# Homework Help: Fluid dynamics

1. Aug 23, 2006

### Idea04

i'm stuck on a problem. Lets say you have a large water tower. and the pressure is basically static pressure from the weight of the water. But when the water starts moving through pipes is the pressure then dynamic pressure? In which would you have to calculate the pressure from 1/2 density of water times velocity ^2.

And another thing, I was studying hydraulic multiplication briefly. What I studied was you have a 2 inch diameter piston moving nine inches downward with a hundred pounds force. And it pushes a 6 inch diameter piston upward 1 inch with 900 pounds force. My question is if you pushed down the large piston with a hundred pounds force, then what kind of force will the small piston get.

2. Aug 23, 2006

### Andrew Mason

Bernoulli's principle applies. Pressure is a measure of the energy density of the fluid. Pressure can be converted to kinetic energy density (KE/unit volume), in which case the pressure must decrease by the amount of the increase in kinetic energy density.

I think you can figure that out. Put a 900 lb downward force on the large piston. and a 100 lb downward force on the small piston. Does either piston move? What is the force being exerted by the large piston, then, on the small one? What is the force exerted by the small on the large?

AM