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Units

Units

(OP)
I had a request this morning for information on compressing 16,900 Nm^3/hr.  I could deal with kg/hr or m^3/hour (possibly even with a prefix as long as there were parenthesis), but as far as I know "N" is "Newton" not any sort of prefix.  If I assume that it is was a typo and he meant to type "M(m^3)" (10^6*m^3), then he's asking for 16.9*10^9 m^3/hr or 14 BCF/d (billion standard cubic feet/day) or 2.7 billion hp to meet his compression needs.

So the "N" isn't Mega.  I've asked him for clarification, but anyone have any idea what it might be?  He's talking about a CO2 sequestration project taking exhaust gas from atmospheric to about 14 atmosphres.

David

RE: Units

zdas,

Some confusion here. 'N' stands for Normal (not Newton in this case). It simply means that the air is at normal atmosperic pressure (not compressed) and temperature (usually room temperature).

An example: 1000 Nm^3 compresses to roughly 140 m^3 at 100 PSI (which is close to 7 kp/cm^2 or 700 kPa).

But you are right, 'N' is Newton - most of the time.

RE: Units

(OP)
skogsgurra,
That helps a lot.  I didn't think the guy really wanted to install more compression than there already was in the world.  I guess I haven't seen the "normal" indication, mostly I see "Standard" (for standard conditions, I've even seen SCM for "Standard Cubic Meter" which is about weird).  

Since the project is in the U.S. I wonder if his "Normal" conditions are 1 atm, and 60F or 0C (or 15.6C)?  I'll have to assume it's 15.6C (because of your "room temperature" ref), but ...

Looks like he only needs 3,000 hp.

I just love the "standardization" in SI units.  It's no worse (and in many ways a lot better) than the U.S. mess, but the way folks take sortcuts is exactly as confusing in either system.  I still hate it when people tell us that all of our units confusion will go away with SI.

Thanks

David

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com

RE: Units

David,

I will tell you something that will make you hate me; all of your units confusion will go away with SI

The N for Normal is not SI. Just something that is being used ocassionally.

RE: Units

(OP)
I can hate the sentiment without reference to its conveyor.  How could I ever hate you?  You helped me clear up a confusion.  I don't really think that SI will ever do that for me (SI is great, people are stupid and lazy).

David

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com

RE: Units

You will see the terms Nm^3/hr, Nm^3/s, Am^3/hr and Am^3/s very frequently in calculations, codes and design manuals etc dealing with flow of hot gases. (e.g. chimney stack design, gas handling systems, etc)

As others have pointed out above, the "N" means "Normal", as in one atmosphere at ambient temperature. I am not 100% sure if "ambient temperature" has a universal defined value. We tend to use 20 degrees C, but others might use 15 degrees C or even 0 degrees C. Does anyone know the definitive meaning of "ambient" temperature in this application. (If you are dealing with gases at some hundred of degrees, the error in assuming 0 degrees, 15 degrees or 20 degrees Celsius will be pretty small.)

The "A" means "Actual", as in the flow rate at the actual temperature and pressure conditions in the gas duct. (Higher temperatures will lead to an increase in Am^3).

I don't think the use of the N or A are strictly SI conventions, but are essential to ensure everyone knows how much gas you are dealing with. For example, 16,900 Nm^3/hr is a LOT more gas than 16,900 Am^3/hr at 300 degrees Celsius and 1 atmosphere. However, whether you use metric or Imperial units, you need to make sure that you specify the applicable temperature and pressure, as well as the volumetric flow rate. The "N" and "A" are just a convenient short-hand way of doing so.

The main reason for using Nm^3 is that for a given gas stream (with no inflows or outflows), the value stays constant regardless of pressure and temperature changes along the duct. Also, when mixing two gas streams with different temperatures and pressures, you can simply add the Nm^3 to get the total gas flow rate (assuming no chemical reaction takes place etc). you then need to calculate the mixed gas temperature and pressure conditions to get the Am^3 value.

Hope this helps.

RE: Units

Normal flow rates are generally used in Europe as far as my knowledge is concerned and particularly by the Germans(every machine we imported from Germany states the compressed air requirement in NM3/Hr). As suggested by JulianHardy, there is no single definition for Normal flow rate. The temperature ranges from 00C to 200C. The manufacturer is the better option to let us know what exactly the temperature considered was.

Regards,

Eng-Tips.com : Solving your problems before you get them.

