Re: [Phys-l] United States Geological Survey

[Jacques Rutschmann <jr@colororange.com>]

Bernard Cleyet wrote:


> No is typo. For an homogenous earth there are two sets of tides the
sun tides ~ 1/3 those of the moon. Periodically superimposed there's
only one set called spring tides, etc.

I think that you are saying that the moon and the earth have a tidal
effect. This is not what I am disputing.

The USGS at

http://sfports.wr.usgs.gov/SFPORTS/glossary.html#MLLW says:
> > "There are two low tides in each tidal cycle (so usually two low tides in each day). These two low tides are not quite the same height  because one tide is generated by the gravitational interaction with the sun
(which is small), and the other is generated by the gravitational interaction with the moon (which is not so small)"

Maybe I am reading this totally wrong but my only understanding of the USGS text is that:
1) There are two low tides a day because one is generated by the moon and the other by the sun (Therefore if the sun was not around, we would only observe one low tide a day).
2) One low tide is due to the gravitational interaction with the sun (with the moon having no effect at that point) and the other is due to the gravitational effect of the moon (the sun having no effect).

> No is typo
Are you saying that that my reading is wrong, that the USGS is saying something else, that happens to be correct?
Or are you saying that my reading is correct, that the statements 1) and 2) are also correct?


Bernard Cleyet wrote:

> bc  couldn't find Bernoulli

The USGS at:
http://ga.water.usgs.gov/edu/spcities.html
and also here (see question 7):
http://ga.water.usgs.gov/edu/qahome.html 
States 
"Gravity provides water pressure to send water on its way to you. The water pressure must be strong enough to sometimes go uphill to get to your house, and as the water flows in the pipes, friction between the water and the pipe walls reduces the water pressure. Water pressure is kept up by gradually reducing the diameter of the pipe".
and
"One way to assure that you have enough water pressure at your house is to make the pipes smaller and smaller as they get further away from the water tower".

I am not sure that I understood your answer... do you agree with the USGS?
I think that Bernoulli predicts that for frictionless incompressible flow, pressure decreases as a pipe narrows (not increase as the USGS says). In addition any friction/turbulence in the pipes would also lower the pressure more with smaller pipes.