The Relationships Between the Variables

Oct 13, 2025 by wbrum.org 49 views

Introduction

The problem of a man walking away from a street light pole and the movement of his shadow is a classic example of related rates in calculus. In this article, we will analyze the situation and determine the speed at which the tip of the man's shadow is moving when he is a certain distance away from the pole.

The Problem

A street light is mounted at the top of a 15-ft-tall pole. A man 6 feet tall walks away from the pole with a speed of $$4 , \text{ft/s}$$ along a straight path. We need to find the speed at which the tip of his shadow is moving when he is a certain distance away from the pole.

The Variables

Let's define the variables involved in the problem:

$x$$: the distance between the man and the pole$
$y$$: the length of the shadow$
$v$$: the speed of the man$
$s$$: the speed of the tip of the shadow$

The Relationships Between the Variables

We can establish the following relationships between the variables:

The distance between the man and the pole is increasing at a rate of $$v = 4 , \text{ft/s}$$.
The length of the shadow is related to the distance between the man and the pole by the similar triangles formed by the pole, the man, and the shadow.
The speed of the tip of the shadow is related to the speed of the man and the length of the shadow.

The Similar Triangles

The similar triangles formed by the pole, the man, and the shadow are:

\frac{15}{x} = \frac{6}{y}

We can rearrange this equation to solve for $$y$$:

y = \frac{6x}{15} = \frac{2x}{5}

The Speed of the Tip of the Shadow

We can use the chain rule to find the speed of the tip of the shadow:

s = \frac{dy}{dt} = \frac{dy}{dx} \cdot \frac{dx}{dt}

We know that $$\frac{dx}{dt} = v = 4 , \text{ft/s}$$ and $$y = \frac{2x}{5}$$, so we can find $$\frac{dy}{dx}$$:

\frac{dy}{dx} = \frac{d}{dx} \left( \frac{2x}{5} \right) = \frac{2}{5}

Now we can find $$s$$:

s = \frac{dy}{dt} = \frac{dy}{dx} \cdot \frac{dx}{dt} = \frac{2}{5} \cdot 4 = \frac{8}{5} \, \text{ft/s}

Conclusion

In this article, we analyzed the situation of a man walking away from a street light pole and the movement of his shadow. We established the relationships between the variables involved and used the chain rule to find the speed of the tip of the shadow. The result is that the tip of the man's shadow is moving at a speed of $$\frac{8}{5} , \text{ft/s}$$ when he is a certain distance away from the pole.

Discussion

This problem is a classic example of related rates in calculus. It requires the use of similar triangles and the chain rule to find the speed of the tip of the shadow. The result is a simple but elegant solution to a seemingly complex problem.

Applications

This problem has several applications in real-world situations. For example, it can be used to analyze the movement of shadows in architecture and engineering. It can also be used to model the movement of objects in physics and engineering.

Future Work

There are several directions for future research on this problem. One possible direction is to extend the analysis to more complex situations, such as a man walking away from a pole with a non-uniform speed. Another possible direction is to use this problem as a starting point for more advanced calculus topics, such as optimization and differential equations.

References

[1] Calculus, 3rd edition, by Michael Spivak
[2] Calculus, 2nd edition, by James Stewart
[3] Related Rates, by Paul Dawkins

Glossary

Related Rates: a technique in calculus used to analyze the rates of change of related quantities.
Similar Triangles: triangles that have the same shape but not necessarily the same size.
Chain Rule: a technique in calculus used to find the derivative of a composite function.
Optimization: the process of finding the maximum or minimum of a function.
Differential Equations: equations that involve an unknown function and its derivatives.

Introduction

In our previous article, we analyzed the situation of a man walking away from a street light pole and the movement of his shadow. We established the relationships between the variables involved and used the chain rule to find the speed of the tip of the shadow. In this article, we will answer some frequently asked questions about the problem and provide additional insights.

Q&A

Q: What is the relationship between the distance between the man and the pole and the length of the shadow?

A: The relationship between the distance between the man and the pole and the length of the shadow is given by the similar triangles formed by the pole, the man, and the shadow. Specifically, we have:

\frac{15}{x} = \frac{6}{y}

where $$x$$ is the distance between the man and the pole, $$y$$ is the length of the shadow, and $$15$$ and $$6$$ are the heights of the pole and the man, respectively.

Q: How do we find the speed of the tip of the shadow?

A: We can use the chain rule to find the speed of the tip of the shadow:

s = \frac{dy}{dt} = \frac{dy}{dx} \cdot \frac{dx}{dt}

We know that $$\frac{dx}{dt} = v = 4 , \text{ft/s}$$ and $$y = \frac{2x}{5}$$, so we can find $$\frac{dy}{dx}$$:

\frac{dy}{dx} = \frac{d}{dx} \left( \frac{2x}{5} \right) = \frac{2}{5}

Now we can find $$s$$:

s = \frac{dy}{dt} = \frac{dy}{dx} \cdot \frac{dx}{dt} = \frac{2}{5} \cdot 4 = \frac{8}{5} \, \text{ft/s}

Q: What is the significance of the speed of the tip of the shadow?

A: The speed of the tip of the shadow is an important quantity in this problem because it represents the rate at which the shadow is moving. This is a fundamental concept in physics and engineering, where the movement of objects and shadows is often critical to understanding the behavior of systems.

Q: Can we extend this analysis to more complex situations?

A: Yes, we can extend this analysis to more complex situations. For example, we can consider a man walking away from a pole with a non-uniform speed, or a man walking in a circular path around the pole. In these cases, we would need to use more advanced techniques, such as optimization and differential equations, to analyze the movement of the shadow.

Q: What are some real-world applications of this problem?

A: This problem has several real-world applications in architecture, engineering, and physics. For example, it can be used to analyze the movement of shadows in building design, or to model the movement of objects in physics and engineering.