Radii in Cylindrical Shells

  • Thread starter kazafire93
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So what is the radius now?In summary, the conversation is about finding the volume of a solid by rotating a region bounded by given curves. The person is having difficulty with finding the correct radius for cylindrical shells. The correct height is determined to be 2 - (y2+1) and the correct radius is y + 1 when rotating about y = -1. However, when rotating about y = -2, the correct radius would be y + 2. The person is seeking an explanation for how to find the right answer.
  • #1
kazafire93
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Homework Statement



Hi, I'm having a tremendous amount of difficulty with finding the radii in problems using cylindrical shells.

Here's the question: find the volume of the solid found by rotating the region bounded by the given curves:

x = y2+1, x = 2, about y = -2

I got 2 - (y2+1) for the height, which I know is correct. For the radius, though, I got 1 + y...which is incorrect. Can anyone explain how to get the right answer? Thanks!
 
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  • #2
kazafire93 said:

Homework Statement



Hi, I'm having a tremendous amount of difficulty with finding the radii in problems using cylindrical shells.

Here's the question: find the volume of the solid found by rotating the region bounded by the given curves:

x = y2+1, x = 2, about y = -2

I got 2 - (y2+1) for the height, which I know is correct. For the radius, though, I got 1 + y...which is incorrect. Can anyone explain how to get the right answer? Thanks!
Hello kazafire93. Welcome to PF !

If you were rotating about y = -1, the radius of the cylindrical shell would be y - (-1) = y + 1.

But you're rotating about y = -2.
 

1. What is the formula for finding the radius of a cylindrical shell?

The formula for finding the radius of a cylindrical shell is r = h/2π, where h represents the height of the cylindrical shell and π is the mathematical constant pi.

2. How is the radius of a cylindrical shell related to its volume?

The radius of a cylindrical shell is directly proportional to its volume. This means that as the radius increases, the volume of the cylindrical shell also increases.

3. Can the radius of a cylindrical shell be negative?

No, the radius of a cylindrical shell cannot be negative. The radius represents the distance from the center of the cylinder to its outer surface, and distance cannot be negative.

4. How do you calculate the surface area of a cylindrical shell?

The formula for calculating the surface area of a cylindrical shell is A = 2πrh + 2πr², where r is the radius and h is the height of the cylindrical shell.

5. Can the radius of a cylindrical shell be larger than its height?

Yes, the radius of a cylindrical shell can be larger than its height. This is often the case with objects such as cans or tubes, where the radius is significantly larger than the height.

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