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One dimensional inelastic collision; bullet through block.

  1. Jul 7, 2015 #1
    1. The problem statement, all variables and given/known data
    In the figure here, a 12.8 g bullet moving directly upward at 930 m/s strikes and passes through the center of mass of a 8.3 kg block initially at rest. The bullet emerges from the block moving directly upward at 520 m/s. To what maximum height does the block then rise above its initial position?

    20150707_211326_zpsglwxmosy.jpg

    2. Relevant equations
    m1v1i + m2v2i = m1v1f + m2v2f

    3. The attempt at a solution
    20150707_210809_zpsywi6ntir.jpg
    not sure where i went wrong with this one.
     
  2. jcsd
  3. Jul 7, 2015 #2

    SammyS

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    Velocity for block is correct.

    Bad algebra in solving for y-y0 .
     
  4. Jul 7, 2015 #3

    Nathanael

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    Instead of y-y0 I will call it h and instead of a, I will call it g.

    You wrote the equation v2=2gh... good so far... but then you wrote h=v2-2g :confused:
     
  5. Jul 7, 2015 #4
    I need to take an algebra class, again.
     
  6. Jul 7, 2015 #5
    why is g positive in this problem? because the bullet is fired straight up and so the accerleration of g is -9.8m/s/s. I can see it works out in the equation for it to be positive, i just dont know the reason why.
     
    Last edited: Jul 7, 2015
  7. Jul 7, 2015 #6
    Thank you
     
  8. Jul 7, 2015 #7

    SammyS

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    So I'm assuming that you resolved the above question, because of your following post.
    It may be helpful to you and us, if you could learn to use the edit feature and/or the delete feature to alter or to remove a post like Post #5.
     
  9. Jul 7, 2015 #8
    Sure, i didnt know about the delete feature. I'll edit it above.
     
  10. Jul 7, 2015 #9

    Nathanael

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    Because it is really Vf (which you called V) that is zero, and the V0 is what is 0.6323. So g would be negative but you would also have to subtract V02 over to the other side and then the negative sign would cancel out.
     
  11. Jul 7, 2015 #10
    Ok, i think i get it now because the problem is treated as two problems and the v_f i got from the first problem becomes v0 in the next. Thanks.
     
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