1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Related rates waliking away from light towards building

  1. Oct 13, 2009 #1
    1. The problem statement, all variables and given/known data

    a spotlight won the ground shines on a wall 12 m away if am man 2 m tall walks from the spotlight towards the building at a speed of 1.6 m/s how fast is the length of his shadow on the building decreasing when he si 4 m from the building?

    2. Relevant equations
    using relative triangles

    3. The attempt at a solution
    trying to learn related rates as well, this is what i got tell me if i am wrong here...

    draw triangle ABC with A being the light, B being the base of building, and C being top of shaddow/building.

    the second triangle is formend with the man and the light, using ADE, D being the mans feet and E being his head at 2 m height.

    using the 2 meter horizontal from the mans height we have two relative triangles.

    call the range from the light (line AD) x and call the building/shaddow (line BC) y

    using the two triangles we can infer that 2/x=y/12 or xy=24

    if we then differentiate relative to time 0=dx/dt * y + dy/dt * x

    we then plug in 1.6 for dx/dt and 8 for x(range from the light not the building); and y=3 (using xy=24 @ x=8) and solve for dy/dt= -.6m/s which should be negative.

    Is this correct?
  2. jcsd
  3. Oct 13, 2009 #2


    User Avatar
    Homework Helper

    wasn't this in the other posting?
  4. Oct 13, 2009 #3
    yes i had already posted this, i have a new posting out titled related rates kite
  5. Oct 13, 2009 #4


    User Avatar
    Science Advisor

    Yes, that is correct.
  6. Oct 13, 2009 #5


    User Avatar
    Science Advisor

    Please don't post the same thing more than once!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook