Physics 111 Elementary Mechanics ndash; Lecture 7.ppt

October 17, 2006 Center for Solar-Terrestrial Research Physics 111: Elementary Mechanics – Lecture 7 Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research Introduction Potential Energy and Conservation of Energy Conservative Forces Gravitational and Elastic Potential Energy Conservation of (Mechanical) Energy Potential Energy Curve External Forces Work and Potential Energy (Non-)Conservative Forces The system consists of two or more objects. A force acts between a particle–like object in the system and the rest of the system. When the system configuration changes, the force does work W1 on the particle–like object, transferring energy between the kinetic energy K of the object and some other form of energy of the system. When the configuration change is reversed, the force reverses the energy transfer, doing work W2 in the process. W1 = –W2 ? conservative force Path Independence of Conservative Forces The net work done by a conservative force on a particle moving around every closed path is zero. The work done by a conservative force on a particle moving between two points does not depend on the path taken by the particle. Conservation of Mechanical Energy Potential Energy Curve Turning Points Equilibrium Points Neutral Equilibrium Unstable Equilibrium Stable Equilibrium Conservation of Energy The total energy of a system can change only by amounts of energy that are transferred to or from the system. The total energy E of an isolated system cannot change. * * Potential Energy General Form Gravitational Potential Energy Elastic Potential Energy Mechanical Energy Conservation of Mechanical Energy In an isolated system where only conservative forces cause energy changes, the kinetic and potential energy can change, but their sum, the mechanical energy Emec of the system, cannot change. A plot of U(x), the potential energy function of a system containing a particle confined to move along the x axis. There is no friction, so mechanical ener