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Homework Help: Kinetic Energy and Work energy theorem

  1. Oct 28, 2011 #1
    1. The problem statement, all variables and given/known data
    A toboggan is initially moving at a constant velocity along a snowy horizontal
    surface where friction is negligible. When a pulling force is applied parallel to
    the ground over a certain distance, the kinetic energy increases by 47%. By what
    percentage would the kinetic energy have changed if the pulling force had been
    at an angle of 38° above the horizontal?

    2. Relevant equations

    w= Fd cos theta
    w= 1/2mv^2

    3. The attempt at a solution
    so Ek = 1.47 Ek intial Ek = kinetic energy
  2. jcsd
  3. Oct 28, 2011 #2
  4. Oct 28, 2011 #3
    yeah can you explain what you did?
  5. Oct 28, 2011 #4
    First find an expression for the change in KE when the force is applied along the horizontal.
  6. Oct 28, 2011 #5
    you mean a expression for both cases?
    we know that in the first case the force and displacement act in the same direction meaning no work is done.
    and the second case i dont get, how can you get W = (Fcos38)d where F and d is not even given
  7. Oct 28, 2011 #6
    Work is done in BOTH cases. This work results in an increase in KE.
    No more numerical data is necessary. Just assume initial velocity is u in both cases and final is V for the first case and v for the second (symbols chosen so that big V is for the greater value and small v for the smaller value - I hate to call them v1 and v2!) since the final will be different for the two cases.
  8. Oct 28, 2011 #7
    how am i suppose to find the velocity if they give so little information?
  9. Oct 28, 2011 #8
    Assume that applied force is F in first case and Fcos38 in the second case.
    Initial velocity is u in both.
    Distance travelled is assumed to be constant e.g. x in both.
  10. Oct 28, 2011 #9
    What is V in terms of u, x and acceleration?
  11. Oct 28, 2011 #10
    i did this instead , this should make sense right? cause the change in W = the final kinetic energy - the intial kinetic energy, and W = Fd in first case and second is W=fdcos38

    Attached Files:

  12. Oct 28, 2011 #11
    I think it is OK.
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