# Shadows and distance, I need a formula to solve for Object 2

• March 20th 2012, 06:25 AM
crah28
Shadows and distance, I need a formula to solve for Object 2

A shadow from object 1 projects on a wall 1.75 X larger than its original size. Object 1 is 4.5 ft from the wall the shadow has been casted on. Object 2 has a known size which cast a shadow of 1.25 X its original size. How far is object 2 from the wall.
• March 20th 2012, 09:39 PM
princeps
Re: Shadows and distance, I need a formula to solve for Object 2
Quote:

Originally Posted by crah28

A shadow from object 1 projects on a wall 1.75 X larger than its original size. Object 1 is 4.5 ft from the wall the shadow has been casted on. Object 2 has a known size which cast a shadow of 1.25 X its original size. How far is object 2 from the wall.

Let :

$r_1=1.75~ , r_2=1.25~ ,x_1=4.5 ~ , x_2 = ?$

and denote distance of the source of the light from the wall as d . Therefore :

$d=r_1(d-x_1) \Rightarrow d=1.75(d-4.5) \Rightarrow d=10.5$

Hence :

$d=r_2(d-x_2) \Rightarrow x_2=\frac{dr_2-d}{r_2} \Rightarrow x_2=2.1$
• March 21st 2012, 07:20 AM
crah28
Re: Shadows and distance, I need a formula to solve for Object 2
(Bow)Thanks for your help, the formula was spot on.