# Homework Help: Rate of Flow Q

1. Nov 11, 2004

Hi! This is the first question on my homework, so it's suppost to be the easiest, but I'm not sure how to tackle it. It's a Bernoulli's principle problem. Here it is:

A horizontal pipe 11.6 cm in diameter has a smooth reduction to a pipe 4.72 cm in diameter. If the pressure of the water in the larger pipe is 8.1E4 Pa and the 6.82E4 Pa pressure in the smaller pipe is at what rate does water flow through the pipes?

So I thought I'd use the equation:
P1 + .5pv1^2+pgy1 = P2 + .5pv2^2+pgy2

I'm not sure how to apply this to my problem because the equation has two unknowns: v1 and v2. Is there another bernoulli's equation i should be using?? There are several variations in my text, but none seem to work. Thanks, Brad

2. Nov 12, 2004

### Staff: Mentor

equation of continuity

You need to apply the continuity equation, which says that the same amount of mass flows past each point. It can be written as $A_1 v_1 = A_2 v_2$, where A is the cross-sectional area.