Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Block being shot upwards

  1. Oct 4, 2007 #1
    1. The problem statement, all variables and given/known data
    question: a 2kg block rests over a small hole in a table. janelle is breathe the table and shoots a 15g bullet through the hole and into the block, where it lodges. How fast was the bullet going if the block rises 75cm above the table?

    2. Relevant equations
    m1v1i +m1v2i = m1v1f + m2v2f

    3. The attempt at a solution
    i tired working this out with the straight line motion equation of s= vt -at^{2}
    but this did not workout

    i tired solving for t but got .153s
    from there i am lost
  2. jcsd
  3. Oct 5, 2007 #2


    User Avatar
    Homework Helper

    There are 2 parts to the problem...

    1) bullet being fired into block... this requires conservation of momentum.

    2) bullet+block rising up to 75cm... this requires conservation of energy.

    try to write the equations for part 1) and part 2)... then you should be able to solve for the initial velocity of the bullet.
  4. Oct 5, 2007 #3
    okay so m1v1i = m1v1f + m2v2f
    but (because they are combined and therefor have the same combined mass and velocity), couldn't it be written as: m1v1i = (m1 + m2) vf

    the conservation of energy? as in Ke=Ep or 1/2 mv(squared) = mgh?
    or as in initial Ke= final Ke?

    there are two unknows here. i guess what your saying is work them (the the separate equations) out and substitute it for one unknown. but could you please give me a bit of guidance? one i see it once i'll be good from there on.
  5. Oct 5, 2007 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Last edited: Oct 5, 2007
  6. Oct 5, 2007 #5


    User Avatar
    Homework Helper

    you're almost there:

    equation 1:

    m1v1i = (m1 + m2)vf

    equation 2:

    (1/2)(m1 + m2)vf^2 = (m1 + m2)gh (as Astronuc mentioned, all the energy is converted to gravitational potential energy)

    solve these two equations... you can cancel the masses in the second equation. your two unknowns are v1i and vf...
  7. Oct 6, 2007 #6
    right, that makes a lot of sense to me now
    just rearranged for vf and subs into v1i and i get the right answer
    thanks to both of you
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook