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Relationship between work and kinetic energy

  1. Apr 4, 2009 #1
    1. The problem statement, all variables and given/known data
    A block of ice slides down a frictionless ramp at angle q = 50° while an ice worker pulls on the block (via a rope) with a force that has a magnitude of 50 N and is directed up the ramp. As the block slides through distance d = 0.50 m along the ramp, its kinetic energy increases by 80 J. How much greater would its kinetic energy have been if the rope had not been attached to the block?
    http://www.lowellphysics.org/beta/Textbook%20Resources/Chapter%207.%20Kinetic%20Energy%20and%20Work/Problems/c07x7_11.xform_files/nw0315-n.gif

    2. Relevant equations
    W = Fd, delta K = Kf - Ki

    3. The attempt at a solution
    I found W = Fd = (50)(0.5) = 25 J. Then I tried to figure out the kinetic energy at the top and bottom of the ramp, but I got stuck because the mass of the block wasn't given. Then I checked the back of the book and found that the answer to the increase in kinetic energy was actually 25 J, and in some vague way this makes sense to me, but not totally, and not in a way I would have been able to figure out on my own. Can someone give me some helpful words to explain this?
     
  2. jcsd
  3. Apr 4, 2009 #2

    rl.bhat

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    Homework Helper

    Work done = change in KE.
    From this find the net down ward force (F-Fr).
     
  4. Apr 5, 2009 #3
    thanks. i feel stupid.
     
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