# Writing equations in cylindrical coordinates (need work checked again please)

1. Sep 9, 2008

### clickcaptain

Could someone tell me what I'm doing wrong? thanks!!

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

Write the equation is cylindrical coordinates

7x2 + 7y2 = 2y

r = ? (has to be in the r = ? format)

2. Relevant equations

r2 = x2 +y2
x = rcos(θ)
y = rsin(θ)

3. The attempt at a solution

7x2 + 7y2 = 2y

7(x2 + y2) = 2y

7(r2 = 2rsin(θ)

r2 = (2rsin(θ))/7

r = sqrt((2rsin(θ))/7)

2. Sep 9, 2008

### HallsofIvy

I think it would be better to leave it as $r^2= (2/7) sin(\theta)$ rather than taking the square root, but, yes, that is correct.

3. Sep 9, 2008

### clickcaptain

its an online submission that has r = "enter here" , but i keep getting that its a wrong answer, is there any other way that this could be written?

4. Sep 9, 2008

### Defennder

Note that you forgot to cancel out r on both sides of the equation. Don't express r in terms of r in the final answer.

5. Sep 10, 2008

### crisy

Anybody have any idea about tramsforming the momentum equation into 2-D cylindrical co-ordinates...i've already derived the momentum equation from first principle but have difficulty with the transformation to cylindrical co-ordinates

6. Sep 11, 2008

### Defennder

Hi, if you have a separate question you should post it in a new thread so that others can aid you. Sometimes people don't bother reading through a thread which already has several replies.