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? on change in mechanical energy

  1. Jun 20, 2008 #1
    I am having problems with this physics problem.
    Starting from rest a 6.0 kg block slides 2.2m down a rough 30deg incline in 2.0s. find the change in mechanical energy due to friction. answer in J.

    I know that change in mechanical energy is obtained by E=Ef-Eo or Wnc=(KEf+PEf)-(KEo-PEo) and I have used that but must be using the wrong #s.
    The given values are easy to plug in but I am not sure on heights and velocities. the block travels down the incline 2.2 m which is only a height drop of 1.1m do I use 2.2 or 1.1m?, and I figure the velocity to be 2.2m/2.0s= 1.1 m/s ,but since the block starts at rest is my initial velocity is 0 and final 1.1 right and I have tried using 1.1 and 2.2 as starting heights and 0 as final height, but cannot get the right answer.
    any help is appreciated
  2. jcsd
  3. Jun 20, 2008 #2


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    Staff: Mentor

    Thread moved to Homework Help.

    Welcome to the PF, Ken. I'm not familiar with the term "mechanical energy". How is mechanical energy related to potential energy (PE) and kinetic energy (KE)? The wording of the proble is a bit confusing, but it sounds like the friction force on the block is not enough to keep it from starting to slide on its own when released, so there will be some acceleration down the incline due to gravity, and hindered by the friction force. You can write a FBD equation for that interaction of forces and derive the total acceleration, which will head you in the direction of getting your answers.
  4. Jun 20, 2008 #3

    Doc Al

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    Staff: Mentor

    That looks OK. Think of it this way: ME(initial) = ME(final) + Wnc.
    Compare the initial mechanical energy to the final mechanical energy.
    Which distance affects gravitational PE?

    That would be the average velocity--you need the final velocity at the bottom of the incline. How does the average velocity relate to the initial and final velocity?
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