1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    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!

Rate of Change question

  1. Nov 22, 2009 #1
    A man 1.8 m tall walks at speed of 10m/s towards a street lamp which is 7m above the ground. How fast is the length of the man's shadow decreasing?


    My attempt:

    Let the man's shadow and the lamp's length be l, and the distance be d and time be t. Given, dd/dt = 10.

    I am supposed to find dl/dt.

    dd/dt= dl/dt X dd/dl

    So how do I find dd/dl?:confused:
     
  2. jcsd
  3. Nov 22, 2009 #2
    You can imagine the man walking on the x-axis towards the y-axis. Call his position x and the length of his shadow s.
    The key to these types of problems is using similar triangles. So we can get a relation between s and x.

    (1.8/s) = (7/x+s)

    Get s on one side and differentiate with respect to t. And remember dx/dt = -10 since x is decreasing.

    I hope i didn't make any mistake. Check the book to make sure if you have the answer.
     
  4. Nov 22, 2009 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Draw a picture and think "similar triangles".
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Rate of Change question
Loading...