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Homework Help: Change in Potential Energy: What am I doing wrong?

  1. Oct 2, 2007 #1
    1. The problem statement, all variables and given/known data
    A 4.20 kg block is set into motion up an inclined plane with an initial speed of v0 = 7.70 m/s (Fig. P7.25). The block comes to rest after traveling 3.00 m along the plane, which is inclined at an angle of 30.0° to the horizontal.

    1. For this motion, determine the change in the block's kinetic energy.
    DeltaK = 1/2m(vf^2-vi^2)^2=-1/2mvi^2= -124.5J

    This next question is the portion I am having problems with:

    2. For this motion, determine the change in potential energy of the block-Earth system.

    2. Relevant equations

    DeltaU = mg(3.00)sin30.0

    3. The attempt at a solution

    =(4.20)(9.8)(3.00)(0.454)
    =56.1

    So what is wrong with my answer??
     
  2. jcsd
  3. Oct 2, 2007 #2

    Doc Al

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    Staff: Mentor

    What's the sine of 30 degrees?
     
  4. Oct 2, 2007 #3

    dynamicsolo

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    Homework Helper

    Before you begin calculations involving trig functions, always check the mode your calculator has been left in. The sine of 30 grads is 0.454; you need to reset the input mode to degrees...
     
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