# Typical engineer's life problems

1. Aug 13, 2004

### Clausius2

Let's discuss the next problem:

An unfortunated engineer has one heavy stone that has to be moved a small distance. Thus, he thinks of pushing it away by means of a water jet, pointing it towards stone centre of mass. But he has to types of jet available:

-One has 50 bar of static pressure and 5 m/s of water velocity.
-The other has 10 bar of static pressure and 89.5 m/s of water velocity.

One jet has to be choosen in order to provide the neccesary force to push the stone a short distance, as engineer's boss is claiming all day it has to be done.

What type of jet would you choose?.

2. Aug 13, 2004

### LENIN

I would personaly choose the first jet. Becouse the stone is heavy and it would probably be easyer to move with a jet that has more static pressure.

3. Aug 13, 2004

First one, more static pressure.

4. Aug 13, 2004

### pervect

Staff Emeritus
The first one has a very small advantage when you figure out the total pressure by my calculations. I think the point of the exercise is to use Bernouli's law, though.

5. Aug 13, 2004

### Clausius2

My opinion is similar to yours. It is certain that if you compute total presure is aprox. the same in both cases. I have not the universal answer, but in my opinion, if you pay attention to the leading stagnation point that is formed in the interface stone-water, it seems pressure total is what exerts the pushing force. Those who had said static pressure has not written the momentum equation in the jet direction.

Any opinion counteracting what I have just said is welcomed, but it is logical to think velocity provides the exchange of momentum to the stone, although static pressure acts like a "total pressure" reservoir ready to be released when the water stagnates eventually at the zone of frontal contact. So that both of the jets would be valid, isn't it?.