# Finding the volume - Polar coordinates

1. Jan 2, 2017

### Dilemma

Hello everyone,

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

Question : Find the volume of the region which remains inside the cyclinder x 2 + y 2 = 2y, and is bounded from above by the paraboloid surface x 2 + y 2 + z = 1 and from below by the plane z = 0

2. Relevant equations

3. The attempt at a solution

This looks like a pretty straightforward question. First thing to do is converting equations to polar coordinates. When they are converted, the cylinder's region can be written as r = 2sinθ. Parabolaid surface's eq. is (1-r2).Through these, a double integral can easily be set up as the following:

∫(from 0 to π)∫from 0 to 2sinθ) (1-r2)rdrdθ.

However, the solution to this problem proposes a different approach which does not return the same answer.

Here it is:

Last edited: Jan 2, 2017
2. Jan 2, 2017

### Staff: Mentor

No.
If you convert the cylinder's equation to polar coordinates, you get $r^2 = 2r\sin(\theta)$, which is different from what you show.
What you show here is not an equation. What equation do you get?

3. Jan 2, 2017

### Dilemma

Hello Mark,

I am sorry for the inconvenience I caused.

I simplified $r^2 = 2r\sin(\theta)$.

Then, I converted parabolaid's surface equation.

$z = 1 - x^{2}-y^{2}$
$z = 1 - r^{2}$​

4. Jan 2, 2017

### Staff: Mentor

No problem. That's what we're here for.
You should get a different result from these equations.

5. Jan 2, 2017

### Dilemma

Here is my reasoning :

By polar coordinates definition $x = r\cos(\theta)$ and $y = r\sin(\theta)$. Therefore, $x^{2}+y^{2} = r^{2}$ I do not see any problem. Also, the answer confirms my logic. The only problem is that the answer divides the integral into two, one of them is from 0 to $\pi/6$, and the other is from $\pi/6$ to $\pi/2$. However, second part of the integral's "r" range is 0 to 1, and I am having difficult times understanding this. My coursebook has a similar question solved in a similar manner as mine.

6. Jan 2, 2017

### Eclair_de_XII

You know, I feel like you're missing an extra integral sign. Shouldn't you use cylindrical coordinates, or something?

7. Jan 2, 2017

### LCKurtz

Look at the first quadrant in this picture, which shows the circle traces in the $xy$ plane:

The brown region is the polar area over which you need to integrate. The $30^\circ$ line divides the area into two parts. In the lower sliver under the line $r$ goes from $0$ to the pink circle. Above the line $r$ goes from $0$ to the brown circle.

8. Jan 2, 2017

### Dilemma

Thank you. I now understand that I did not consider the region which is limited by the surface x 2 + y 2 + z = 1.