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Homework Help: Distance between 2 particles

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data

    The position of a particle at time t is given by r(t) = 3(t2 - sin t)i +
    tj - (cos t)k , and the position of another particle is R(t) = t2i + (t3 +
    t)j+(sin t)k . At time t = pi, what is the rate of change of the distance
    between the two particles? Are they getting closer to one another, or
    are they getting farther apart?

    2. Relevant equations

    3. The attempt at a solution

    I'm actually not sure how to attempt this problem. I think that if the rate of change is postive they are getting farther apart and if it is negative they are getting closer, but I'm not sure.
    Last edited by a moderator: Oct 26, 2011
  2. jcsd
  3. Oct 26, 2011 #2


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    Homework Helper

    The first thing you need to do is write an expression for the distance between the two particles. Think pythagorean theorem.
  4. Oct 27, 2011 #3
    I'm not sure I understand. Do I subtract the square of the i,j,k components from each other to get d^2 and then take the directional derivative?
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