Shadow Rate of Change: Solving a Geometry Problem

[squall325]

Homework Statement

A street light is on top of a 20-foot pole. A person who is 5 feet tall walks away from the pole at the rate of 5 feet per second. At what rate is the tip of the person's shadow moving away from the pole when he is 20 feet from the pole?

2. The attempt at a solution

20/5 = y/y-x (similar triangles)
y = 4x/3 (to solve for x)

im stuck here...

i also checked another solution i got here but i don't know how he got dy/dt = (6/9)*5
https://www.physicsforums.com/showthread.php?t=114901

 

[lanedance]

Homework Statement

A street light is on top of a 20-foot pole. A person who is 5 feet tall walks away from the pole at the rate of 5 feet per second. At what rate is the tip of the person's shadow moving away from the pole when he is 20 feet from the pole?

2. The attempt at a solution

20/5 = y/y-x (similar triangles)
y = 4x/3 (to solve for x)

im stuck here...

i also checked another solution i got here but i don't know how he got dy/dt = (6/9)*5
https://www.physicsforums.com/showthread.php?t=114901

well that has different numbers for a start...

you have
x = 4y/3

what do you want to know? what is the rate of change of the length of shadow (y') in terms of the rate of change of the person distances from the pole (x')

so differntiate both sides of your expression w.r.t t