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Homework Help: Work-energy theorem

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data
    A 16 g bullet is accelerated in a rifle barrel
    72.3 cm long to a speed of 1010 m/s.
    Use the work-energy theorem to find the
    average force exerted on the bullet while it is
    being accelerated. Answer in units of N.


    2. Relevant equations
    W=changeKE


    3. The attempt at a solution
    Fx=(mvf^2)/2-(mvi^2)/2
    F=((16*1010^2)/2*.723
    F=1.1287e7
    Did I do this correctly?
     
    Last edited: Oct 20, 2008
  2. jcsd
  3. Oct 20, 2008 #2
    Check your units. Looking at your equation

    F=((16*1010^2)/2*.723

    you have units of

    g * m3 / s2


    You seem to have the right equation for Kinetic Energy (although wrong units). That is, KE = (mass) * (velocity2) / 2, which should result in units of N * m.
     
  4. Oct 20, 2008 #3
    The way I had it in units was:

    N * m = (kg *(m^2/s^2) )/ 2
    N = (kg *(m^2/s^2) )/ 2 m
    N = (kg *(m/s^2) )/ 2
    N= kg * m/s^2

    The method in units was correct but i don't know if there is an alternative method to solve this problem.
     
  5. Oct 21, 2008 #4
    almost correct: you should convert 16g to kg, that's all.
     
  6. Oct 21, 2008 #5
    ok, i got 11, 287.4 N thanks
     
  7. Nov 10, 2008 #6
    I have a problem that is quite similar.
    But im confused in the way you were able to find your answer.
    when I multiplied your .016kg by 1010^2, I came up with 16321.6
    which is not your 11,287.4 N

    please explain.
     
  8. Nov 10, 2008 #7
    Remember that F=Wx, so find the work done by kinetic energy

    N * m = (kg *(m^2/s^2) )/ 2
    N = (kg *(m^2/s^2) )/ 2 m
    N = (kg *(m/s^2) )/ 2

    Use the distance given to find the force
     
  9. Nov 11, 2008 #8
    Yeah, shortly after I studdied the thread and got it all kinked out.
    I love this place.
    Thank you anyways.
     
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