Volume of two pieces of a sphere cut by a plane

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Homework Help Overview

The discussion revolves around finding the volume of two segments of a unit sphere defined by the equation x² + y² + z² = 1, which are created when the sphere is intersected by a plane at z = a. Participants are exploring the implications of the intersection and the necessary calculations to determine the volumes of the segments formed.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the interpretation of the variable a as a point on the z-axis and the implications for calculating the height of the segments. There are questions about the integration process needed to find the volume, with some suggesting the need to clarify the limits of integration. Others raise concerns about the distinction between volume and area in the context of the problem.

Discussion Status

The discussion is active, with participants providing guidance on how to approach the volume calculation and clarifying assumptions about the variable a. Some participants have noted the need to ensure that a is within the bounds of the unit sphere, and there is an ongoing exploration of the necessary formulas and coordinate systems for the calculations.

Contextual Notes

There is an assumption that a must be between -1 and 1, with specific focus on the case where 0 ≤ a ≤ 1. Participants are also considering the implications of calculating the volume above and below the plane, as well as the transition to surface area calculations using spherical coordinates.

ParoxysmX
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Homework Statement



Consider the unit sphere x^{2} + y^{2} + z^{2} = 1

Find the volume of the two pieces of the sphere when the sphere is cut by a plane at z=a.

The Attempt at a Solution



My interpretation is that a is a point on the z-axis that the plane cuts at. So the height of the segment is r-(r-a). After that, I'm not sure how to proceed. Should you somehow integrate the volume of the segment between r=1 and r=a?
 
Last edited:
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Volume or area? You say volume in the title and area in the question.
 
LastOneStanding said:
Volume or area? You say volume in the title and area in the question.

Volume. Edited accordingly.
 
ParoxysmX said:
My interpretation is that a is a point on the z-axis that the plane cuts at. So the height of the segment is r-(r-a). After that, I'm not sure how to proceed. Should you somehow integrate the volume of the segment between r=1 and r=a?

Well, you will have ##a<1## (since it's a unit sphere) so you will want to go the other way around; but, yes, what you are proposing should work. If you give a try and get stuck, show your work in lots of detail and we'll be able to help more.
 
The height of the segment is r-a. Your expression of the height, r - (r - a), equates to just a.
 
Obviously a must be between -1 and 1. It is sufficient to assume that 0\le a\le 1 and calculate the volume above z= a and below the sphere. (Of course, the volume of the entire sphere is (4/3)\pi so the volume low z= a is (4/3)\pi minus the volume above. And if a< 0, just flip it over.)

In cylindrical coordinates the base is given by r^2+ a^2= 1 so the cover that base r goes from 0 to 1- a^2 and \theta from 0 to 2\pi. For each r and \theta, the height is z- a= \sqrt{1- r^2}- a
 
Ok I believe I have that one figured out. The next problem now is to calculate the surface area of those segments using spherical coordinates. I'm told the formula S = \int^{b}_{\phi=a}\int^{d}_{\theta=c} sin\theta d\theta d\phi should be used. What are a, b, c and d here?
 

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