Source of light

1. Oct 3, 2007

bdb1324

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. Oct 3, 2007

learningphysics

How do you get that v*t is h?

3. Oct 3, 2007

bdb1324

no since they are similar triangles and v/t is the starting point of h than the larger triangle must be v*t

4. Oct 3, 2007

learningphysics

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.

5. Oct 3, 2007

bdb1324

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

6. Oct 3, 2007

7. Oct 3, 2007

bdb1324

thanks I got it. It was the same problem. (L*h)/vt^2