You guy are about a million times smarter than I, but I got a question
wrong on a test years ago and it nags at me.
What would seem to be the more destructive force.
High pressure water or:
Massive water flow. IE volume
I guessed volume but it was wrong but I think the test was wrong.
It wasa long time ago and before I croak I need somee finality on this.
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:
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..
"daniel peterman" wrote in
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.
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
I answered the entire exam with qualitative "here is how I would do it"
answers, describing my logic and thought process, but never supplying
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.
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
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
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!
Total force, sure, because there is more area. Not necessarily
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.
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
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
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.