# Fluids rate of flow homework help

1. Apr 17, 2012

### domyy

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

Two cylindrical pipes both carry an ideal fluid (zero viscosity) in laminar flow. Pipe A has radius Ra = 2 and flow velocity Va = 8cm/s. Pipe B has radius Rb = 4cm and flow velocity Vb = 4cm/s.

A) Calculate the cross section of pipe A and pipe B.
B) Calculate the rate of flow in liters/second (1l = 1000cm cubic)

2. Relevant equations

maybe v1a1 = v2a2 ?

3. The attempt at a solution

v1a1 = v2a2
= (8cm/s)(2cm) = (4cm/s)(4cm) = 1 ?

It`s wrong.

Last edited: Apr 17, 2012
2. Apr 17, 2012

### darkxponent

Re: Fluids

okay. Now tell me what is a1, a2?

3. Apr 17, 2012

### tms

Re: Fluids

Just what do $a_1$ and $a_2$ represent?

Also, are these pipes connected end to end, or are they in parallel?

4. Apr 17, 2012

### domyy

Re: Fluids

you mean...

Last edited: Apr 17, 2012
5. Apr 17, 2012

### tms

Re: Fluids

What has pressure got to do with it?

Go back to your first equation: what is $a_1$? This is not a trick question.

6. Apr 20, 2012

### domyy

Re: Fluids

I have to calculate cross section of pipe A and B.

My book says cross-sectional area is VA.

I am confused with these formulas because I have the following:

Flow rate = VA
Flow rate = V/T

So, flow rate = cross sectional area = VA?

Last edited: Apr 20, 2012
7. Apr 20, 2012

### domyy

Re: Fluids

R = 2 = area 1 = 4
R = 4 = area 2 = 16
?

Last edited: Apr 20, 2012
8. Apr 20, 2012

### domyy

Re: Fluids

I think I know it...

Last edited: Apr 20, 2012
9. Apr 20, 2012

### domyy

Re: Fluids

would that be correct?

10. Apr 20, 2012

### domyy

Re: Fluids

I just need this to finish my homework.

11. Apr 20, 2012

### domyy

Re: Fluids

I think I know it:

I think I am supposed to use pi. So, could it be: 3.14 x 4^2 = 50.24 for cross section pipe B?

Last edited: Apr 20, 2012
12. Apr 20, 2012

### domyy

Re: Fluids

I know the answer! I got the answer! I got the answer!! Just a question: how should I leave the flow rate considering the answer for B = 0.20096, should I say the answer is 0.20 or 0.200 l/s ?

13. Apr 21, 2012

### tms

Re: Fluids

Assuming $V$ is volume and $A$ is area, something is wrong, either the book or your reading of it. The units are odd, too: $\mathrm{L}^5$. If $V$ and $A$ mean something else, then I don't know.

What is $T$?

Think units. What units must flow rate have?

14. Apr 21, 2012

### tms

Re: Fluids

Since the problem, as you gave it, uses only one significant figure, then your answer should have no more than one significant figure.