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Related RA

  1. Jul 13, 2011 #1
    edit: meant to put rates of change as the title, not related rates.
    1. The problem statement, all variables and given/known data
    A particle moves on a vertical line so that its coordinates at time t is s(t) = 4t^3 - 12t^2 +7, where t≥0
    a) when is the particle moving upward and when is it moving downward
    b) find the total distance that the particle travels in the time interval 0≤t≤ 3

    3. The attempt at a solution
    so i'm pretty sure I would take the derivative to find the velocity function, but how do I find out when it's moving upward and when it is moving downward? positive = upward and negative = downward?
  2. jcsd
  3. Jul 13, 2011 #2


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    Sure, they are sort of implying since the motion is along a vertical line that s(t) is distance along that line upward from the origin. So positive derivative is upwards.
  4. Jul 13, 2011 #3
    When I take the derivative I get 12t^2 -24t. Are you saying this can only be a positive function?
  5. Jul 13, 2011 #4


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    No! I'm saying for values of t where the derivative is positive motion is upwards, for values of t where the derivative is negative, it's downwards. The question is when (for what values of t) is it upwards and for which is it downwards.
  6. Jul 13, 2011 #5
    Gotcha! I'm stuck on which values of t is would be upward and downward. Other than plugging in random values of t into the velocity function, how would I go about doing this?
  7. Jul 14, 2011 #6


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    Factor it. [itex]12t^2- 24t= 12t(t- 2)[/itex]. That will be positive when the two variable factors, t and t-2, have the same sign, negative when they have different signs.

    Also, "a- b" is positive if and only if a> b, negative if and only if a< b so the signs of t- 2 and t= t- 0 depende upon whether t> 2 and t> 0. If t< 0, then t< 2 also so t and t- 2 are both negative. If 0< t< 2, t is positive and t- 2 is negative. If t> 2 then t> 0 also so t and t- 2 are both positive.
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