Validity of using h (time) with KN (force) in SI unit system 1

[Cansand]

You know that in SI units
by definition 1N/m^2= 1kgf/m^2=1kg/(m.S^2)
I am using software to analyze a problem where it is more practical to use Hour rather than Second as the time unit.


I do not know if I can use KN as a force unit with Hour as the time unit for the same probelm .
or should I stay with the Second

 

[CoryPad]

According to NIST Special Publication 330 The International System of Units (SI), hour is a member of the group "non-SI units accepted for use with the International System".

If you are going to perform calculations that require appropriate units, then you need to use seconds or use a conversion between seconds and hours.

If you are only displaying data, then use hours.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[eromlignod]

First of all, since when does a N/m^2 equal a kgf/m^2?

The left and right members of your equation are correct, but only if you are talking about kilograms of mass, not force. If you use kgf, there are no time units.

You'd better get the rest of your physics units straightened out before you worry about time.

Don
Kansas City

 

[Cansand]

eromlignod
Thanks for correcting this printing mistake
Sorry I wanted to write
1N/m^2= 0.1kgf/m^2=1kg/(m.S^2)

 

[RPstress]

Whenever an equation is unfamiliar or complicated (e.g., involving both mass and force) it pays to reduce it to m, kg, s. Forces are in newtons (N) and stresses are in pascals (Pa).

 

[katmar]

You can mix your units in whatever combination you like. Just don't call it SI if you are using hours instead of seconds. People adjust their units to fit the reality of their problems every day.

I suppose to be strictly correct you could use ks (i.e. kiloseconds) as a longer unit of time and stay true to SI in the same way as the waterworks people use kilolitres, but I have never seen it done.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[KENAT]

Does the software actually ask for hours?

I'd usually just muliply hours to get the equivalent seconds before starting the actual calculation.

It's funny, at uni & highschool almost everything was base SI units with 10^x as required.

In my 'real world' I don't think I've ever used a meter in a calculation!

 

[Cansand]

The software doesnot ask for hour. However,I am analyzing the behavior of a system (mechanical-heat diffusion) under several years (20 years). So i found it practical to analyze it under hourly time scale where made-sense change can be abotained
so my inputs are the following (please correct if wrong):

Modulus of Elasticity E = KN/m^2
Density = t/M^3
Speed =m/h
Acceraltion gravity= 9.8*3600^2 (m/h^2)

 

[KENAT]

I'd still be tempted to use seconds, 1hr = 3600

 

[JStephen]

If you have a fairly simple situation with all units in a consistent system of measurement, you can usually ignore the units and just crank through the problem. But if the problem gets very complicated, or odd units are used anywhere in it, you need to keep the units intact as you work through the whole thing.

The slip up there with the kg-f is a perfect example. Plug that in where it should be newtons, and the whole thing is off. With english units, you can have similar issues confusing lb-f with lb-m, or ft-lb with BTU, etc.

 

[Cockroach]

Yeah, JStephen hit the old perverbial nail on the head.

The confusion most likely comes from the imperial system, pounds mass and pounds force, for example. Mass is measured in "slugs", not pounds mass/force! In comparison, the metric system measures mass in grams and force as dynes, OR mass in kilograms and force in newtons. They are the same, only the decimal shifts.

I have recently seen an Italian pressure relief valve company expressing pressure as kg/mm^2. Clearly the acceleration of gravity need be multiplied in to give the correct unit of pressure measurement as MPa. I think it is a leap in faith to expect a targeted audience to interprete your datum and would suggest refraining from the mix of odd units.

Afterall, the oilfield is wierd enough! Pipe with imperial threads are measured off in meters, torque is measured in decaNewton-meters, pressure is kiloPascals but valves are in psi. I keep a small conversion table in my wallet, "don't leave home without it"!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[drawoh]

helppoorpeople,

I once tried to do some structural calculations using millimeters. Things got messed up when I squared them. I strongly recommend using standard units for calculations, i.e., meters, kilograms and seconds.

