Related Rates: Circles and Changing Circumference

In summary, the rate of change of the diameter with respect to time is 4/π inches per second, given that the circumference of a circle is increasing by 4 inches per second and the radius is 2 inches. The information about the radius is irrelevant as the circumference and diameter are constant multiples of each other.
  • #1
Qube
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Homework Statement



The circumference of a circle is increasing by 4 inches per second. What is the rate of change of the diameter with respect to time if the radius is 2 inches?

Homework Equations



C = 2∏r = ∏d.

The Attempt at a Solution



dC/dt = 4 = ∏dd/dt.

dd/dt = 4/∏

Is this it? It seems as if the information given about the radius is irrelevant.
 
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  • #2
Hi Qube! :smile:
Qube said:
The circumference of a circle is increasing by 4 inches per second. What is the rate of change of the diameter with respect to time if the radius is 2 inches?

It seems as if the information given about the radius is irrelevant.

yup! :biggrin:
 
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  • #3
Qube said:

Homework Statement



The circumference of a circle is increasing by 4 inches per second. What is the rate of change of the diameter with respect to time if the radius is 2 inches?

Homework Equations



C = 2∏r = ∏d.

The Attempt at a Solution



dC/dt = 4 = ∏dd/dt.

dd/dt = 4/∏

Is this it? It seems as if the information given about the radius is irrelevant.
Yes, this is it. The circumference is a constant multiple of the diameter, so the rates of change of circumference and diameter are also going to be constant multiples of one another.
 
  • #4
Mark44 said:
Yes, this is it. The circumference is a constant multiple of the diameter, so the rates of change of circumference and diameter are also going to be constant multiples of one another.

Alright, thanks guys. Thanks for the explanation; this provides an easy way to check!
 

What is the relationship between related rates and circles?

The concept of related rates involves finding the rate of change of one variable with respect to another. In the context of circles, this means determining how the radius, circumference, and area of a circle change as its dimensions change. This relationship is important in many real-world applications, such as calculating the speed of a rotating wheel or the growth rate of a circular object.

How do you find related rates for circles?

To find related rates for circles, you must first identify the variables that are changing and the rate at which they are changing. Then, you can use the formulas for circumference and area of a circle to determine the relationship between these variables and their rates of change. Finally, you can use the chain rule to express the rate of change of one variable in terms of the other.

What is the chain rule and how is it used in related rates?

The chain rule is a calculus rule that states how to find the derivative of a composite function. In the context of related rates and circles, it is used to express the rate of change of one variable (such as the radius) in terms of the rate of change of another variable (such as the circumference or area). This allows us to solve for the unknown rate of change by substituting in the known values.

What are some real-world applications of related rates and circles?

As mentioned earlier, related rates and circles are used in many real-world scenarios, such as calculating the speed of a rotating wheel, finding the growth rate of a circular plant or tumor, and determining the volume of a spherical balloon as it is being inflated. They are also important in physics and engineering, as they can be used to analyze the motion and forces involved in circular objects and systems.

What are some common mistakes when solving related rates problems involving circles?

One common mistake is not properly identifying the variables and their rates of change. It is important to clearly define what each variable represents and how it is changing. Another mistake is not using the correct formulas for circumference and area of a circle, which can lead to incorrect results. Finally, it is important to carefully apply the chain rule and properly set up the equation to solve for the unknown rate of change.

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