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Homework Help: Cannonball velocity problem

  1. Nov 10, 2008 #1
    1. The problem statement, all variables and given/known data
    A 30 kg cannonball strikes a 150 kg stationary wooden block and embeds itself in the block. The block and cannonball move together at 4 m/s. What was the original velocity of the cannonball?


    2. Relevant equations
    m1v1=m2v2


    3. The attempt at a solution
    using the equation i gave i got:

    30v=150(4)
    v=20

    the answer my teacher my teacher gave me was 24 m/s. and i was thinking if you add the '4 m/s' to the velocity i just found it would be '24 m/s'. but i dont know if that was right. i was thinking it might be right because they both moved together after the ball hit the wood for 4 m/s. so is that right? or am i supposed to do something else?

    thank you!
    sweedeljoseph
     
  2. jcsd
  3. Nov 10, 2008 #2
    The mass after the collision would be the masses of both objects added together since they are now considered to be one object moving at the same velocity.

    m1V1 + m2V2 (equals 0) = v'(m1 + m2)

    Solve for V1 before
     
  4. Nov 10, 2008 #3
    so i was right? you add the '4 m/s' to the velocity that i found?
     
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