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Homework Help: Dymanics motion/energy/power, any help would be great

  1. Aug 13, 2008 #1
    I state my problen in my solution
    A big gun had a range of 120 km, a 37.5 m barrel, a muzzle velocity of 1550 m/s and fired a 120 kg shell.
    a) If you assume the shell’s acceleration to be constant, what maximum power was transferred to the shell as it travelled along the barrel? [5 Marks]
    b) What average power was transferred to the shell? [3 Marks


    MY solution

    okay so i went about it finding acceleration
    u=0
    v=1550
    s=37.5
    m=120

    v^2=u^2+2as
    a=(v^2-u^2)/(2s)

    so now i have a=32033.33m/s^2

    then using Newtons law

    F=ma
    F=120*32033.33

    then

    F=3844000N

    W=Fd
    this is my first question is it fair to say d=37.5m or is it 120km??
    well thats what i have done

    W=144150000J

    P=w/t
    second question how do i find time?

    third question is the above finds the total power yes? how do i find max power, does it mean its the power at a point? and is average pwoer just total power/distance? to find part b
     
  2. jcsd
  3. Aug 13, 2008 #2

    Kurdt

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    For part a you would use the length of the barrel. The other info given is that the shell undergoes constant acceleration in the barrel and has a final speed of 1550m/s at the end of the barrel. One can work out the time from that. Maximum power I think will be instantaneous power, which is the force multiplied by the velocity. Average power is what you've been calculating.
     
  4. Aug 14, 2008 #3
    ahh so you mean the v=u+at and solve for t

    and that makes sense about the max power thanks
     
  5. Aug 14, 2008 #4

    Kurdt

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    Yes, that is what I was getting at. :smile:
     
  6. Sep 13, 2008 #5
    This looks familiar "yes or no?"
     
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