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Collision and Impulse

  1. Mar 30, 2009 #1
    1. The problem statement, all variables and given/known data
    Suppose a gangster sprays Superman's chest with 3 g bullets at the rate of 100 bullets/min, and the speed of each bullet is 500 m/s. Suppose too that the bullets rebound straight back with no change in speed. What is the magnitude of the average force on Superman's chest?

    2. Relevant equations
    J = FavgT
    p = mv
    J = pf - pi

    3. The attempt at a solution
    I don't really understand how to get the delta T. I got J = -3 Ns from equation 3, don't know where to go from there.
  2. jcsd
  3. Mar 30, 2009 #2


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    Homework Helper

    You have 300 grams of bullets a minute hitting at 5 grams a sec on average.

    Taking rebound into account you have a change in velocity of 1000 m/s then don't you have 5k grams/s*m/s? That looks like Newtons to me.
  4. Mar 31, 2009 #3
    hmm interesting. So how would I know to play with the units rather than using typical linear momentum equations?
  5. Mar 31, 2009 #4


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    Insight. Regardless of the equations you must end with the same units they are asking. Units are often a guide to the answer. But it's not like I'm suggesting you not use the equations, so much as suggesting you understand what the equations are relating to begin with.

    F = m*a = m*Δv/Δt = Δmv/Δt = Δp/Δt
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