RE: Units

Ok, time for a german voice to be heard ;)

Nm³ is in fact a purely german naming convention (stemming from the old german "DIN" standards. it has nothing to do with SI system. however, there is also an iso standard incorporatiung this naming, but defining different standard conditions. I will give you both.

pressure p = 1.013 bar (a)
humidity is 0% (dry gas)

temperature (DIN 1343) = 0°C
temperature (ISO 2553) = 15 °C

When dealing with a german customer or with a customer insisting on german nomenclature, in 90 % of the cases, 0°C is meant as reference temperature.

hth,
chris

RE: Units

I apologize for the huge number of mistakes in my previous post. I've been in a hurry, just quickly jotted that down.

chris

RE: Units

Zdas04 is right, there is reason for confusion in the use of the letter N.

One practical way I found of avoiding confusion when the N is attached to -or combined with- other units is to write a dot for multiplication or a slash mark for division attached to the N when meant to be equal to 1 kg.m/s2. In this case there wouldn't be a doubt that N is the unit for force, the Newton.

So, for example, Pa=N/m2 as the unit of Pressure; N.m, for Torque; N.m2/C, for Electric flux, etc.

Just as an addendum, a small n attached to  other units without a multiplication or division symbol means nano which is 10-9.

RE: Units

25362

One reason that there is some (or a lot of) confusion seems to be that Americans don't differentiate between upper case and lower case letters. I very often se KB for kilobytes instead of kB* and MF for microfarads - which is a double fault - since mF would actually mean millifarads and not microfarads. Confusing 'N' and 'n' can probably lead to very interesting complications. I think that it is worth-while to memorize these few letters and their corresponding multipliers: (I leave out the first four Y, Z, E and P)

T   tera   10^12  it is tera not terra!

G   giga   10^9   a billion in US  a milliard in EU

M   mega   10^6   a million

k   kilo   10^3   a thousand

m   milli  10^-3  a thousandth

u   micro  10^-6  a millionth  (should be a greek 'mu')

n   nano   10^-9  a billionth in US etc

p   pico   10^-12

f   femto  10^-15  (from Danish Femtan = fifteen)

a   atto   10^-18  (also Danish, Atten = eighteen)

Keep the upper/lower case and remember that prefixes are multipliers and no units. That will make things a lot easier.

The less common prefixes hekto, deka, deci and centi are for old-timers. Don't bother to learn them if you do not need them. That also applies to the Y (yotta), Z (zetta), E (exa) and P (peta) that I left out.

There is another source of confusion. I hope that we all agree that ppm means parts per million, but what does ppb stand for? One part in 10^9 or one part in 10^12? I have not found any definitive answer to that. Has anyone? Or is it country-dependent?


* The difference between the "bakers" or binary kilobyte and the decimal kilobyte is sometimes handled by using K for 1024 and k for 1000, but that is not what I am talking about and that would not work for the megayte (M for binary megabyte and m for decimal megabyte would really confuse people).

RE: Units

(OP)
skogsgurra,
Good post.  Notice Quark's post above "every machine we imported from Germany states the compressed air requirement in NM^3/Hr".  What does a capital "M" stand for?

Another big confusion is parenthisis.  Is kg^2 a thousand squared grams (whatever that might physically be) or a squared kilogram?  Typically it will be the later, but how do you know with confidence?  I like 25362's approach of showing unit relations explicitly.  N/m^2 is clearly a Pa and N/mm^2 is probably a MPa, N/m(m^2) is most likely a kPa, but could be taken as a Pa/m (a reasonable pressure-gradient term).

I don't think I'm being picky here.  A huge proportion of the time the context makes the usage clear, but exceptions are no better in SI than the system we use in the Rocky Mountains.

David

RE: Units

"compressed air requirement in NM^3/Hr".  What does a capital "M" stand for?"

My guess (and yours, I presume) is that 'M' should be an 'm'. If one wanted to be mean and nasty one could point out that 'NM^3/Hr' is as far from SI as you can get. 'N' is not SI, 'M' is not SI and 'Hr' is not SI.

Trying to squeeze 'NM^3/Hr' into SI clothes could result in the following: N->n (nano), M->m (metre), H = Henry, r = ? Not much meaning in such an excersise.

OK, you built the Rockies using the imperial system. And the Egypts built their pyramides using their system. I admit that our Eiffel tower is less impressive. But it was built using the SI - and so was the Statue of Liberty!

(There might be some (un)intentional misunderstanding in this post)

RE: Units

skogsgurra:

With respect to capitalization, I've got one question. I believe that I was taught that when dealing with units, those that are based on a person's name are to be capitalized but those that are not are not. So, for example, it should be kb (not kB), kPa, kW, km, etc. Do you know if this is correct?

