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# Triple Integral

Posted on 2006-10-28
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A spherical cloud of gas of radius 3km is more dense at the center that toward the edge, at a distance p km from the center the density is d(p) = 3 - p, write an integral representing the total mass of the cloud and evaluate it.

My problem is not evaluating the integral it is setting it up.

Clearly the best option is a triple integral in spherical coordinates.

/pi  / 2pi / 3
|    |      |        d(p)p^2 sin(x) dp dy dx
|    |      |
/0  /0    /0

Here is my problem, I've tried with d(p) = 3-p like the problem states and I get a very strange answer, the homework solution uses p - 3, and I dont see why that make this change. Please help.

-Brian
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Question by:BrianGEFF719
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LVL 18

Accepted Solution

JR2003 earned 100 total points
ID: 17828977
If you take the surface area of concentric spheres 4*pi*r^2
The mass of a thin spherical shell of radius r1 and thickness dr is:
(3-r1)4*pi*r1^2 dr

So I think you just need to integrate this from 0 to 3 wrt r.

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LVL 19

Author Comment

ID: 17829395
Hi JR2003,

Thanks for the response. I'm pretty sure that a sitution such as this setup in cartesian would be quite a bit messier.

-Brian
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LVL 19

Author Comment

ID: 17829401
disregard last statment.
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LVL 23

Assisted Solution

Mysidia earned 150 total points
ID: 17829736
There are a lot of ways to set up the integral.  I think you have the right idea.

And  p-3 is just  -d(p), by the way; if you set the limits up differently, that can turn up.

Whether they picked the same limits matters.

In what way is that a strange answer?
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LVL 19

Author Comment

ID: 17830826
>>In what way is that a strange answer?

It is completly wrong. Should I post my complete work?
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LVL 22

Assisted Solution

ID: 17830974
Doing a spherical integral is just way too complex.  Take advange of the simple geometry of the situation.
Surface of a sphere of radius r is 4*pi*r^2
density is 3-r

So, total mass of the cloud is the integral of (3-r)*(4*pi*r^2)dr from 0 to 3.

(3-r)*(4*pi*r^2)dr
(12*pi*r^2 - 4*pi*r^3)dr
This integrates to:
4*pi*r^3 - pi*r^4
Evaluate this from 0 to 3
4*pi*27 - pi*81 - (0 - 0)
27pi
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LVL 19

Author Comment

ID: 17831002
>>Doing a spherical integral is just way too complex.

I have to do it in either cylindrical or spherical coordinates, for this section i cannot use cartesian coordinates.
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LVL 23

Assisted Solution

Mysidia earned 150 total points
ID: 17831477
Not complicated at all.

G(x,y,p) =  d(p)p^2 sin(x)
== ( 3p^2 - p^3 ) sin(x)

Integrate[ G(x,y,p),  {p from 0 to 3} ]  =>
( p^3 - p^4/4 ) sin(x)  |{p from 0 to 3}   =>
H(x) =>  ( 27 -  81/4  ) sin(x)  => 27/4  sin(x)

Integrate [ H(x) ]{y from 0 to 2pi} =>  2pi * H(x)

Integrate [ 27/4  sin(x) * (2pi) ] {p from 0 to pi}   =>
[ 27/4  * -cos(x) * (2pi) ]_x=0^\pi  => ...

-27 pi / 2  - - ( 27pi / 2 ) =  2 * (27/pi) =  27pi

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LVL 19

Author Comment

ID: 17831575
@Mysidia:

I'm having a hard time following the solution you posted. I'm not familiar with the notation you're using.
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LVL 23

Assisted Solution

Mysidia earned 150 total points
ID: 17831707
What i'm saying is you only need a straightforward integration, a multiplication by 2pi, and another very straightforward integration.

It's not as if there are different orders to execute these steps in,  start evaluating the inner integral first,
then the middle one, then the outer integral.

a => b => c  is just a mixed up way of saying   a leads to b, and then a=c.

E(q)|_q=q0^q1   is just a shorthand for ' ( E(q1) - E(q0) )'.
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LVL 19

Author Comment

ID: 17831728
@Mysidia:

-27 pi / 2  - - ( 27pi / 2 ) =  2 * (27/pi) =  27pi

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LVL 23

Expert Comment

ID: 17833015
Ok, last lines should in fact read

[ 27/4  * -cos(x) * (2pi) ] {from 0 to pi} =
(27/2 * -cos(pi) * pi)  - (27/2 * -cos(0) * pi)  =  2 * (27pi/2)
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LVL 27

Assisted Solution

BigRat earned 125 total points
ID: 17833935
I don't understand the complexity of the solutions posted so far.

Taking JR2003's point about concentric shells, each of area 4*pi*r² (using r rather than p for the radius), each of thickness dr (delta r) and of density (3-r) gives a mass of 4*pi*r²*(3-r)*dr. This needs to be integrated from r=0 (the middle) to r=3 (the outer limit).

