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Homework Help: Work and energy (change in potential)

  1. Oct 25, 2012 #1
    1. The problem statement, all variables and given/known data

    part a and b are solved(is it correct), but I am not sure how to solve part c.
    A single constant force F = (3.02i + 5.45j)N acts on a 3.69kg particle.

    a) Calculate the work done by this force if the particle moves from the origin to the point having the vector position r = (1.97i - 2.77j)m

    b) What is the speed of the particle at r if its speed at the origin is 3.51m/s?

    c)What is the change in the potential energy of the system?
    2. Relevant equations

    3. The attempt at a solution
    a) F dot r = (3.02i + 5.45j) dot (1.97i +-2.77j) = (3.02x1.97)+(5.45x -2.77)= -9.15J

    b)1/2mvi^2 - (F dot r) = 1/2mvf^2
    (0.5x3.69kgx3.51^2 - 9.15J = 1/2(3.69kg)vf^2
    v= 2.71m/s

    c)W= Ek-Eg
    -9.15J = 1/2mvf2-1/2mvi2 - Eg
    -9.15J = 1/2(3.69kg)(2.71^2-3.51^2) - Eg
    Eg= 0.033J
    can anyone tell me what Im doing wrong for part c?
  2. jcsd
  3. Oct 26, 2012 #2


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    I think the result you got in part (c) should have been zero. What you actually got looks like rounding error.
    I would have thought that the lost KE has all gone into the PE (i.e. total conserved), no? A uniform force field would be conservative.
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