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**Unformatted text preview: **1 CHAPTER 6 WORK and KINETIC ENERGY Work Done by a Constant Force Work Done by a Variable Force Work- Kinetic – Energy Theorem Center of Mass Work 2 The energy is one of the most important concept that helps us describe many processes in the world around us. • Falling water releases stored “gravitational potential energy” turning into a “kinetic energy” of motion. This “mechanical energy” can be used to spin turbines and alternators doing “work” to generate electrical energy. • Human beings transform the stored chemical energy of food into various forms necessary for the maintenance of the functions of the various organ system, tissues and cells in the body. • Burning gasoline in car engines converts “chemical energy” stored in the atomic bonds of the constituent atoms of gasoline into heat that then drives a piston of combustion engine. • Stretching or compressing a spring stores ‘elastic potential energy’ that can be released as kinetic energy. • The process of vision begins with stored “atomic energy” released as electromagnetic radiation (light) that is detected by exciting atoms in the eye. 3 WORK DONE BY CONSTANT FORCE In physics Work it is transfer energy by force. The Work is scalar value and can be positive negative and zero. For a constant force, the work done equals the force component in the direction of the displacement times the magnitude of the displacement. Or another definition Work is scalar (or dot) product of vector force and vector displacement. θ ∆ ∆ ∆ cos r F r F W ⋅ ⋅ = ⋅ = Assuming r is in x direction. θ ∆ ∆ ∆ cos x F x F W x ⋅ = ⋅ = 4 The SI unit of work is the joule (J), which is equals The product of Newton and meter. 1J = 1N*m In the U.S. customary system, the unit of work is foot-pound. 1ft*lb = 1.356J Unit of work that use in nuclear and quantum physics. is electron volt (eV). 1 eV = 1.62*10^-19J For the multiple forces, that act on object we will have. ..... 2 2 2 2 1 1 x F x F x F W total ∆ + ∆ + ∆ = 5 When force F net applied to object and object move under influence of this force with acceleration a from A to B along the x axis. The physical value 2 2 mv is scalar represented energy associated with motion and call Kinetic Energy Work Kinetic Energy Theorem K mv mv W i f ∆ =- = 2 2 2 2 Completed work is equal change in kinetic energy of object 2 2 2 1 2 2 2 A B v v v v B A AB net B A net AB mv mv mv dv v m dt v dt dv m W dt dx v dt dv m ma F dx F W B A B A- = = ⋅ ⋅ = ⋅ ⋅ ⋅ = = ⋅ = = ⋅ = ∫ ∫ ∫ A B 6 Negative work means A the kinetic energy of the object increases. B the applied force is variable. C the applied force is perpendicular to the displacement....

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