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Homework Help: Kinetic Energy, Work Done, and Inclines

  1. Dec 14, 2009 #1
    UPS delivery man is loading his truck by shoving the parcels up a ramp. The ramp is inclined at an angle of 30 degrees above the horizontal. He sends a parcel of mass 15 kg, up this ramp. the coefficient of sliding friction is .10 and gravitational force is class standard (10).

    A The magnitude of the kinetic friction acting on the parcel is?
    B He sends parcel up ramp with initial Kinetic Energy, E, so that by the time, the parcels up the ramp, it has 0 KE. Ramp is length 1.0m.
    C To give this parcel this initial KE, he most do WORK on the parcel. Assuming he pushed it a short distance of .75m what is the average force he needs to apply. Ignore friction for this part.


    The work I have done is:

    A
    Y Components: N-MG Cos 30 --> N=150 Cos 30 --> N=129.9
    Friction = [tex]\mu[/tex]k * N --> .10 * 129.9 ---> Magnitude =12.99

    B

    1/2 M Vo2+ [STRIKE]MGH[/STRIKE]= [STRIKE]1/2 M Vf2[/STRIKE] + MGH

    1/2 M Vo2 = MGH


    H= Sin30 -->.5


    1/2 (15) (V)2= 75
    V=3.16

    So initial KE = 75 ?????


    C

    Is related to B so if that's wrong this is wrong.

    Work = Force * Displacement
    Work = KE

    KE: 75 / Disp: .75 = Force: 100N


    Thanks for your help.
     
  2. jcsd
  3. Dec 14, 2009 #2
    Part a looks fine. Part B is missing the effects of frictional dissipation which will rob some of the PE.

    C is done by ignoring friction but asks that you compute force from the work done and the displacement over which force is appplied. Assuming he pushes parallel to the ramp--this will be F*d=energy required to get to top. That help?
     
  4. Dec 14, 2009 #3
    So for B:

    Energy - Friction= 75-12.99 = 62.01


    For C:

    Would I use the new energy 62.01/ (.75) = 86.68N
     
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