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Work-Kinetic Energy Contrasting Newton's Laws?

  1. Nov 13, 2007 #1
    1. The problem statement, all variables and given/known data


    This is a question that's supposed to prove to us that you can use the work-kinetic energy theorem as a second theory of motion. Except I keep getting different answers when I work it out first w/ KET and second with Newton's Laws. So I'm definitely NOT fulfilling the objective here...

    In a rifle barrel, a 15 g bullet is accelerated from rest to a speed of 780 m/s.
    a) Find KE, at rest AND when it reaches 780 m/s.
    b) Find work done.
    c) Barrel = 72 cm long. Find mag. of average net force with Fnet = W/delta'r'cos(theta), where delta'r'cos(theta) is the displacement.
    And then you find acceleration w/ Newton's Laws and the net force.
    2. Relevant equations
    KE = 1/2mv^2, W = kf - ki, Fnet = ma


    3. The attempt at a solution
    At initial position, KE = 0
    At final speed, KE = 5.85 J

    Net force = 5.85 / (0.72m)(1) = 8.125 N (??)

    Constant acceleration. This is so wrong.... 422500 m/s^2
    Using vf^2 = vi^2 + 2a(x)

    So my Fnet here = (0.015kg)(422500 m/s^2)...

    Thanks!
     
  2. jcsd
  3. Nov 13, 2007 #2
    your KE final is wrong. it should be much bigger than that.
     
    Last edited: Nov 13, 2007
  4. Nov 13, 2007 #3
    D'oh! I totally forgot to square the velocity. Um. Yeah. Now everything works. Thanks...
    Bullets have a friggin' huge acceleration! Zowee! Wouldn't want to get hit by one of those!
     
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