Math Problem Statement

The volume of a sphere is decreasing at a constant rate of 6031 cubic centimeters per second. At the instant when the volume of the sphere is 32923292 cubic centimeters, what is the rate of change of the radius? The volume of a sphere can be found with the equation V, equals, start fraction, 4, divided by, 3, end fraction, pi, r, cubed, .V= 3 4 ​ πr 3 . Round your answer to three decimal places.

Solution

To solve this problem, we need to relate the rate of change of the volume of the sphere to the rate of change of the radius. Let's break it down step by step.

Step 1: Given Information

We know the following:

  • The rate of change of the volume dVdt\frac{dV}{dt} is 6031-6031 cm³/s (the negative sign indicates the volume is decreasing).
  • The volume of the sphere at the given instant is V=32,923,292V = 32,923,292 cm³.
  • The volume of a sphere is given by the formula: V=43πr3V = \frac{4}{3} \pi r^3
  • We are asked to find the rate of change of the radius drdt\frac{dr}{dt} at the instant when the volume is 32,923,29232,923,292 cm³.

Step 2: Differentiate the Volume Formula with Respect to Time

To relate dVdt\frac{dV}{dt} and drdt\frac{dr}{dt}, we can implicitly differentiate the volume formula with respect to time.

V=43πr3V = \frac{4}{3} \pi r^3

Differentiating both sides with respect to tt:

dVdt=43π3r2drdt\frac{dV}{dt} = \frac{4}{3} \pi \cdot 3r^2 \frac{dr}{dt}

Simplifying:

dVdt=4πr2drdt\frac{dV}{dt} = 4 \pi r^2 \frac{dr}{dt}

Step 3: Solve for drdt\frac{dr}{dt}

Now, plug in the known values. We know dVdt=6031\frac{dV}{dt} = -6031 cm³/s, and V=32,923,292V = 32,923,292 cm³. To find rr, we first solve for rr using the volume formula.

V=43πr3V = \frac{4}{3} \pi r^3

Rearrange to solve for r3r^3:

r3=3V4πr^3 = \frac{3V}{4 \pi}

Substitute V=32,923,292V = 32,923,292 cm³:

r3=3×32,923,2924πr^3 = \frac{3 \times 32,923,292}{4 \pi}

Now, calculate rr.

r3=98,769,87612.56647,868,161.92r^3 = \frac{98,769,876}{12.5664} \approx 7,868,161.92

Taking the cube root:

r7,868,161.923198.4 cmr \approx \sqrt[3]{7,868,161.92} \approx 198.4 \text{ cm}

Step 4: Substitute Known Values into the Rate of Change Formula

Now that we know r198.4r \approx 198.4 cm, substitute this value into the differentiated equation to find drdt\frac{dr}{dt}.

dVdt=4πr2drdt\frac{dV}{dt} = 4 \pi r^2 \frac{dr}{dt}

Substitute dVdt=6031\frac{dV}{dt} = -6031, r=198.4r = 198.4, and solve for drdt\frac{dr}{dt}:

6031=4π(198.4)2drdt-6031 = 4 \pi (198.4)^2 \frac{dr}{dt}

6031=4π×39,393.16×drdt-6031 = 4 \pi \times 39,393.16 \times \frac{dr}{dt}

6031=493,742.21drdt-6031 = 493,742.21 \frac{dr}{dt}

Now, solve for drdt\frac{dr}{dt}:

drdt=6031493,742.210.0122 cm/s\frac{dr}{dt} = \frac{-6031}{493,742.21} \approx -0.0122 \text{ cm/s}

Final Answer:

The rate of change of the radius is approximately 0.012-0.012 cm/s (rounded to three decimal places).


Would you like a more detailed explanation or further clarification? Here are a few related questions:

  1. How would the rate of change of the radius change if the volume of the sphere was increasing instead of decreasing?
  2. What would happen to the rate of change of the radius if the volume decreased at a faster rate?
  3. How would the equation change if we were dealing with a cylinder instead of a sphere?
  4. How can we calculate the surface area of the sphere given this radius?
  5. What are some real-world scenarios where this type of calculation could be applied?

Tip: When differentiating a formula with respect to time, make sure to apply the chain rule correctly, especially when the variable is inside another function like r3r^3.

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Math Problem Analysis

Mathematical Concepts

Related Rates
Differentiation
Geometry

Formulas

Volume of a sphere: V = (4/3)πr³
dV/dt = 4πr²(dr/dt)

Theorems

Chain Rule
Implicit Differentiation

Suitable Grade Level

Grades 11-12