Problem 68
Question
Assume that the radius \(r\) and the volume \(V=\frac{4}{3} \pi r^{3}\) of a sphere are differentiable functions of \(t .\) Express \(d V / d t\) in terms of \(d r / d t\)
Step-by-Step Solution
Verified Answer
\( \frac{dV}{dt} = 4 \pi r^2 \frac{dr}{dt} \)
1Step 1: Understand the Relationship Between Variables
The volume of a sphere is given by the formula \( V = \frac{4}{3} \pi r^3 \), where \( V \) is the volume and \( r \) is the radius. Both \( V \) and \( r \) are functions of time \( t \) which implies \( V = V(t) \) and \( r = r(t) \). Our goal is to find \( \frac{dV}{dt} \) in terms of \( \frac{dr}{dt} \).
2Step 2: Differentiate the Volume Formula
Differentiate both sides of the volume equation with respect to \( t \). Using the chain rule on the right side, we get:\[ \frac{dV}{dt} = \frac{d}{dt} \left( \frac{4}{3} \pi r^3 \right) = \frac{4}{3} \pi \left( 3r^2 \right) \frac{dr}{dt}. \]
3Step 3: Simplify the Expression
Simplify the expression obtained from differentiation:\[ \frac{dV}{dt} = 4 \pi r^2 \frac{dr}{dt}. \]This expresses the rate of change of the volume with respect to time \( \frac{dV}{dt} \) in terms of the rate of change of the radius with respect to time \( \frac{dr}{dt} \).
Key Concepts
DifferentiationChain RuleRelated Rates
Differentiation
In calculus, differentiation is the process of finding the derivative of a function. The derivative represents the rate at which one quantity changes with respect to another, usually time. In the context of our exercise, differentiation helps us understand how the volume of a sphere changes as its radius changes over time.
When differentiating a function, we are essentially computing the slope of the tangent line to its graph at any given point. This is an important concept that allows us to calculate instantaneous rates of change.
In our sphere example, the volume function, given by \[V = \frac{4}{3} \pi r^3,\]is differentiated with respect to time, \( t \), to find how the volume changes as the radius changes. Differentiating each part of the equation enables us to link the change in radius with the change in volume.
When differentiating a function, we are essentially computing the slope of the tangent line to its graph at any given point. This is an important concept that allows us to calculate instantaneous rates of change.
In our sphere example, the volume function, given by \[V = \frac{4}{3} \pi r^3,\]is differentiated with respect to time, \( t \), to find how the volume changes as the radius changes. Differentiating each part of the equation enables us to link the change in radius with the change in volume.
Chain Rule
The chain rule is an essential tool in calculus used to differentiate compositions of functions. It allows us to find the derivative of a function with respect to a variable when the function is composed of other functions.
In our exercise, to find \( \frac{dV}{dt} \), we implicitly use the chain rule on the volume function \( V = \frac{4}{3} \pi r^3 \). Here, \( r \) is a function of time, so we need to apply the chain rule to account for the fact that \( r \) depends on \( t \).
In our exercise, to find \( \frac{dV}{dt} \), we implicitly use the chain rule on the volume function \( V = \frac{4}{3} \pi r^3 \). Here, \( r \) is a function of time, so we need to apply the chain rule to account for the fact that \( r \) depends on \( t \).
- First, differentiate \( r^3 \) with respect to \( r \) to get \( 3r^2 \).
- Then, multiply this by \( \frac{dr}{dt} \), the derivative of \( r \) with respect to \( t \).
Related Rates
Related rates problems involve finding the rate at which one quantity changes by relating it to other quantities whose rates of change are known. These problems occur frequently in dynamic systems where various components depend on one another.
In the exercise, we explored how the volume of a sphere, which depends on the radius, changes as the radius changes over time. By understanding this relationship, we can solve for unknown rates given certain information.
The formula we derived, \[\frac{dV}{dt} = 4 \pi r^2 \frac{dr}{dt},\]illustrates this concept by expressing the rate of change of the sphere's volume \( \frac{dV}{dt} \) in terms of the known rate of change of the radius \( \frac{dr}{dt} \) and the current radius size, \( r \). This type of analysis is valuable in numerous applications, such as physics, engineering, and real-life problem-solving situations where variables are interconnected.
In the exercise, we explored how the volume of a sphere, which depends on the radius, changes as the radius changes over time. By understanding this relationship, we can solve for unknown rates given certain information.
The formula we derived, \[\frac{dV}{dt} = 4 \pi r^2 \frac{dr}{dt},\]illustrates this concept by expressing the rate of change of the sphere's volume \( \frac{dV}{dt} \) in terms of the known rate of change of the radius \( \frac{dr}{dt} \) and the current radius size, \( r \). This type of analysis is valuable in numerous applications, such as physics, engineering, and real-life problem-solving situations where variables are interconnected.
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