Tony Moss wrote: > Fellow shadow watchers, > No doubt the following is well known but it came > as a surprise to me. > While filling a watering can with a garden hosepipe adjusted to give a > powerful single jet* I have to PUSH the hose firmly into the can to > overcome the reaction from the water jet BUT as soon as I dip the nozzle > into the water a smaller force takes over and seems to PULL it gently > down into the water. (no measurements made so purely subjective > conclusions)
I wish I were home where I could conduct some experiments. Since I am in a hotel room I have to do some speculation I hope what I say is not too wrong. I would guess this is the result of the Bernoulli effect. Not when it produces the airplane lift, but when it does NOT produce it. As everybody knows, when the stream of air flows more rapidly on the upper surface of the wind the pressure diminishes and the pressure bellow the wing pushes the airplane up. No, if the air speed gets too high, it "detaches" itself from the surface and the lift is lowered. At the same time, on the trailling edge you have a tourbillion or a vortex. When you insert the hosepipe into the the bucket pressure diminishes on the sides of the jet. It tends to drag the water down, as it pierces it. Now as the water moves down and, at the same time, inwards, the jet becomes an inverted funnel. Towards the bottom of the bucket it moves sideways faster and faster, losing force downwards. Before or after touching the bottom the water has to go up, by the sides, in order to replace the water that is moving inwards. In the water surface, on the sides of the hosepipe, what you have now is a downward motion that "attracts" the hosepipe down. There are other components we should consider. Take, for instance, the weight of the hosepipe as you move it closer to the water and possibily put it in a more vertical fashion. This changes the center of gravity. On the other hand, let me ask you a question: if you but your hose pipe in the bottom of a swimming pool and than you open and close the water flow, what happens? Does it move in the counter-flow when you open it? It should, I guess. Has the difference of density between the air and the water anything to do with the phenomenum you describe? If I were a scientist instead of a speculator, I would not have written this message: I would have made some tests, taken some measurements and than talk as an expert. Alas, I am only a poor inquirer buried in a hotel room with a connection to the Internet! - fernando > > > i.e. the force reverses on entry! > > Why is this so? and, if I'm right, how do 'jet' boats, working on the > reaction principle, function at all I wonder? > > * The nozzle is one of those variable things which screw back and forth > around a central pin. The 'single jet' is more of a tight spray pattern > really. > > Tony (should have paid more attention in Physics lessons) Moss -- Fernando Cabral Padrao iX Sistemas Abertos mailto:[EMAIL PROTECTED] http://www.pix.com.br mailto:[EMAIL PROTECTED] Fone: +55 61 321-2433 Fax: +55 61 225-3082 15º 45' 04.9" S 47º 49' 58.6" W 19º 37' 57.0" S 45º 17' 13.6" W