I share your consternation about the sloppy use of caps when writing units, especially since m & M are both legitimate prefixes.

RE: Units

Beggar,

I haven't heard that before. Sounds like a good principle, but does it hold? Most units are named after dead persons and most units are abbreviated using capital (first) letter so it seems to be right.

Even less frequent units like gray (Gy) and siemens (S) follow that rule. And units like lumen (lm) and radian (rad) that are not named after persons use small letters throughout.

The unit for electrical resistance seems to be a problem since the greek omega symbol is used, but a clever friend just told me that it is capital omega so your rule seems to be right. Which makes me blush and change kB to kb.

I think that you are perfectly right there. Thanks.

RE: Units

Beggar, that sounds right to me. But, is it enshrined in law? or just a convention? or had the distinction become blurred.
So many units are named after some one or other.
cP centi-poise (mPas; milli-pascal seconds)Poise and Pascal
cSt centi-stokes Stokes
°re; Redwood never was degrees R, so far as I can tell, possibly because anything named for someone with a name like Sir Boverton Redwood is too good to initialise.
°ree;C(Celcius), F (Farenheit), R(Rankine) and K(Kelvin) Degrees C was, course, Centigrade which also had a capital C despite not being named for someone.

Now we come to something else.
"What is the temperature?" "126°rees;C"
"Waht is the diference in temperature?" "16 Centigrade°rees;" (I'm old-fashioned, i drink them too) i.e. i always learned that a specific temperature was an identity and a difference was so many degrees on a particular scale. So "16°rees;Centigrade is a temperature and "16 Centigrade °rees; is not a specific temperature.
So, am i right or have i been deluding myself all these years? This is part and parcel of my other pet irritation mentioned elsewhere of people who say "one hundred and eleven point seventy two" in stead of "point seven two".

JMW
www.viscoanalyser.com

RE: Units

jmw,

Getting somewhat off track: Do you put some Angostura in them? And some fruit?

RE: Units

I must be doing something, look what happened to my "insert symbol" I got °rees; when i wnated °

JMW
www.viscoanalyser.com

RE: Units

Capitalization of proper names for SI units is a style convention for SI only.

there is a factor of 8 between
kB = kilobyte
kb = kilobit

TTFN

RE: Units

As far as I remember, Beggar is right. Besides, millipascal second is written mPa.s with a dot, and centistokes, cS, not cSt. oC goes for degrees Celsius, oF, for degrees Fahrenheit; K, for degrees Kelvin without o, for the basic scale of absolute temperatures. cP (=mPa.s), for centipoise as unit of absolute, aka dynamic, viscosities; J for joule (heat, work or energy); A for amperes, current; C for Coulomb, charge; T for Tesla, magnetic field, and so forth.

RE: Units

One teeny little correction.  

Kelvin is the SI unit, not degrees nor degrees Kelvin.  

So its 298 Kelvin, period.

TTFN

RE: Units

2
In the SI, capitalization is used only for ABBREVIATIONS of units derived from proper names.  This is a written rule, as shown in detail in NIST SP330, available at:

http://ts.nist.gov/ts/htdocs/200/202/mpo_pubs.htm

One teeny litte correction to IRstuff's previous post:

it is 298 kelvins (notice the plural), or 298 K

Regards,

Cory

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

RE: Units

IRstuff,
If this holds then how do you differentiate between an absolute temperature vs an absolute delta temperature as jmw alluded in which case it would be degrees K and K degrees respectively (in every sense of the word)?

Jesus is THE life,
Leonard

RE: Units

I know that, but it just doesn't look right.

TTFN

RE: Units

It is amazing.
All those scientists and engineers who have contributed to the creation of the various systems and not one of them, SI or otherwise, has any more consistency than the rules of grammar or the better logic than in spelling.

JMW
www.viscoanalyser.com

RE: Units

Absolute delta temperature is still kelvins.  

What you're referring to is degree Celcius.  

The only place for the degree or degree symbol is degree Celcius (ºC).  (This is the inconsistency in the SI style guide -- Celcius is the only proper name that remains capitalized)  

Seems only fitting that Lord Kelvin would at least get the same courtesy, eh?

TTFN

RE: Units

(OP)
I rest my case.

David

RE: Units

To IRStuff, thanks for the correction on K. My little contribution: it's Celsius -not Celcius- for Anders Celsius (1850).

RE: Units

Thanks... At least I was consistent in spelling it wrong.  

TTFN

RE: Units

To make matters worse there are (at least?) four definitions for ton.

