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Homework Help: Finding kinetic energy

  1. Feb 20, 2010 #1
    1. The problem statement, all variables and given/known data

    a 20 kilogram block is placed at the top of a 10 meter long inclined plane. the block starts from rest and slides without friction down the length of the incline

    determines the kinetic energy of the block just as it reaches the bottom of the incline

    2. Relevant equations

    mass = 20 kg
    distance = 10 m
    height = 10 m

    ke = 1/2 mv square

    3. The attempt at a solution

    is there another equation i can use to solve it?
  2. jcsd
  3. Feb 20, 2010 #2


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    Homework Helper

    There are two different approaches. One is to start with
    energy at the top = energy at the bottom
    and use your KE formula for one side of that.

    The other is to use accelerated motion formulas to find how fast it is going after falling the given distance.

    Is the height given? The question says "a 10 m long inclined plane" but does not give the height or angle of incline. I don't think you can find the answer without knowing the height or angle.
  4. Feb 20, 2010 #3
    oh there was a diagram with the question showing that the height was 10m so which equation i can use if i know height?
  5. Feb 21, 2010 #4
    what is the other formula?
  6. Feb 21, 2010 #5
    The other formula you need is for gravitational potential energy. Do you know it? If so, you can simply use conservation of energy and get the solution.
  7. Feb 21, 2010 #6
    gravitational is mgh? is conservation of the energy same formula
  8. Feb 21, 2010 #7
    The principle of conservation of energy states that the total energy of a closed system must remain the same at all times. This system is totally isolated, except for gravity, and there is no internal friction. So the total energy of the system is:

    [tex]E_T = E_K + E_P[/tex].

    Does that help? Don't forget, there is a sign convention to take into account.
  9. Feb 21, 2010 #8
    oh so using that forumula kienetic energy should be the same as the gravitational?
  10. Feb 21, 2010 #9
    Exactly. Because energy is conserved, the gravitational potential energy lost as it slides down the slope should equal the kinetic energy gained. :)
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