Colt Steele
Frequent poster

Post Number: 174
Posted on Friday, 29 October, 2004 - 07:48 pm:

I do not understand what to do for this question:

Evaluate the following triple integral as a repeated integral using the appropriate coordinate system:

S x2 .dV where S is the unit ball {ρ,ϕ,θ):ρ<1}.

Any help would be great. :-)

Francis Woodhouse
Prolific poster

Post Number: 299
Posted on Friday, 29 October, 2004 - 07:56 pm:

Make a change of variables to spherical coordinates. What will x become, and what will the limits be, for the integral over the unit ball when using spherical coordinates? (Don't forget to multiply the integrand by the determinant of the Jacobian of the transformation.)
Colt Steele
Frequent poster

Post Number: 175
Posted on Friday, 29 October, 2004 - 09:04 pm:

i do not know how to change the variables to spherical coordinates. I do not really know what to do with most of the questions you have posed.

Thanx
Francis Woodhouse
Prolific poster

Post Number: 300
Posted on Friday, 29 October, 2004 - 09:22 pm:

In a multiple integral, if you change variables from (x,y,z) to (u,v,w), then

Latex image click or follow link to see src

where

Latex image click or follow link to see src

and you multiply the integrand by the absolute value of this determinant.

Also, remember that a change from Cartesian coordinates (x,y,z) to spherical coordinates (ρ,θ,ϕ) is given by

x=ρsinϕcosθ

y=ρsinϕsinθ

z=ρcosϕ.

Think about what limits you would need to cover the entire sphere of radius ρ after making this transformation. Be careful that each unit volume is only covered once.

Colt Steele
Frequent poster

Post Number: 176
Posted on Saturday, 30 October, 2004 - 12:04 am:

cheers man, your the best.
Colt Steele
Frequent poster

Post Number: 177
Posted on Saturday, 30 October, 2004 - 05:20 pm:

I got an answer of 1/20*pi. Could somebody verify this.

Thanx
Francis Woodhouse
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Post Number: 307
Posted on Saturday, 30 October, 2004 - 05:36 pm:

Bad news, Colt; I got 4pi/15.

The final integral I ended up with was
Latex image.

Make sure you've got the correct limits, and that you calculated the Jacobian correctly.
Colt Steele
Frequent poster

Post Number: 178
Posted on Saturday, 30 October, 2004 - 06:02 pm:

I have got a different limit for d(phi). but i do not know why it is 0-> pi and not 0-> 2*pi.
Francis Woodhouse
Prolific poster

Post Number: 308
Posted on Saturday, 30 October, 2004 - 06:11 pm:

We only want to traverse each unit volume of the sphere once. As q varies from 0 to 2pi, covering the whole radius, we only need vary phi from 0 to pi to enable all slices of the circle to be covered (think of a circle being moved up and down the sphere, with a line from the centre of the sphere to the circle).
Colt Steel
Frequent poster

Post Number: 180
Posted on Saturday, 30 October, 2004 - 06:52 pm:

thanx for all the help