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Energy with friction

  1. Mar 21, 2008 #1
    Two masses are connected by a string via a pulley, m1=45kg is on a ramp while m2=103kg is hanging off the edge. The pulley is frictionless and of negligible mass. The coefficient of kinetic friction between the 45.0 kg block and incline is 0.250. Determine the change in the kinetic energy of the 45.0 kg block as it moves from A to B, a distance of 20.0 m. Theta is 37 degrees

    I first tried to find velocity (V) to use in a DKE equation

    My equation for V was sqrt((2(m2ghi-m1ghf+um1gdcos@))/(m1+m2))

    u = coefficient of kinetic friction
    hi = initial height, 20m in this case
    hf = 20sin@
    @ = theta

    I then used DKE = 0.5m1Vf^2-0.5m1Vi^2

    where Vi = 0

    Using this method I got an answer of DKE of 5059.76J, which was wrong.
    I do not know if I am over simplifying the problem or thinking too much or if I'm completely thinking in the wrong direction. Help would be much appreciated.
  2. jcsd
  3. Mar 21, 2008 #2

    Doc Al

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    Staff: Mentor

    Check the signs of your terms.
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