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Work done (1)

  1. Jul 14, 2013 #1
    1. The problem statement, all variables and given/known data

    Doug pushes a 5.30 kg crate up a 2.20-m-high 20 degrees frictionless slope by pushing it with a constant horizontal force of 27.0 N. What is the speed of the crate as it reaches the top of the slope?

    a) Solve this problem using work and energy.



    2. Relevant equations
    Wd=delta KE, ke=.5mv^2 and pe=mgh

    3. The attempt at a solution
    Conservation of energy
    W.d=Pe+KE
    Fcos20=mg(h/sin20)+.5mv^2
    v= 3.9m/s
    but I consider another way about the work done
    work done = delta KE
    work done on force = Fcos20
    work done against the gravitational force = mg(h/sin20)
    so Work done by force - work done against the gravitational force =Change of KE .
    which one is a corrected concept or both make sense. thanks so much.
     
  2. jcsd
  3. Jul 14, 2013 #2
    This is not correct.
     
  4. Jul 14, 2013 #3
    why is that not corrected ?
    Since work done must be equal to delta KE , so we consider workd done on gravitational force is not a conserved work done ?
     
    Last edited: Jul 14, 2013
  5. Jul 16, 2013 #4
    Left side of the equation is force, right is energy. Potential energy is ##mgh##, not ##mg\frac{h}{\sin 20^o}##
     
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