# Finding the volume of an ellipsoid by using the volume of a sphere

• Fractal20
It can output in a variety of formats including BBCode for Physics Forums.In summary, the conversation discusses finding the volume of an ellipsoid using the volume of a unit ball in R^3. The conversation mentions using a change of variables and the Jacobian to solve the problem. It is eventually determined that the Jacobian is equal to abc, leading to the solution of the problem.

## Homework Statement

So I keep coming across problems that suggest finding the Volume of an ellipsoid using the volume of a ball ie:

Find the volume enclosed by the ellipsoid:

(x/a)^2 + (y/b)^2 + (z/c)^2 = 1

by using the fact that the volume of the unit ball in R^3 is 4pi/3

## The Attempt at a Solution

I keep getting stuck on this one. It seems like it will be some change of variables in which abc will eventually just pop out front of the usual expression for the volume of a ball. I keep thinking that I should be able to use spherical coords and a substitution like u = x/a, v = y/b and w = z/c. But then you just get the normal unit ball integral. I feel like it might come out of the limits of the integral in terms of P. That is, the maximum length of p is a function of $\phi$ and $\theta$ in the ellipsoid case. But with the above substitution then P is 1. Or I keep thinking it might come out of dP, but that hasn't been clear either.

I also keep thinking of plugging in for p -> p = $\sqrt{u2 + v2 + w2}$ = $\sqrt{(psin \phi cos \theta)2/a + (psin \phi sin \theta)2/b + (pcos \phi cos \theta)2/c}$ but then the p2 term would cancel out.

Anyway, I am just going in circles. Does anybody have any suggestions?

Fractal20 said:

## Homework Statement

So I keep coming across problems that suggest finding the Volume of an ellipsoid using the volume of a ball ie:

Find the volume enclosed by the ellipsoid:

(x/a)^2 + (y/b)^2 + (z/c)^2 = 1

by using the fact that the volume of the unit ball in R^3 is 4pi/3

## The Attempt at a Solution

I keep getting stuck on this one. It seems like it will be some change of variables in which abc will eventually just pop out front of the usual expression for the volume of a ball. I keep thinking that I should be able to use spherical coords and a substitution like u = x/a, v = y/b and w = z/c. But then you just get the normal unit ball integral. I feel like it might come out of the limits of the integral in terms of P. That is, the maximum length of p is a function of $\phi$ and $\theta$ in the ellipsoid case. But with the above substitution then P is 1. Or I keep thinking it might come out of dP, but that hasn't been clear either.

I also keep thinking of plugging in for p -> p = $\sqrt{u2 + v2 + w2}$ = $\sqrt{(psin \phi cos \theta)2/a + (psin \phi sin \theta)2/b + (pcos \phi cos \theta)2/c}$ but then the p2 term would cancel out.

Anyway, I am just going in circles. Does anybody have any suggestions?

The volume of the ellipsoid is$$\iiint_V 1dxdydz$$in the ##xyz## plane. Don't think in terms of spherical coordinates, think more about what the ellipsoid becomes in ##uvw## space under the transformation you are thinking about which I have highlighted. And the Jacobian has something to do with it.

LCKurtz said:
The volume of the ellipsoid is$$\iiint_V 1dxdydz$$in the ##xyz## plane. Don't think in terms of spherical coordinates, think more about what the ellipsoid becomes in ##uvw## space under the transformation you are thinking about which I have highlighted. And the Jacobian has something to do with it.

Is it simply that x/a = u -> dx = adu and etc?

I think I can see it from the Jacobian in Spherical but not Cartesian. Like if u,v,w are defined like normal in spherical coordinates, then making the jacobian makes a factor of a across the first row, b on the second and c on the third which pull out in the determinant.

But the Jacobian in cartesian still seems weird. So:

∫∫∫R(xyz)dV = ∫∫∫R(uvw) J dV

But in cartesian x=ua, y=vb and z=wc only defines the "functions" in terms of one other variable (x = f(u), y=g(v) etc...) then fv(u) is 0 etc. I am pretty new to Jacobians so I am problem just have a misunderstanding with them.

Ohhh! I see the Jacobian will just be a diagonal matrix, so it won't be 0. Thanks, sorry I can be a little slow.

Fractal20 said:
Ohhh! I see the Jacobian will just be a diagonal matrix, so it won't be 0. Thanks, sorry I can be a little slow.

So do you see how to finish it?

I think so. Just that J then equals abc. Right?

Fractal20 said:
I think so. Just that J then equals abc. Right?

Yes. Assuming you mean ##\left |\frac {\partial(x,y,z)}{\partial(u,v,w)}\right|##. So you get ##abc## times the volume of the unit sphere, which you already know.

I'm not too familiar with the notation. So just to be sure. If I saw we have x=f(u), y=g(v), z = h(w) then is what you wrote equivalent to determinate of

fu fv fw
gu gv gw
hu hv hw

? Sorry, I still can't figure out how to write matrices in this forum.

Fractal20 said:
I'm not too familiar with the notation. So just to be sure. If I saw we have x=f(u), y=g(v), z = h(w) then is what you wrote equivalent to determinate of

fu fv fw
gu gv gw
hu hv hw

? Sorry, I still can't figure out how to write matrices in this forum.

Yes. Right click on this to show the tex:$$J = \left |\begin{array}{ccc} a & 0 & 0\\ 0 & b & 0\\ 0 & 0 & c \end{array}\right| = abc$$

Alternatively, you can use a Tex editor. I've found Daum equation editor (a free Chrome app) to be excellent for complicated Tex formatting.

## What is an ellipsoid?

An ellipsoid is a three-dimensional shape that resembles an elongated sphere, with three unequal axes. It is often referred to as an "egg-shaped" object.

## How is the volume of an ellipsoid calculated using the volume of a sphere?

The volume of an ellipsoid is calculated by multiplying the volume of a sphere with the ratio of the ellipsoid's three semi-axes. This ratio is known as the eccentricity.

## What is the formula for finding the volume of a sphere?

The formula for finding the volume of a sphere is V = (4/3)*π*r^3, where "V" is the volume and "r" is the radius of the sphere.

## Can the volume of an ellipsoid be calculated using different units of measurement?

Yes, the volume of an ellipsoid can be calculated using different units of measurement, as long as the units are consistent throughout the calculation.

## Are there any real-life applications of calculating the volume of an ellipsoid using the volume of a sphere?

Yes, finding the volume of an ellipsoid using the volume of a sphere is a useful concept in various fields such as engineering, astronomy, and geology. It can be used to model and calculate the volume of natural objects like planets, asteroids, and even cells in biology.