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Homework Help: Related rates of man walking

  1. Feb 23, 2007 #1
    A man starts walking north at 6 ft/s from a point P. Five minutes later, a woman starts walking south at 2 ft/s from a point 500 ft due east of P. At what rate are the people moving apart 30 min after the woman starts walking?

    so i was ableto set up a right triangle using the x-y plane (y being north and x being east)

    so i was trying to relate the components with distance^2=x^2+y^2
    then where i differientate i get dd/dt=dx/dt+dy/dt

    i know dy/dt=8 and i want to know dd/dt

    there dx/dt=0 becuase there is no change in x.....x is always 500 ft

    so would dd/dt=dy/dt=8 ????????
     
  2. jcsd
  3. Feb 23, 2007 #2

    HallsofIvy

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    Then you differentiated wrong! If [itex]dd^2= x^2+ y^2[/itex] then, differentiating, 2dd d(dd)/dt= 2x (dx/dt)+ 2y (dy/dt). Since dx/dt= 0,
    2dd d(dd)/dt= 2y(dy/dt) so dd d(dd)/dt= 2y dy/dt. Now, 30 minutes after the woman starts walking (35 minutes after the man starts walking) what are dd and y?

    Very close but not exactly!
     
  4. Feb 23, 2007 #3
    Pyramid shaped tank...?

    Does anyone know how to do this problem on related rates?:

    The base of a pyramid-shaped tank is a square with sides of length 8 meters, and the vertex of the pyramid is 8 meters above the base. The tank is filled to a depth of 2 meters, and water is flowing into the tank at the rate of 4 cubic meters per minute. Find the rate of change of the depth of water in the tank. (Hint: the volume of a pyramid is given by V=1/3bh, where b is the base area and h is the height of the pyramid)
     
  5. Feb 23, 2007 #4

    HallsofIvy

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    Please, please, please! Do not "hijack" someone else's thread for a different question. Start your own thread- it's not that difficult!

    Okay, can you determine how h is related to b for different heights of the water?
     
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