Lec10_Sum_f

# Lec10_Sum_f - Summary Lecture 10 The work done on an object...

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1 Lecture 10 Mechanical Energy Energy Conservation Power Physics 111, Summer 2011, June 29, Lecture 10 Summary The work done by a force at some angle between the directions of the force and the displacement: The work done on an object is equal to the product of constant force acting the object and distance in the same direction as its motion: d F W II = d F W ) cos ( θ = p Kinetic energy (KE): Work-energy theorem : the net work done on an object is equal to the change in the object’s kinetic energy: 2 2 1 2 2 mv mv KE = = KE KE KE W Δ = = 1 2 ravitational potential nergy: gh E 2 Elastic potential energy (PE) of spring ( k is spring stiffness constant, x is compressed distance): The unit of work and energy : joule (J) Gravitational potential energy: mgh PE 2 2 1 kx PE Elastic = Physics 111, Summer 2011, June 29, Lecture 10 A spring stored elastic potential energy of 10 J. If the spring has stiffness constant of 250 N/m, by what distance it was compressed? Elastic Potential Energy 1) 0.1 m 2) 0.2 m 3) 0.3 m 4) 0.4 m 2 2 1 kx PE Elastic = 3 m N J k PE Elastic x / 250 ) 10 )( 2 ( ) )( 2 ( = = m m 3 . 0 28 . 0 = Physics 111, Summer 2011, June 29, Lecture 10 Conservative and Nonconservative Forces Conservative forces (e.g. gravity): the work done does not depend on the pass taken but only on the initial and final positions. A B 1 y 2 y 3 y 4 Nonconservative forces (e.g. friction) : the work done depends on the pass taken. ) ( 1 2 y y mg PE = Δ A B Physics 111, Summer 2011, June 29, Lecture 10

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2 Conservation of Energy Examples of energy conversion: Light bulb: ar engine: Electrical energy to energy of light hemical nergy of fuel to inetic nergy of car Battery: Car engine: Nuclear power plant: Thermal power plant: Chemical energy of fuel to kinetic energy of car Chemical energy of substances to electrical energy Nuclear energy to heat and then to electrical energy Chemical energy to heat and then to electrical energy Law of conservation of energy:
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## This note was uploaded on 02/11/2012 for the course PHYSICS 111 taught by Professor B during the Summer '08 term at Iowa State.

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Lec10_Sum_f - Summary Lecture 10 The work done on an object...

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