An example Fig 13-2 Let us consider a block of mass m attached to a vertical spring near the Earths surface 1. system=block. Here the spring force and gravity are external forces, there are no internal forces within the system and thus no potential energy Using Eq(13-1), △K=W+诉 Spring graI Earth Ch 13 Energy III: Conservation of energyCh.13 Energy III: Conservation of energy Let us consider a block of mass m attached to a vertical spring near the Earth’s surface. 1. system=block. Here the spring force and gravity are external forces; there are no internal forces within the system and thus no potential energy. Using Eq(13-1), K = Wspring +Wgrav Earth Fig 13-2 An example