this way they develop “tennis elbow”. Also it is easy to hurt a knee in a football game. These injuries（损伤）happen while the player is having fun, but they still hurt. A few months ago, Kathleen Simmons, who loves playing volleyball, hurt her knee in a volleyball game. Her doctor told her that she needed a very difficult opera列举实例说明能量的转移和能量的转化有方向性？