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Conservation of Energy

  1. Apr 6, 2016 #1
    1. The problem statement, all variables and given/known data

    The coefficient of friction between the block of mass m1 = 3.00kg and the surface in Figure P7.22 is μk = 0.400. The system starts from rest. What is the speed of the ball of mass m2 = 5.00 kg when it has fallen a distance of h = 1.50m?

    2. Relevant equations
    This is in the chapter on conservation of energy, so
    Ui + Ki + ΔEmech = Uf + Kf

    3. The attempt at a solution
    Using downward direction as positive so g is positive

    m2gh - fh = 1/2(m1 + m2)v2
    (5.00kg)(9.8m/s2)(1.5m) - (-11.76N)(1.5m) = 1/2(3.00kg + 5.00kg)v2
    91.14Nm = 4kg * v2
    22.785Nm2/s2 = v2
    4.77m/s = v

    That's not the answer that I'm seeing on the answer sheet. I'm not sure what I did wrong.
  2. jcsd
  3. Apr 6, 2016 #2


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    Think conceptually and decide if you want to have two minus signs here.
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