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Kinematics: force along x-axis problem

  1. Oct 12, 2004 #1
    The only force acting on a 2.7 kg body as it moves along the positive x axis has an x component Fx = - 6x N, where x is in meters. The velocity of the body at x = 3.0 m is 8.0 m/s.

    What is the velocity at x = 4?

    I found it to be 0.22m/s but my friends swear up and down it's not. Could I get a second opinion on this answer? My work is something like this:

    F = -6x
    a = -6/2.7 * x
    v = -3/2.7 * x^2 +c

    v(3) = 8

    8 = -3/2.7 * 3^2 + c
    c = 18

    v(4) = -3/2.7 * 4^2 + 18
    v(4) = 0.222

    Corrections please? Thanks in advance.
  2. jcsd
  3. Oct 12, 2004 #2
    W = d(Kinetic energy) = (Integral from inital to final ) F dx

    that's waht you're missing
  4. Oct 12, 2004 #3
    heh that makes sense. I had forgotten the point of the entire hw assignment I was helping with was work-related. I still can't figure out why my way didn't work though oh well.
  5. Oct 12, 2004 #4
    It didn't work BECAUSE!

    First of all what are you thinking about here how did you figure (ASSUME) that a = 6 / 2.7 x
    and how did you derive v from that ???

    Seems you tried to find the difference betwee nthe two points on force

    Remember that is W is the integral because it is the area under the graph of force and the only way to do that is to find the integral
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