# Liquids involving continuity equation

Any help would be appreciated -

The water flowing through a 1.9 cm (inside diameter) pipe flows out through three 1.3 cm pipes. (a) If the flow rates in the three smaller pipes are 28, 15, and 10 L/min, what is the flow rate in the 1.9 cm pipe?

The basic continuity idea is A1v1 = A2v2.
The flow rate equation is R = Av.

For each of the 3 pipes, the flow rate is given (R) and the area can be calculated (pi*r^2).

To find the flow rate of the 1.9cm pipe, we need to know the speed of water flow (v, since R =Av).

But I don't know how to put them all together and relate them. I tried things but I always got the wrong answer. Please give me a hint or help. Thanks.

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Doc Al
Mentor
What does "flow rate" mean? Hint: All the water in the main pipe must end up in the smaller pipes.

Pyrrhus
Homework Helper
The conservation of mass is the concept from which the continuity equation is derived. Therefore, if you have a Q from one main pipe and it divides into 3 other pipes, the Q on the main pipe must be the sum of the Qs on the 3 smaller pipers, so mass is conserved.

Okay,

I understand what both of you guys said, and you're thinking, "well...if she understands it then why can't she do the problem???" It is because I'm a dumbass.

Anyhow, I understand the concept (at least, I think). I'm not asking for a completely solved problem, but can you help me a bit more with how to set up the problem? I promise I will try my hardest to solve it...I just need a starting point.

I set the initial pipe as A1v1, and the 3 respective pipes as A2v2, A3v3 and A4v4. I did everything I know but still got it wrong.

Thank you.

Never mind! I got it: you just add all the flow rates of three pipes together. This makes sense, although I hate it when they put problems like this...it seems too easy.