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Homework Help: Work done on a block

  1. Feb 27, 2013 #1
    Say we have a friction-less inclined plane in the shape of a right triangle, sloping downward to the right. The angle above the horizontal axis is inconsequential for these specific purposes aside from the fact that we know it is inclined above said axis.

    We then have a block sliding down (to the right) the plane. My question is about the work done on the block by the incline plane and gravity.

    Is the work done by the inclined place on the block considered to be positive, negative, or zero? I think it is zero as what would be considered the normal force has a direction perpendicular to the movement of the block.

    I am also hung up on the same question regarding the work done by gravity. It would make sense to think the work by gravity would be positive as that is what moves the block down the inclined plane without outside forces having acted upon it. I just wanted to verify these if anyone could possibly clear things up a bit.

    Thanks in advance.

  2. jcsd
  3. Feb 27, 2013 #2
    If you lift an object, you are giving it gravitational potential energy.

    So, to put it at the top of that incline, it will have gained potential energy.

    When it is released, it is losing potential energy, but gaining gravitational kinetic energy.

    KE = PE


    When it's frictionless, it's best to think of these problems with the equations

    KE = [itex]\frac{1}{2}[/itex]mv[itex]^{2}[/itex]

    PE = mgh
  4. Feb 27, 2013 #3


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    Hello TheNotoriousWM. Welcome to PF !

    You are correct on both accounts.
  5. Feb 27, 2013 #4
    Thanks for the help all, never hurts to check.
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