Finding Velocity of Top of Object's Shadow

[bdb1324]

Homework Statement

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

Homework Equations

it is a similar triangle problem

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

 

[learningphysics]

How do you get that v*t is h?

 

[bdb1324]

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

 

[learningphysics]

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

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.

 

[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

 

[learningphysics]

The source of light and the wall are fixed at a distance L apart from each other.

The object is moving from the source towards the wall at a speed of v. At t = 0, it is at the source.

So the object's distance from the source at a time t is v*t.

Have a look at this thread:

https://www.physicsforums.com/showthread.php?t=184707&highlight=light+source+shadow

 

[bdb1324]

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