1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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)...

  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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook