A growing sand pile Sand falls from a conveyor belt at the rate of 10 m³/min onto the top of a conical pile. The height of the pile is always three-eighths of the base diameter. How fast are the (a) height and (b) radius changing when the pile is 4 m high? Answer in centimeters per minute.
Table of contents
- 0. Functions7h 52m
- Introduction to Functions16m
- Piecewise Functions10m
- Properties of Functions9m
- Common Functions1h 8m
- Transformations5m
- Combining Functions27m
- Exponent rules32m
- Exponential Functions28m
- Logarithmic Functions24m
- Properties of Logarithms34m
- Exponential & Logarithmic Equations35m
- Introduction to Trigonometric Functions38m
- Graphs of Trigonometric Functions44m
- Trigonometric Identities47m
- Inverse Trigonometric Functions48m
- 1. Limits and Continuity2h 2m
- 2. Intro to Derivatives1h 33m
- 3. Techniques of Differentiation3h 18m
- 4. Applications of Derivatives2h 38m
- 5. Graphical Applications of Derivatives6h 2m
- 6. Derivatives of Inverse, Exponential, & Logarithmic Functions2h 37m
- 7. Antiderivatives & Indefinite Integrals1h 26m
- 8. Definite Integrals4h 44m
- 9. Graphical Applications of Integrals2h 27m
- 10. Physics Applications of Integrals 3h 16m
- 11. Integrals of Inverse, Exponential, & Logarithmic Functions2h 34m
- 12. Techniques of Integration7h 39m
- 13. Intro to Differential Equations2h 55m
- 14. Sequences & Series5h 36m
- 15. Power Series2h 19m
- 16. Parametric Equations & Polar Coordinates7h 58m
4. Applications of Derivatives
Related Rates
Problem 3.8.36
Textbook Question
Moving along a parabola A particle moves along the parabola y = x² in the first quadrant in such a way that its x-coordinate (measured in meters) increases at a steady 10 m/sec. How fast is the angle of inclination θ of the line joining the particle to the origin changing when x = 3 m?

1
First, understand that the angle of inclination θ is the angle between the line joining the particle to the origin and the positive x-axis. This angle can be expressed using the tangent function: θ = arctan(y/x).
Since the particle is moving along the parabola y = x², substitute y = x² into the expression for θ: θ = arctan(x²/x) = arctan(x).
To find how fast θ is changing, differentiate θ = arctan(x) with respect to time t. Use the chain rule: dθ/dt = (dθ/dx) * (dx/dt).
Calculate dθ/dx for θ = arctan(x). The derivative of arctan(x) with respect to x is 1/(1 + x²). Therefore, dθ/dx = 1/(1 + x²).
Substitute dx/dt = 10 m/sec (given) and x = 3 m into the expression for dθ/dt: dθ/dt = (1/(1 + 3²)) * 10. Simplify this expression to find the rate at which the angle θ is changing.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Derivative
The derivative represents the rate of change of a function with respect to a variable. In this problem, it is used to find how fast the angle of inclination θ changes as the x-coordinate of the particle increases. Calculating derivatives helps determine instantaneous rates of change, which is crucial for understanding motion along curves.
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Derivatives
Chain Rule
The chain rule is a fundamental technique in calculus used to differentiate composite functions. It is essential here because the angle θ is a function of x, which itself changes over time. By applying the chain rule, we can relate the rate of change of θ with respect to time to the rate of change of x with respect to time.
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Intro to the Chain Rule
Trigonometric Functions
Trigonometric functions, such as tangent, are used to relate angles to side lengths in right triangles. In this problem, the tangent function helps express the angle θ in terms of x and y coordinates. Understanding how these functions work is crucial for translating geometric relationships into algebraic expressions that can be differentiated.
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Introduction to Trigonometric Functions
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