# Triple integral of sphere

#### -EquinoX-

1. The problem statement, all variables and given/known data

The figure below shows part of a spherical ball of radius 5 cm. Write an iterated triple integral which represents the volume of this region.

http://img19.imageshack.us/img19/9237/sphereu.th.jpg [Broken]

http://img19.imageshack.us/img19/1699/inte.jpg [Broken]

2. Relevant equations

3. The attempt at a solution

I already got A, B, C and D correctly. However when I try E and F to be -3 and -sqrt(25-x^2-y^2) respectively and vice versa, the answer is wrong.. why is this.. what should be E and F then?

Last edited by a moderator:

#### gabbagabbahey

Homework Helper
Gold Member
Is -3 really the minimum value of z for that volume? #### -EquinoX-

I guess so... z = -3 is just a plane right... other than that I really don't know what it is

Last edited:

#### gabbagabbahey

Homework Helper
Gold Member
Sure, but the region your interested in lies below the plane z=-3, so z=-3 should be the maximum not the minimum......likewise, the region of interest lies above the spherical surface z=-sqrt(25-x^2-y^2), so that should be the minimum value of z.

Did you try E=-sqrt(25-x^2-y^2) and F=-3?

Last edited:

#### -EquinoX-

yes I did and as I said on my original posting "vice versa" and it doesn't work as well..

#### gabbagabbahey

Homework Helper
Gold Member
yes I did and as I said on my original posting "vice versa" and it doesn't work as well..
That's odd; assuming that E and F are the lower and upper limits on z, it should work.....how do you know that it doesn't work? Are you imputing these values into an online submission? Or are you just getting an incorrect final answer for the volume?

#### -EquinoX-

yes, into an online submission

#### -EquinoX-

and shouldn't it be -sqrt(25-x^2-y^2) instead of -sqrt(25-x^2-z^2)

#### gabbagabbahey

Homework Helper
Gold Member
Hmmm, what were your A,B,C and D?

Are you using the convention that $$\int_A^B\int_C^D\int_E^F dzdydx$$ means first integrate over z from E to F, then over y from C to D, and finally over x from A to B? Or are you using some other convention?

#### -EquinoX-

A = -4
B = 4
C = -sqrt(16-x^2)
D = sqrt(16-x^2)

#### gabbagabbahey

Homework Helper
Gold Member
and shouldn't it be -sqrt(25-x^2-x^2) instead of -sqrt(25-x^2-z^2)
Sorry, I meant to write -Sqrt(25-x^2-y^2)

#### gabbagabbahey

Homework Helper
Gold Member
A = -4
B = 4
C = -sqrt(16-x^2)
D = sqrt(16-x^2)
That looks fine; you are either making a mistake/typo when typing in your E and F values, or the online submission program has a bug

#### -EquinoX-

hmm...that's what I thought as well.. but are you sure that E=-sqrt(25-x^2-y^2) and F=-3?

#### gabbagabbahey

Homework Helper
Gold Member
hmm...that's what I thought as well.. but are you sure that E=-sqrt(25-x^2-y^2) and F=-3?
Yes, why wouldn't that be correct? (Unless the volume is supposed to also include the cone that subtends that section of the sphere)

#### -EquinoX-

Just making sure, as that's what I had in mind as well.. I'll have to ask my instructors about this then.. thanks for confirming

#### -EquinoX-

Hmm.. there's a hint to this question, not sure if it helps

Hint: If you flip this solid upside down, you can view it as an upper portion of a sphere that is centered at the origin.

#### gabbagabbahey

Homework Helper
Gold Member
Hmm.. there's a hint to this question, not sure if it helps

Hint: If you flip this solid upside down, you can view it as an upper portion of a sphere that is centered at the origin.
Hmmm.... maybe they are expecting you to do that (even though it is clearly unnecessary); if so, then z would go from +3 to +Sqrt(25-x^2-y^2), so you could try those as your E and F values

#### -EquinoX-

well..it makes no sense at all though... I only have one more chance to get this right

#### gabbagabbahey

Homework Helper
Gold Member
Why doesn't it make any sense? The volume of the spherical section is the same no matter where it is right?...If the problem suggests that you reflect it over the xy plane, then you might as well...

#### -EquinoX-

Why doesn't it make any sense? The volume of the spherical section is the same no matter where it is right?...If the problem suggests that you reflect it over the xy plane, then you might as well...
yeah, I just realized that the volume is the same.. thanks for pointing that out.. now the answer is correct

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving