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Homework Help: Rate of change problem very hard please help

  1. Oct 2, 2008 #1
    Rate of change problem...very hard...please help!!!

    1. The problem statement, all variables and given/known data
    The hands on a clock are of length 5 inches(minute hand) and 4 inches(hour hand). How fast is the distance between the tips of the hands changing at 3:00?

    3. The attempt at a solution

    Let the coordinates of the two hands be (x1,x2) for the minute hand, and (y1,y2) for the hour hand. Then,

    s(distance) = sqrt((x2-y2)^2 + (x1-y1)^2)


    25=(x1)^2 + (x2)^2
    16=(y1)^2 + (y2)^2

    Then ds/dt = (-2(x1 - x2y1/y2)dy1/dt - 2(y1 - y2x1/x2)dx1/dt)/sqrt((x2-y2)^2+(x1-y1)^2).
    But I don't know how to calculate these rates.

    Please Help!!!
  2. jcsd
  3. Oct 3, 2008 #2
    Re: Rate of change problem...very hard...please help!!!

    The problem involves things in a circular motion. I would convert x and y into parametric values involving sine and cosine with an angular velocity. That should tell you all those rates you don't know.
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