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Homework Help: Vehicle Momentum problem

  1. Mar 30, 2008 #1
    1. The problem statement, all variables and given/known data
    The Army of the Nation of Whynot has a plan to propel small vehicles across the battlefield by shooting them, from behind, with a machine gun. They've hired you as a consultant to help with an upcoming test. A 100 kg test vehicle will roll along frictionless rails. The vehicle has a tall "sail" made of ultrahard steel. Previous tests have shown that a 20.0g bullet traveling at 400 m/s rebounds from the sail at 100 m/s. The design objective is for the cart to reach a speed of 19.0 m/s in 30.0 s . You need to tell them how many bullets to fire per second from the machine gun. Note that your answer should be a rate, which can be fractional, rather than a quantity, which must be an integer.

    2. Relevant equations
    p=mv J=change in momentum

    3. The attempt at a solution
    I found the impulse exerted on a bullet.
    J= -10 Ns

    Then I set up an equation to find what impulse needs to be exerted on the cart to speed up to 19 m/s:

    0kgm/s + J = (100kg)(19 m/s)
    J=1900 Ns

    Then I assumed the impulse exerted on a bullet is the same as the impulse exerted on the sail by a single bullet :

    1900Ns* (1 bullet/10Ns) =190 bullets
    I then divided that by 30 sec. & came up with 6.33 bullets/sec.

    Is this correct?
  2. jcsd
  3. Mar 30, 2008 #2
    Your math is correct and I ran through your procedure and it looks sound.
  4. Mar 30, 2008 #3


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    … Because … Because … Because … !

    The Nation of Because regards your militaristic experiments as a threat to our beliefs! :frown:

    The ancient Nation of Because will always defeat the Nation of Whynot!

    :mad: Go to your room! :mad:
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