Shadow Rate of Change: Solving a Geometry Problem

Click For Summary
SUMMARY

The discussion centers on a geometry problem involving a 20-foot pole and a 5-foot tall person walking away from it at 5 feet per second. The key equation derived from similar triangles is y = 4x/3, where y represents the length of the shadow and x represents the distance from the pole. The participants are attempting to find the rate at which the tip of the person's shadow is moving away from the pole, specifically when the person is 20 feet from the pole. The differentiation of the equation leads to the expression dy/dt = (6/9)*5, which requires clarification on how to derive this rate of change.

PREREQUISITES
  • Understanding of similar triangles and their properties
  • Basic knowledge of calculus, specifically differentiation
  • Familiarity with related rates in geometry
  • Ability to set up and solve equations involving rates of change
NEXT STEPS
  • Study the concept of related rates in calculus
  • Learn how to differentiate equations involving multiple variables
  • Explore applications of similar triangles in real-world problems
  • Practice solving geometry problems involving shadows and light sources
USEFUL FOR

Students studying calculus, particularly those focusing on related rates and geometry, as well as educators looking for examples to illustrate these concepts.

squall325
Messages
2
Reaction score
0

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
 
Physics news on Phys.org
squall325 said:

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
 

Similar threads

  • · Replies 14 ·
Replies
14
Views
4K
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 1 ·
Replies
1
Views
12K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
5K