UK: mass = 2,240 pounds; shipping:register = 100 cubic feet = 2.832 cubic metres
USA: short = 2,000 pounds; long = 2,240 pounds
Metric ton (tonne) = 1,000 kilograms = 2,204.62 pounds
Spain: corta = 2,000 libras = 2,028.7 pounds; larga = 2,240 libras = 2,271.2 pounds

Not to mention the stone and the quintal, as additional mass measures.
 

RE: Units

Completely agree, 25362.

I am always confused when someone uses "tons" to convey a weight to me.

Are you still in touch with 24601 (Jean Valjean - Les Mis.)?

tg

RE: Units

Tons as a measure of weight! That's a novel idea!

To throw a spanner into the works, shipping tons are a measure of volume:
 One (1) U.S. shipping ton is equivalent to 40 cubic feet, and one (1) British shipping ton is equivalent to 42 cubic feet.

Good Luck
johnwm

RE: Units

25362,


Quote (25362):

Metric ton (tonne) = 1,000 kilograms = 2,204.62 pounds
Small correction:
Metric ton (tonne) = 1,000 kilograms = 2,204.62 pounds at sea-level on earth

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey


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

RE: Units

Umm, I'm not sure I agree:

1 tonne = 1000 kg.

The kilogram is a unit of mass, not a unit of weight. By implication, the tonne is also a unit of mass because 1 tonne = 1000kg. 1kg is still 1 kg, whether it is on the earth or on the moon. Weight, measured in Newtons because weight is a force, will vary according to location, but the mass will remain the same.



-----------------------------------

Start each new day with a smile.

Get it over with.

RE: Units

ScottyUK

Yes, you are right, but is the pound in question a unit of mass or a unit of force (weight)?

If you are talking about the pound mass, then 1 kg = 2.20462 pounds mass wherever you are in the universe. (Discounting the possibility of mass changes due to motion at relativistic near-light-speed velocity!)

If you are talking about a pound weight, then you need to specify your local gravitational acceleration to determine the weight of an object, once you know its mass. This changes with your elevation.

Chalk up another one for then SI system - used properly, you should never confuse the mass of an object in kilograms with its weight in newtons.

RE: Units

Someone said something early in the thread about standardization.  I wish there was some in the metric world.

I deal with spanish, german, south african, brazillian, mexican, etc., manufactured products that all use different metric units (cm vs mm, kPa vs bara, tonnes/hr, etc.,) and the clients I deal with are imperial units USA types, with imported technical help from central and south american countries, with a smattering of australians and south africans mixed in.  It is like trying to speak 3 languages at once to communicate it all amongst the players.

I use websites from all of the above, and hate trying to change from kpa to bara to kg/M^2 to psia (don't correct my capitalization or errors in the foregoing) for a discussion with all involved about the same piece of equipment or process.

That is my two cents worth.

rmw

RE: Units

Weight and mass are commonly used as meaning the same thing. However, ScottyUK is right: in physics a body's mass is an intrinsic property; it doesn't depend on location.

With my 75-kg mass, my weight near Earth's surface is then 75kgx9.8m/s2 or 735 N. On the moon , where the acceleration of gravity is only 1.6 m/2, I would weigh only 120 N. In the remote reaches of intergalactic space, far from any gravitating object, my weight would be essentially zero.

Let's not discuss here the mass-energy relativistic equivalence as well as inertial (unaccelerated) mass concepts.

RE: Units

rmw,

There IS standardization in the metric world - the SI.  The problem is not that standardization doesn't exist, rather it is because people don't use the standards as they should.  I won't correct your errors, but I would like to comment on the units you mention.

cm, mm, kPa, and tonnes/hr all are acceptable within the SI.  Conversions are simple and shouldn't cause anyone much confusion.

bara, kg/M2 and psia are not acceptable units or abbreviations within the SI.  I am baffled why anyone would want to use these.

Your hate for conversion among these units is the reason why everyone should start using the SI right now and stop using anything outside the SI.

Regards,

Cory

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

RE: Units

Well said!

RE: Units

It appears that as a result of an EU directive to force metrication through, that the New English Bible (or something similar) has been updated with measurements in the metric system.

One notes the intrusion of "calculator syndrome"; that is to say that when converting from one system to another the result is quoted without considering if it is at all rational to do so. I have noticed a tendancy of people, when converting from one system to another, to quote all the deciemal places the calculator shows them. To me, if someone quotes three decimal places and does not express a tolerance i assume +/-0.001 is the best. Unfortunately many people don't round the converted figure properly.

