# Homework Help: Speed of water flowing through pipe

1. Nov 7, 2012

### thatshowifeel

1. The problem statement, all variables and given/known data
The speed of water flowing through the "influent" 4-inch diameter section of the piping system below is 3.0 ft/s. What is the volume flow rate of water in the piping system? Express the volume flow rate in $ft^3/s$

2. Relevant equations

3. The attempt at a solution
I really don't know what to do. I know pi*r^2 comes into play here, but I need help. Just tell me how to start and guide me through it please.

2. Nov 7, 2012

Flow rate = area of the pipe multiplied by the velocity.

You have everything but the flow rate. The area of the pipe is pi*r^2 as you said. Since you're not given a second density we can assume the flow is incompressible, and due to the conservation of mass, the flow rate in the 3 in dimater pipe will be the same. Q1 = Q2. I can't give you anymore without solving it for you.

3. Nov 7, 2012

### SteamKing

Staff Emeritus
Given that the fluid is water, it's safe to assume that the fluid is incompressible.

4. Nov 9, 2012

### thatshowifeel

Can the speed of the water be different even though the flow rate is the same? Part two says to find the speed of the water in the 3-inch diameter section in ft/s

5. Nov 9, 2012

### SteamKing

Staff Emeritus
Yes.

6. Nov 9, 2012

### bigfooted

Yes, this is connected to the equation given by vadar2012: flow rate = area x velocity.
The flow rates are the same everywhere in the pipe (it must be, the water doesn't disappear). Therefore, if the area of the pipe increases, the velocity decreases.

So:
step 1: calculate flow rate = area x velocity or Q1=A1 x V1
step 2: use Q1 = Q2 to calculate V2