If I have English units, I ignore slugs and just replace m with w/g. This makes the SI equations work. So far, I have always used inches for length, instead of feet. G=386in/sec^2. Trying to use slugs could result in wrong unit conversions.

One of my mechanics of machines teachers insisted that we all do unit balances with our calculations. This has kept me out of a lot of trouble, including the case noted above.

JHG

 

[eromlignod]

Drawoh wrote:

"So far, I have always used inches for length, instead of feet. G=386in/sec^2. Trying to use slugs could result in wrong unit conversions."

Not really, but using inches instead of feet certainly will. Calculations in engineering units are based on feet, slugs and seconds. Just like with MKS or CGS, that's what units should be used during calculations.

1 lb = 1 slug-ft/s^2

I also avoid using all bizarre hybrid units like "pounds mass" and "kilograms force".

Don
Kansas City

 

[rb1957]

your problem is consistency ...

"Modulus of Elasticity E = KN/m^2
Density = t/M^3
Speed =m/h
Acceraltion gravity= 9.8*3600^2 (m/h^2)"

KN includes time units ...
convert all your units to the correct fundamental units (including slugs if you're going imperial) and adjust for your time units.

could you define your own second as being 3600 "real" seconds ?

isn't this just a graphing exercise ? I mean you have lots of seconds of data ... graph each 4000th one; there isn't much change using SI units, magnify the axis by a factor of 1,000 ...

 

[katmar]

Don, although I am an enthusiastic SI user I must take exception to you calling the pound mass a "bizarre hybrid unit". The pound mass (lb) has been around a very long time and is the unit of mass in the very widely used foot-pound-second system of units (which uses the poundal as the unit of force).

When a pound is written without the qualification of whether it is mass or force the general convention is that it is mass. If it is force then (in my experience) it is always written as pound force or lb.f. So I would say 1 lb is certainly not equal to 1 slug.ft/s² because a lb is a unit of mass and the compound unit slug.ft/s² is a unit of force. However it would be correct to say 1 lb.f = 1 slug.ft/s²


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JStephen]

I think I had one textbook from way back when that used slugs for mass. Everywhere else, it is has been mass in lb-m with a g-c factor included where necessary. I've never used dynes or poundals, thankfully.

We still see kg-f/cm^2 used as stress on occasion. It makes perfect sense, just not SI.

 

[drawoh]

katmar,

If you assume that m = 2.2lbm = 1kg, you need a procedure for deciding if and where to insert g into the equations. There are several ways to do this, which gets unmethodical and confusing very quickly.

The pound is a unit of force or weight, equivalent to Newtons.

F = ma = (w/g)a.

All the handbooks quote English stresses and densities in inch units, so I don't like to use feet, and so I stay away from slugs. In the SI system, you have to stick with meters because these are used to work out Newtons.

I know the pound-mass works in thermodynamics, but I have done very little of that since college. I would really have to read up to get back into that stuff now.

JHG

 

[JStephen]

Simply treating the lbf and lbm as two separate units is how you keep them separate.

Insert 1-lbf, 1-lbm, and 32.174 ft/s^2 into F=ma/gc is an easy way to remember the units on gc.

The kg-f is no more or less awkward, and easily understandable by those used to dealing with lbm/lbf.

Even the SI system has its inconsistencies. A cubic centimeter of water is a gram, but to be consistent, it should be a cubic meter. A cubic decimeter is a liter, to be consistent, it also should be a cubic meter.

 

[rb1957]

1lbf = 1lbm*32.174ft/sec^2
1lbf = 1slug*1ft/sec^2
1poundal = 1lbm*1ft/sec^2 (i think)
1kgf = 1kgm*9.807m/sec^2
1N = 1kgm*1m/sec^2

what's the big deal ...

returning to the OP, redefine the units in the fundamental dimenions (above you used N for E and mph for speed, but N includes a time unit so oon the face of it you're being incompatable) and be consistent

OR use different dimensions for different units and be very careful combining them !