phy2048-ch7_new - Chapter 7 Potential energy and...

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1 Chapter 7 – Potential energy and conservation of energy I. Potential energy b Energy of configuration II. Work and potential energy III. Conservative / Non-conservative forces IV. Determining potential energy values: - Gravitational potential energy - Elastic potential energy I. V. Conservation of mechanical energy VI. External work and thermal energy VII. External forces and internal energy changes VIII. Power I. Potential energy Energy associated with the arrangement of a system of objects that exert forces on one another. Units: J Examples: - Gravitational potential energy: associated with the state of separation between objects which can attract one another via the gravitational force. - Elastic potential energy: associated with the state of compression/extension of an elastic object. II. Work and potential energy If tomato rises b gravitational force transfers energy “ from tomato’s kinetic energy “ to ” the gravitational potential energy of the tomato-Earth system. If tomato falls down b gravitational force transfers energy from ” the gravitational potential energy “ to ” the tomato’s kinetic energy.
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2 W U - = Δ Also valid for elastic potential energy Spring force does –W on block b energy transfer from kinetic energy of the block to potential elastic energy of the spring. Spring force does +W on block b energy transfer from potential energy of the spring to kinetic energy of the block. General: - System of two or more objects. - A force acts between a particle in the system and the rest of the system. f s f s Spring compression Spring extension - When the configuration change is reversed b force reverses the energy transfer, doing W 2 . III. Conservative / Nonconservative forces - If W 1 =W 2 always b conservative force. Examples: Gravitational force and spring force b associated potential energies. - If W 1 W 2 b nonconservative force. Examples: Drag force, frictional force b KE transferred into thermal energy. Non-reversible process. - When system configuration changes b force does work on the object (W 1 ) transferring energy between KE of the object and some other form of energy of the system. - Thermal energy: Energy associated with the random movement of atoms and molecules. This is not a potential energy.
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3 - Conservative force: The net work it does on a particle moving around every closed path, from an initial point and then back to that point is zero. Conservative force b W ab,1 = W ab,2 W ab,1 + W ba,2 =0 b W ab,1 = -W ba,2 W ab,2 = - W ba,2 IV. Determining potential energy values Δ - = = f i x x U dx x F W ) ( Force F is conservative Gravitational potential energy: Change in the gravitational potential energy of the particle-Earth system. [ ] Δ
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phy2048-ch7_new - Chapter 7 Potential energy and...

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