1. Not finding help here? Sign up for a free 30min 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!

Quick Solve - Literally Stuck at the Last Step

  1. Apr 11, 2010 #1
    The position of a particle moving in the xy-plane is given by the parametric equations x = t3 - 3t2 and y = 2t3 - 3t2 - 12t. For what values of t is the particle at rest?

    So we're trying to find when the slope (dy/dx) is equal to 0.

    dy = 6t2 - 6t - 12
    dx = 3t2 - 3t

    dy/dx = (6t2 - 6t - 12) / (3t2 - 3t)
    dy/dx = [6(t2 - t - 2)] / [3t(t - 2)]
    dy/dx = [2(t + 1)(t - 2)] / [t(t -2)]
    dy/dx = 0 @ t = -1 or 2

    So, i would assume the answer is "-1 or 2" but the answer is really just "2 only". Can anyone explain this to me?
     
  2. jcsd
  3. Apr 11, 2010 #2
    Assuming t is the time, if you find multiple values in the result you will discard the ones that are not useful, like the idea of something taking a negative amount of time.
     
  4. Apr 11, 2010 #3
    Wow, that was simple. I'm sorry but i should've seen how obvious that was. Thanks a bunch!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Quick Solve - Literally Stuck at the Last Step
Loading...