New (to me) "metric" unit

[zdas04]

There was a homework question this morning in Piping (thread378-388505) that is pretty sure to vanish by Monday. The guy actually photocopied his text book. Here is a screen shot:

In the first line the text's author says "... enters the bearing at 250 N/hr". Does anyone have any idea what an "N/hr" might be?

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

I was wondering the same thing.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[TheTick]

If I was a teacher and a student returned this problem with "WTF is a N/hr?", I would give that student an A for the day.

 

[TD2K]

N/hr is just another way of expressing the mass flow but in units of force/time

F = m * a with each side divided by time.

250 N/hr = m * 9.81 m/sec^2

m = 25.5 kg/hr

Never seen it done that way when I've worked with SI or metric units but it's clear to me that is what they are doing.

 

[Compositepro]

I guess that using Newtons as a unit of weight makes as much sense as using kg as a unit of force, which is fairly common.

So, a Newton of weight is equal to a mass that exerts a force of 1N at 1g of acceleration (about 0.1 kg)? In that case it should at least be called Nw in the same way that kgf is used.

 

[zdas04]

So it is a "weight flow rate"? Just what the world needed, another idiotic way to state flow rates. Gosh, people don't even express their own weight in Newtons (I'm not sure how the "stone" survived metrification, but everywhere that doesn't use lbf or stone seems to use kg to express the magnitude of their physical presence on earth).

Text book authors should be peer-reviewed by people who have to live with the crap the book is trying to teach.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

Oh, that makes lots of sense! Does anyone see how this makes SG-0.91 = 8909 N/m³?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[TD2K]

I wonder if the professor is trying to give an exercise in thinking through units? But no, I hope I never see a pump data sheet with the flow in N/hr.

 

[zdas04]

It looks like the textbook uses that unit so it goes farther than just the classroom guy.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[VEBill]

Wolfram Alpha says it's "force onset rate".

I don't think that applies here, which shows how odd this example is.

 

[hokie66]

I suppose it is just an attempt to teach manipulation of units, but it is counterproductive. Metric calculations are supposed to be simple, not difficult. As well as the ridiculous N/hr, solving for velocity in cm/s is another unnecessary complication.

 

[zdas04]

It really feels like the author was thumbing his nose at everyone doing fluid mechanics before or after. And he wants anyone taught using his book to suffer when they find a job. Wish I knew what book it was (and I bet he has the "Modified Bernoulli Equation" that raises my blood pressure every time I see it).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[KENAT]

People in Slug houses shouldn't throw N/hr.

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(probably not aimed specifically at you)
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[IRstuff]

WTF is a slug, anyway? I'm always just using lbf, because I've nothing that measures slugs. I step on them, but I don't measure them.

TTFN
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[zdas04]

I have never seen anyone actually use slugs in anger. I only had one college class where the text used slug in exercises and the teacher said that we should just mark out "slug" and put "lbf". Wikipedia says 1 slug = 1 lbf*s^2/ft. Which brings in g_c to get rid of the acceleration term.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Pud]

I used to be partial to ergs when they were around..

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[KENAT]

If I recall correctly, this guy's aircraft design text book introduced me to slugs at uni.

http://www.aircraftdesign.com/

 

[IRstuff]

ah yes, ergs, that went along with dyne-cm... That seemed to be the physicists' favorite flavor of SI.

TTFN
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[zdas04]

I am from VERY deep in the Ozark mountains and my high school math was mostly about long division and balancing a checkbook. When I started Nuclear Power School in 1972 I had never even heard of the metric system. In one of the first class sessions the instructor came in and drew a centipede on its back with a speech bubble saying "erg". The instructor said "if you'll remember that a dying centipede says 'erg'" you'll never forget that a dyne-cm is the same thing as an erg and that conversion will make a lot of the next 6 months easier". 43 years later I'm thinking he was right.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[IRstuff]

That's a good one; too bad CGS isn't much taught anymore.

TTFN
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