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19-kinetic energy and work 2

# 19-kinetic energy and work 2 - PHYS 211 Chapter 7 Kinetic...

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PHYS 211 3/4/09 Chapter 7 Kinetic Energy and Work Constant Forces Variable Force (Spring)

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Summary Kinetic Energy K = 1 2 mv 2 Work W = Δ K = F d Work transfers energy - sign indicates direction of transfer
Force and Work A box is pulled up a rough incline by a rope connected to a pulley. How many forces are doing work on the box? 1) one force 2) two forces 3) three forces 4) four forces 5) no forces are doing work

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Example (Sample 7-5) Initially stationary crate of mass m=15.0 kg is pulled, via a cable, a distance d=5.70 m up a frictionless ramp to a height of 2.50 m, where it stops. a) work done by gravitation force Fg? W = F d = mgd cos(90 + θ ) cos(90 + θ ) = sin( θ ) sin( θ ) = h d W = mgd h d = mgh = (15.0 kg )(9.8 m / s 2 )(2.5 m ) = 368 J b) work done on the crate by Tension T Δ K = W g + W T = 0 W T = W g = 368 J
Work and Path Alice and Bill, who happen to have the same mass, both to climb to the top of a mountain. Bill takes the steep path straight up, but Alice takes the path that gently winds around the mountain, and it is 8 times longer than the steep path.

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