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Homework Help: Force and Impulse

  1. Oct 23, 2005 #1
    Please can I have some help with this problem:

    A machine gun in automatic mode fires 20g bullets with vbullet=300m/s at 60 bullets/second.
    a- If the bullets enters a thick wooden wall, what is the average force exerted against the wall?
    b- If the bullets hit a steel wall and rebound elastically,what is the average force on the wall?

    What i did is
    for a:
    I computed the impulse: I= pf-pi= m*v-( 0)=6 N.s
    to calculate the average force: I/(time for one bullet to be fired)= 6/(1/60)=360N

    Am I right?
    However I have no idea for the second question??
  2. jcsd
  3. Oct 24, 2005 #2
    please can someone help me with this problem?
  4. Oct 24, 2005 #3

    Doc Al

    User Avatar

    Staff: Mentor

    It's the same idea. The only difference is the change in momentum. (The final velocity of the bullet is not zero.)
  5. Oct 24, 2005 #4
    I think that for the second question, the average force should be twice as much, cos the collision is modelled as completely elastic, so you have the following maths...
    FT = m(u-v) // m = 0.02 kg, u = 300 m/s, v = -300 m/s ...
    Therefore, the answer to the second question is twice your answer to the first part (I think) someone will correct me if I'm wrong, so if I am wrong, then it doesnt matter
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