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Homework Help: How much energy was lost

  1. Dec 4, 2008 #1
    a cart with a mass of M is moving at velocity V, a body with a mass of m falls onto the cart from a height of H and sticks to it, how much energy was lost (to heat), there is no friction.

    i said that, since the mass freefalls, its velocity at the moment of impact is[tex]\sqrt{2gh}[/tex]

    the momentum doesnt change so
    MV+0=(m+M)Ux
    0+m[tex]\sqrt{2gh}[/tex]=(m+M)Uy

    is this correct? will the cart have velocity on y axis??
    if there is no friction how is Ux<V

    then find the total energy at the start, which is mgh+0.5MV[tex]^{2}[/tex], and subtract it from the energy at the end 0.5(m+M)[Uy[tex]^{2}[/tex]+Ux[tex]^{2}[/tex]]

    is this correct

    the answer in my book is

    Q=mgh+V^2[tex]\frac{Mm}{2(m+M}[/tex]
     
  2. jcsd
  3. Dec 4, 2008 #2

    alphysicist

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    Homework Helper

    Re: momentum

    This last equation is not correct. These equations represent the momentum of the masses m and M, but there is an external force in the y direction (from the ground). So the y-momentum is not conserved because the ground prevents the cart from moving in the y-direction.


    It's the other way around; energy lost is Ei-Ef
     
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