The integration is quite simple and gives 4*pi*r^3 - pi*r^4
With the lower limit of zero this is zero, the upper limit is 4*pi*3^3 - pi*3^4
Factor out pi and 3 cubed (27) gives 27*pi*(4-3) or simply 27pi.

There is no need for triple integrals nor trigonometrical functions.
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LVL 22

Assisted Solution

ID: 17833958
Maybe Brian has a homework constraint that he has to use cylindrical or spherical coordinates.
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LVL 27

Assisted Solution

BigRat earned 125 total points
ID: 17834506
Brain actually wrote :-

Clearly the best option is a triple integral in spherical coordinates.

Now I've no idea where he got this from - it might even be his own - but I'll agrue the daylights out of ANY maths teacher about things like this. The BEST option is the SIMPLEST option. Mathematics is not about showing how clever we are, but about using tools to solve practical problems. There are relatively few problems which can be solved in spherical coordinates and those in 3-d Cartesian are also messy.

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LVL 22

Expert Comment

ID: 17834553
agreed
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LVL 19

Author Comment

ID: 17835931
>>Maybe Brian has a homework constraint that he has to use cylindrical or spherical coordinates.

Well this problem was taken out of the section titled "Triple Integrals in Cylindrical and Spherical Coordinates", so I think it is implied that the problem _should_ be done in one of those two coordinate systems. I should have probably been clearer with this from the start, my apologies. My teacher provided his solution using a triple integral, however, using p - 3 vs 3 - p, that is really where my confusion is coming from.

-Brian
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LVL 19

Author Comment

ID: 17835992
However JR2003, NovaDenizen, and BigRat, you've all posted a method that leads to the correct solution.

I'm confused however by the fact that:

SA = 4pi*r^2
D = 3 - p

SA has units of (distance^2)
D has units of (mass/distance^3)

SA * D = MASS/DISTANCE .... not just mass as we are looking for. Now its clear that it works...i'm just confused how that part works.

-Brian
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LVL 22

Expert Comment

ID: 17836035
The integral is of SA * dr, and dr has units (distance).
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LVL 22

Expert Comment

ID: 17836062
I mean to say that the integral is of SA * D * dr, and the units there work out to MASS.  SA*dr is the infinitesimal volume of 'uniform' density that is being integrated.
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LVL 19

Author Comment

ID: 17836082
>>I mean to say that the integral is of SA * D * dr, and the units there work out to MASS.  SA*dr is the infinitesimal volume of 'uniform' density that is being integrated.

Makes sense, I forgot dr had units of distance.
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LVL 27

Expert Comment

ID: 17840834
>>the problem _should_ be done in one of those two coordinate systems

Hmmmm. The problem is completely symetric about the origin. It is the worst example for triple integration I can think of. Presumably one considers a volume element at (r,theta,phi) with density 3-r and integrate all these over the spheroid.

I'll think out a better example for triple integration and post it as a question.
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LVL 19

Author Comment

ID: 17843557
>>Hmmmm. The problem is completely symetric about the origin. It is the worst example for triple integration I can think of.

How so, in spherical coordinates your phi would go from 0->pi, thetha 0->2pi, and your radius from 0->3, to me it seems pretty basic.

brian
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LVL 27

Expert Comment

ID: 17843642
>>How so,

The density is proportional to the radius, ie: to r. The other two coordinates are not involved with the density only with the space, therefore the two integrations of theta and phi actually produce a shell of area 4*pi*r². As as question to show  triple integration using spherical coordinates it's weak to say the least.
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LVL 22

Assisted Solution

ID: 17843705
Just to prove a point, I'll partially solve it in spherical coordinates.  I'll do it with the p on the outermost integral.

x goes from 0 to pi, y goes from 0 to 2pi, p goes from 0 to 3
(3-p)p^2 (sin(x) dy dx) dp

While we integrate on x and y, the terms with p and dp are just multiplying constants so we can factor them out and ignore them for now.

sin(x) dy dx

Integrate on x
-cos(x) dy   from 0 to pi
(1 - -1) dy
2dy

Integrate this on y
2y from 0 to 2pi
4pi

Merging this back with the original integral, we get:
(3-p)4pi p^2 dp over p from 0 to 3
Which is exactly the simple integral I and others suggested.

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LVL 23

Expert Comment

ID: 17843709
It's true that you don't need triple integration, if you happen to have the formula for the proper area available, the
extra two integrations just boil down to multiplication by the area.

However, I don't see anything erroneous with author's choice to set it up as a triple integral, from what I saw,
he did it correctly -- Whether it was a good choice or not is subjective.

If there was a technical error, it was almost certainly in the evaluation.

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LVL 18

Assisted Solution

deighton earned 25 total points
ID: 17848865
I vote for the autor's triple integral as a good and proper approach.
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LVL 27

Expert Comment

ID: 17857150
>>I vote for the autor's triple integral as a good and proper approach.

I'll send you a sledge hammer for Christmas, so that you can crack your wallnuts.
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LVL 19

Author Comment

ID: 17895660
Ok, i've figured out this problem, will close soon.
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LVL 19

Author Comment

ID: 17896020
Thank you guys for your contributions to this problem.

Brian
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