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Recoil Question

  1. Feb 3, 2014 #1
    A machine gun fires 50-g bullets at the rate of 4 bullets per second. The bullets leave the gun at a speed of 1000 m/s. What is the average recoil force experienced by the machine gun?

    My attempt:
    (1) J = FT
    F= J/T

    so what else should I do????
     
  2. jcsd
  3. Feb 3, 2014 #2

    Simon Bridge

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    Well average velocity is total distance divided by total time ... what would average force be?
    (Hint: newtons laws ... how is force related to change in momentum?)
     
  4. Feb 3, 2014 #3
    F= J/T= mv/T

    Thanks!
     
  5. Feb 5, 2014 #4

    Simon Bridge

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    Well done.
    Just checking:- if you traveled in 10s bursts of 10m/s which occur 90s apart - what's the average speed?
    The problem you have is actually slightly easier than that.
     
  6. May 19, 2014 #5
    Hello, so I get 200000 g m/s,

    what am I doing wrong?
     
  7. May 19, 2014 #6

    Simon Bridge

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    Please show your working.
     
  8. May 20, 2014 #7
    J = ft
    = (mv) t
    = (200g * 1000 m/s) (1s)
    = 200 000 gm

    * its asking me for the recoil which means the impulse right?
    and that is J = ft and what is force? f = mv right? so final formula is J = (mv) (t)
    * mass is 200g because of the 4 bullets and velocity is 1000m/s

    so I dont know why is wrong, can't see it
     
  9. May 20, 2014 #8

    Simon Bridge

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    Check that equation (J=mvt) by dimemsional analysis.

    If "f" is the force in that equation, then you just wrote "force=momentum" ... which is incorrect.
    If J is specific impulse, then ##J=\Delta p##

    No - it specifically asks for the average force of the recoil. $$\bar F = \frac{\Delta p}{\Delta t}$$

    No, ##F=ma##.

    Or just use one bullet averaged over 0.25s ... whatever, should get the same number.
    But use the right equation, you'll get the right units.
     
  10. Jun 1, 2014 #9
    ok

    ok I did

    F = Δp / Δt

    but still I am getting the same bad answer

    F = (200 gr/s) (1000 m/s) / (1s)

    what is wrong with my equation?
     
  11. Jun 1, 2014 #10
    where did 1 second come from? I did't see that anywhere in the problem.
     
  12. Jun 1, 2014 #11
    Besides the fact that you're mysteriously dividing by (1s), there seems to be nothing wrong with it.
     
  13. Jun 1, 2014 #12
    cause my equation is F = mv/t

    so the time is 1 second (all bullets fired in one second), what else should I put then? any ideas?
     
  14. Jun 1, 2014 #13

    Simon Bridge

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    Are you not expected to give Force in SI units?
    How do you know you got he wrong answer?
     
  15. Jun 5, 2014 #14
    @Simon, ok i needed to convert 200 gr into kg which is .20 kg and I got 200 N.
    Thanks!
     
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