1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?