1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Related Rates

  1. Oct 29, 2005 #1
    Q. A street light is mounted at the top of a 15-ft tall pole. A man 6 ft walks awsay from the pole with a speed of 5ft/s along a straight path.How fast is the tip of his shadow moving when he is 40 ft from the pole?

    What I've done so far:
    this scenario can be drawn as similar triangles. from similar triangles i got eh equation 15/6 = (x+y)/y, which is also equal to 6x-9y = 0.

    i found the derivative of that, which is 6(dx/dt) - 9 (dy/dt) = 0. then, i substituted (dx/dt), which gives dy/dt = (10/3) ft/s. however, the textbook states that the tip of his shadow is moving at (25/3) ft/s.

    have i done anything wrong?
  2. jcsd
  3. Oct 29, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    Did you happen to notice that 25/3 - 10/3 = 5? That should be a clue! (I.e. think about what x and y are in your equations!)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook