Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Typical engineer's life problems

  1. Aug 13, 2004 #1


    User Avatar
    Science Advisor
    Gold Member

    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. jcsd
  3. Aug 13, 2004 #2
    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.
  4. Aug 13, 2004 #3
    First one, more static pressure.
  5. Aug 13, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
  6. Aug 13, 2004 #5


    User Avatar
    Science Advisor
    Gold Member

    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?.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook