Ch 13 Energy III Conservation of energy Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy Ch. 13 Energy III: Conservation of energy
Law of conservation of mechanical energy K:+u =ke+u In a system in which only conservative forces do work the total mechanical energy remains constant In this chapter: we consider systems of particles for which the energy can be changed by the work done by external forces(系统外的力) and nonconservative forces Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy In this chapter: we consider systems of particles for which the energy can be changed by the work done by external forces(系统外的力) and nonconservative forces. Ki +Ui = K f +U f Law of conservation of mechanical energy: “In a system in which only conservative forces do work, the total mechanical energy remains constant
13-1 Work done on a system by external forces △K+△U= (13-1) SyS SyS ext Positive external work done by the environment on the system carries energy into the system, thereby increasing its total energy i vice versa The external work represents a transfer of energy between the system and the environment. Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy 13-1 Work done on a system by external forces Positive external work done by the environment on the system carries energy into the system, thereby increasing its total energy; vice versa. The external work represents a transfer of energy between the system and the environment. Ksys + Usys =Wext (13-1)
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 energy
Ch.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
2. System=block spring. The spring is within the system △K+△U Spring gra 3. System=block Earth Here gravity is an internal force △K+△U gra Spring 4. System=block spring Earth The spring force and gravity are both internal to the system, so △K+△U+△U 0 Spring grav Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy 2. System=block + spring. The spring is within the system. 3. System=block + Earth. Here gravity is an internal Force. 4. System=block + spring + Earth. The spring force and gravity are both internal to the system, so K + Uspring = Wgrav K + U grav = Wspring K + Uspring + Ugrav = 0