1. Limited time only! Sign up for a free 30min personal 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!

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


    User Avatar
    Science Advisor
    Homework Helper

    … 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:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook