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Homework Help: Source of light

  1. Oct 3, 2007 #1
    1. The problem statement, all variables and given/known data

    A small source of light S is located at a distance L from a vertical wall. An opaque object with a height of h moves toward the wall with constant velocity v_vec of magnitude v. At time t= 0 , the object is located at the source S.

    Find an expression for v_s, the magnitude of the velocity v_s_vec of the top of the object's shadow, at time t.

    Express the speed of the top of the object's shadow in terms of t, v, L, and h

    2. Relevant equations
    it is a similar triangle problem

    3. The attempt at a solution

    I figured out that v*t gives me H

    So i thought that since it was a similar triangle
    I could use (v*t*L)/h to find the v_s
  2. jcsd
  3. Oct 3, 2007 #2


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    How do you get that v*t is h?
  4. Oct 3, 2007 #3
    no since they are similar triangles and v/t is the starting point of h than the larger triangle must be v*t
  5. Oct 3, 2007 #4


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    how are you getting v/t ?

    The small triangle has sides v*t, h and the hypoteneuse.

    The big triangle has sides L, s(height of the shadow) and the hypoteneuse.
  6. Oct 3, 2007 #5
    isn't v/t the initial starting point of the wall and to get the furthest distance the wall is away wouldn't that be v*t
  7. Oct 3, 2007 #6


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  8. Oct 3, 2007 #7
    thanks I got it. It was the same problem. (L*h)/vt^2
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