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Cube sliding down with frictionless slide

  1. Oct 31, 2014 #1
    1. The problem statement, all variables and given/known data
    A cube with m mass is released from the top of a slide, of h height, with a horizontal distance of d.


    2. Relevant equations
    Assuming there is no friction between the cube and the slide, ¿what is the minimal information i need to calculate the speed of the cube at the end of the slide?



    3. The attempt at a solution

    I tried to solve it using the conservation of mechanical energy, so:


    Initial mechanical energy = mgh
    Final mechanical energy = (mv2
    Therefore

    mgh=(mv2
    gh=(v2
    2gh=v2
    √2gh=v


    So i only need to know g and h to solve this but the solution is not that, according to the website where i got this problem. The solution of the problem would be this:

    We have this:

    v2f=v2i + 2gΔx
    (v2i = 0 because at the beginning is static)

    Where Δx is the displacement of the cube, therefore

    vf=√(2gΔx)

    To find the value of Δx we use phytagoras theorem:

    Δx=√(d2+h2)

    So to calculate vf we need to know d ,h, g

    Why this solution is correct ,since mine requires less information?

    (Sorry for my English, is not my first language)
     
  2. jcsd
  3. Oct 31, 2014 #2

    haruspex

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    The above appears to be taking the acceleration down the slope as g, but it will be less than that.
    Your answer is correct, the website's wrong.
     
  4. Oct 31, 2014 #3
    Ok thank you! :D
     
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