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2053Lecture7

# E for each the total energy is e ke pe work done by a

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Unformatted text preview: KE + PE. Work Done by a Variable Force The work W done by N variable forces F1, F2,..., FN through displacements s1, s2, ..., sN is given by W ( F cos ) s + ( F cos ) s + + ( FN cos N ) s N = N i= ( Fi cos i ) si where i is the angle between the force Fi and displacement si. Geometric Meaning of the Work Done by a Variable Force Force Displacement Two Types of Forces A conservative force is a force acting on a body that obeys the conservation of energy principle. For example, the force of gravity acting on a body. Any force that acts on an body so that the total energy before is not equal to the total energy after is called a non-conservative force. For example, friction, air resistance or a propulsion force acting on a body. Non-Conservative Forces & Work The work Wnc done by nonconservative forces is given by either Wnc = KE + PE or = ( KE after - KE before ) + ( PE after - PEbefore ) Wnc = Eafter - Ebefore = = ( KEafter + PEafter ) - ( KEbefore + PEbefore ) . Another Form of NonConservative Work Another form of the non-conservative work Wnc done by external forces is given by KEafter + PEafter + Energy Output = = KEbefore + PEbefore + Energy Input Average Power The average power P is the rate at which energy is transferred to or from a body over a given time interval t. W P= t The SI unit of power is the Watt=J/s....
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