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Homework Help: 2d Motion, calculating time when v=0 from position vector

  1. Oct 3, 2011 #1
    1. The problem statement, all variables and given/known data
    The position vector of a particle is given by r= ihat(2t^3-21t^2+60t+10) +jhat(3/2)(t^2-10t+9)
    At what time, if any, is the velocity of the particle zero (ie what time is the particle instantaneously at rest)

    2. Relevant equations

    3. The attempt at a solution
    Finding the velocity vector is simple, just taking the derivative of position
    However, when I set that to 0 I have the ihat and the jhat and I don't know how to combine them to solve for the time. Is this even the right step?

    If I take
    v= (6t^2-42t+60)ihat+(3t^2-15)jhat
    and set that to 0
    how do I account for the vx and vy when setting that to 0?
    Do I set vx and vy to 0 separately?

    I'm really confused.

  2. jcsd
  3. Oct 3, 2011 #2


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    Staff Emeritus
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    In order for v to be zero, both components must be zero. (at the same time, of course)
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