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Homework Help: Work-Energy Theorem HW help

  1. May 24, 2012 #1
    Help! I am a starter physicist and I need hlep on this question stated thus:

    1. The problem statement, all variables and given/known data

    A sled is being pulled across a horizontal patch of snow.
    Friction is negligible. The pulling force points in the same direction
    as the sled's displacement, which is along the +x axis. As a result, the kinetic energy of the sled increases by 38%. By what percentage would the sled's kinetic energy have increased if this force had pointed 62° above the +x axis?

    2. Relevant equations

    W = final KE - initial KE
    W = (F cos theta)*displacement

    3. The attempt at a solution

    I tried using the work-energy theorem, but i couldn't solve for any of the variables. Then I turned to the second work formula, but still no dice.
  2. jcsd
  3. May 24, 2012 #2
    Since Friction is negligible, no work done.
  4. May 24, 2012 #3


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    Homework Helper

    You have all the necessary formulae. The work is equal to the change of KE. Write it out for both cases. The angles are different, but the displacements same x.
    Call the intial kinetic energy KEi. Write the change of KE in terms of KEi. It is 0.38KEi in the first case, and ηKEi in the second case when theta=62°. You need to find the unknown factor η.
    Divide the equations with each other: Both KEi and x cancel. Solve for η .

  5. May 24, 2012 #4


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    Homework Helper

    That is not right. The sled is accelerated. The change of kinetic energy needs work.

    Last edited: May 24, 2012
  6. May 24, 2012 #5
    The first time I read the problem I did a different way than was suggested.

    If the initial force parallel to the ground causes an inc in the Kinetic energy by 38% then a force at an angle of 62 degrees, there is only 46.9% of the original force being applied horizontally.

    How I got 46.9% is by F cos(angle)= force in the x direction
    Fcos(62) = ""
    F 0.469 = "" ""
    This is essentially saying that only .469% of the force is in the x direction( the rest in the y-direction).
    In the original case(where there is 38% inc) the angle is zero and cos 0= 1 so 100% of the force is in the x direction.

    So if 100 of the force = 38% , then 46.9 percent of the force = ?

    simple ratio 100 = 38
    ----- ----
    46.9 x
    Therefore what is x this is 17% so there is only a 17% inc in Kinetic energy when the force is applied at 62 degrees to the ground.

    Let me know if you want the algebraic method suggested by the other poster you will still arrive at the same answer, but its longer.
  7. May 24, 2012 #6


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    Homework Helper

    Both methods are essentially the same.

  8. May 24, 2012 #7
    I figured it out by myself, but thank you all.
    The ratio of the initial work to the work w/ the 62° angle is

    Fs/s(f cos 62°)
    = 1/(cos 62°)

    Thus, multiply 38% by cos 62° (0.469).
    Significant digits yield 0.18, or 18%.
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