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Unformatted text preview: Chapter 6: Work and Kinetic Energy What is work done by a force What is kinetic energy workenergy theorem How to calculate work done by a varying force along a curved path The meaning and calculation of power in a physical situation Work, a force through a distance As in the illustration, the pushing (force) is in the same direction that the object moves The work is only done by a force when the force contributes to the motion What if and are not in the same direction? F s Unit: joule 1 J = 1 N m Use the parallel component if the force acts at an angle Only the component parallel to the displacement does the work : the angle between If the force and displacement are known by their compoenents then W = F s = Fs cos F = F x i + F y j = (160 N ) i (40 N ) j s = x i + y j = (14 m ) i + (11 m ) j W = F x x + F y y Scalar product F , s Example 6.1 (b) Scalar product of two vectors Numbers can be multiplied; What about vectors It can be multiplied: Scalar product and cross product (Available in CH1 vectors in the text book) What else do you need to know? What if =90 o ? =0 o ? =180 o Using components (2d) It is a Scalar , and has no direction! Dot product A B = B A i i = 1 i j = A B = A x B x + A y B y A B = AB cos =  A  B  cos How to find angles between two vectors: from 2d to 3d How to find the angle between two vectors in 2d? 3d too difficult? Use scalar products: A B = A x B x + A y B y + A z B z =  A  B  cos cos = A x B x + A y B y + A z B z  A  B  Example 1.11 A = 2 i + 3 j + k B = 4 i + 2 j k A = 2 i + 2 j B = 2 i + 2 j How can it be such a great workout with no work? When positive and negative work cancel, the net work is zero even though muscles are exercising. Q6.1 A. The cable does positive work on the elevator, and the elevator does positive work on the cable. v Motor Cable Elevator An elevator is being lifted at a constant speed by a steel cable attached to an electric motor. Which statement is correct? B. The cable does positive work on the elevator, and the elevator does negative work on the cable. C. The cable does negative work on the elevator, and the elevator does positive work on the cable. D. The cable does negative work on the elevator, and the elevator does negative work on the cable. A6.1 A. The cable does positive work on the elevator, and the elevator does positive work on the cable....
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This note was uploaded on 02/19/2012 for the course PHY 2048 taught by Professor Yifu zhu during the Fall '09 term at FIU.
 Fall '09
 YIFU ZHU
 Physics, Energy, Force, Kinetic Energy, Power, Work, WorkEnergy Theorem

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