Interesting q about water

Well, I won't claim I'm smarter than anybody - particularly in this group.

Here's my two cents on it. Consider a very large slow moving river and how little damage it does (as long as it stays within its banks of course). It erodes its banks very little and in fact may not have sufficient velocity to maintain particulates in suspension causing it to become shallower with time when upstream rains or floods fill it with sediment. As an example, a 100 foot wide river 5 feet deep traveling at a cross sectional average speed of

1 mile/hour (88ft/min) flows 44000 cubic feet per minute.

Contrast that with the output of a high pressure washer - ~four gallons per minute (32 cubic feet/hour) at several thousand psi.

Or better yet, to tie this back to metalworking, compare the river with a waterjet cutter. A simple description of these can be found here:

Regards, StaticsJason

Not an expert on hydrology, but after a snagging operation that led to some erosions problems on a nearby stream I heard a hydrologist testify that when a rivers flow velocity doubles, the erosion quadruples. Croak in peace.....hopefully not soon..

ed

"daniel peterman" wrote in message news: snipped-for-privacy@storefull-3238.bay.webtv.net...

The energy contained in a stream of water ( or any fluid, for that matter) is calculated by multiplying the mass flow rate in Kg/ sec times the square of the velocity in metres / second.. From this calculation, you can come to the conclusion that the faster it goes, the more damage it does.

I hope that wasn't the exact phrasing of the question, because it is unanswerable in that form. This is a problem in energy, i.e., physics, and answers in physics almost always require quantities (*numbers*). So a tidal wave (volume) is more destructive than a pressure washer, but a hydraulic-mining jet is more destructive than a river.

You need the how-fast and the how-much numbers of each to compare. Or relative numbers. E.g., "Which is more destructive: doubling the volume or doubling the speed?"

Hmmm ... you need more than how-much and how-fast. You need "concentration". A tidal wave 10 miles wide and 3 feet high is not the same as one 1 mile wide and 30 feet high (each moving the same speed). But, it's still a matter of numbers.

Bob

Daniel,

Like all open-ended engineering problems, the answer to this question is: "That depends".

I ran into a similar situation many years ago during a thermo-dynamics examination, set by a professor with degrees from Harvard and Princeton. (Impressive but he couldn't teach worth a damn).

The entire exam revolved around the design of gas turbines and my initial thought was that the time available was way too short: 3 hr exam. Closer review of the questions showed serious shortcoming of data.

I answered the entire exam with qualitative "here is how I would do it" answers, describing my logic and thought process, but never supplying one calculation.

The professor was pissed at me but gave me a decent mark when I pointed out what would happen on an official appeal, what with the "errors of omission" on his paper.

Methinks I rambled somewhat?

Short answer: Describe the circumstanced under which each option would prevail, ie. dominate. Even if the exam is of the true/false or multiple choice type, write your answer on the side or back of the sheet. You can then argue and substantiate your position on appeal. Note that even text books get some of the answers wrong.

Wolfgang

question

In school I used to like to call these "what am I thinking" questions. I am assuming the question might have had more detail, but possibly not. But let me put it this way; If the energy in the high volume/low pressure river is harnessed it can likely power 10 high pressure low volume water jet cutters. Or one could approach the question as "what do you mean destructive force? cutting a canyon out of rock and sand over a few thousand years or cutting a .125 thick titanium plate in 10 seconds.

The next question is whether the question is asking for force or power. If I have a tight lug nut on a wheel and I tie a 100 lb weight on a 1 foot wrench I can exert a FORCE of 100ft/lbs of torque for 3 months, but there is no POWER transmission without movement.

Hope that helps

Bla

Bad question. In order to have a clear answer it would have to state destructive to what.

Tsunamis do enormous damage with volume flow. However, a water jet cutter can cut materials that would be imervious to less pressure regardless of volume.

If you direct a garden hose fed by 60 PSI water at your house, the house will merely get wet. If a 30-foot high tidal wave ( 15 PSI at the root) moving at 50 mph hits your house -- bye bye house!

But the tidal wave is exerting way more total pressure on the house.

Total force, sure, because there is more area. Not necessarily more pressure.

If water moving at 50 mph (73 ft/sec) is redirected to move vertically, it would rise to a height of about 85 feet. s = v^2/(2g) from high school physics and conservation of momentum. The obstacle causing this redirection of momentum must therefore exert the pressure necessary to do that, which would only be about 42 PSI. Lower pressure but considerably more destruction.

Still begs the question of "destructive to what?" A pinhole leak in a 2000 PSI hydraulic system, while having very low volume flow, can easily inject hydraulic fluid into your eye. If it's your eye, you may well argue that this is more destructive than getting doused with

10 gallons of fluid thrown from a bucket -- particularly if the bucket isn't thrown along with the liquid.

Look at New Orleans with massive watet flow.

John

Id say speed.

Lets say instead of viewing rivers and such... we put it in vacuum, and in space (like all good theoretical physics questions..) We take two blobs of water, one with 2x the volume (and therefore mass) and one with 2x the speed (or v.. same thing if same direction) answer is simple. 1/2mv^2. v^2 is a second order term, and therefore increases faster than m.

(this assumes that Ke is the only limiting factor ... which Im guessing is pretty close.. so "all else being equal, a higher velocity stream will be more destrictive" )

Yes glaciers can erode mountains, but they're order of magnitudes more massive.. give me a waterjet of that velocity, and i'll cut you your mountain.

Yes sir I just can't quite grasp the entire physics. There was done and documented a process called hydraulic mining here in Calif. These large water cannons would blast away shale and sluice it down to find gold. They could shoot water a quarter mile. I guess the gold deposits ran out. Maybe the had some dry years as often happens around these parts. They were called monitors

============================== Because of the damage downstream the runoff was doing to the farm/pasture land this was banned. This was before "one man - one vote" and the farmers/ranchers still had considerable clout in the California legislature.

Unka George