A cubit is a unit of linear measure, from the elbow to the tip of the longest finger of a man. This unit is commonly converted to 0.46 meters or 18 inches, although that varies with height of the man doing the measurement. There is also a "long" cubit that is longer than a regular cubit by a handbreadth. (Ezekiel 43:13) [source: World English Bible Glossary]

This inexact measurement is faithfully converted, for example, in the dimensions of Noahs Ark as if they were working from the blueprints. So exact conversions were used and all the decimal places expressed.

I am waiting for the same directive to reach the collected works of William Shakespeare.

JMW
www.viscoanalyser.com
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Units

According to NIST, http://physics.nist.gov/cuu/Units/outside.html, bar is still an acceptable unit, although discouraged from new uses.

As for weight, go check out any weight scale or balance;  metric weight is labeled as gm or kg, not N.

TTFN

RE: Units

That's because the weighing scale or balance is not showing our weight but our mass.

RE: Units

Point is, all our instrumentation spits out mass units when we weigh stuff.  

Therefore, we windup using mass units instead of weight units when refering to weight, otherwise, our poor listener will have to figure out what 747 newtons corresponds to on his scale.

TTFN

RE: Units

IRstuff,

You are correct that bar is allowed but strongly discouraged.  But the discussion point should be "What the f@#$%& is a bara?"

Regards,

Cory

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RE: Units

(OP)
Good question Cory.  If I go into the wonderful Uconeer program, 100 atm is 101.33 bar.  Boy that number looks familiar.  14.7 psi is 101 bar.  So if you disregard reality for a minute, 14.7 psia would be 101 bara.

But what do you do in a place like Farmington, NM, USA, where 0 psig = 11.8 psia.  Does zero bara cause a shift so that 0 psig becomes 1.2 bara?

My head hurts.

David

RE: Units

25362,

You said:

"That's because the weighing scale or balance is not showing our weight but our mass."

Totally incorrect! A scale can only show your weight, not your mass. Don't believe me? Try taking your scales up Mt Everest and check your weight. (Of course, you will need to allow for the extra weight of the thermal clothing you are wearing, plus an adjustment for the mass you probably lost due to the exertion of the climb!)

It is one thing for the general public to use mass and weight interchangeably, and I think it is equally acceptable for the general public to use units of mass when they really mean weight, and to weigh something when they really want to determine its mass. Most of the time, at the surface of the earth, a 1 kilogram mass has a weight of 1 kilogram force, to acceptable precision for cooking a cake or monitoring one's fitness and health.

However, I hope all engineers and scientists are quite clear on the distinction between the two, and don't use mass units when they should really be using force units, and vice versa. It is one of the most common basic errors that I see from young inexperienced engineers - plugging a weight unit into a dynamic analysis, when they should have used the object's mass (or vice versa) - if you do this, you can end up with a very convincing looking dynamic analysis - but it will be totally meaningless!

RE: Units

Cory,

I am well aware of SI, but unfortunately, I am on the receiving end of the line, and cannot dictate to those who are preparing the stuff that they use SI units.  Hence, my point about the lack of standardization, and the confusion it causes those of us that did not grow up on the metric, and may not have even encountered it for the first time until later in life.

I deal with european equipment, and if left alone in our shop, where everything is metric, even though it is not SI, I love the system, and use it myself when I have to make something from a clean sheet of paper, and can pick the system of units (who the heck would want to have to deal in sixteenths, thirtyseconds, and sixtyfourths if they did not have to.)

The other thing I hate, and I hate it fervently is those who have not converted completely.  I just finished a shade tree project using my tools, where I had a mix of standard english and metric tools out.  It is maddening to be using one type, and to have to (after finding out the hard way) stop and switch back to the other.  

I would love standardization.  Make them do it for me.

rmw

RE: Units

JulianHardy, this is because one is using a spring scale which compares forces, not masses. Thus, it is calibrated at sea level, or whatever elevation on Earth it is meant to operate; nevertheless, it shows mass, which, BTW, is proportional to weight.

That we confuse mass and weight at all results from the remarkable fact that the gravitational acceleration of all objects at a given location is the same.
A two-pan type of balance would show the same reading at the top of the Everest as at sea level.

To measure mass in a spaceship, one should use, for example, a vibrating (or oscillating) mechanism independent of gravity effects.

RE: Units


25362, Some confusion here perhaps. Mass is an intrinsic property of an object, while weight is the effect of the acceleration due to gravity acting on that mass. A quick review of my early Physics shows Newton's First Law as:
f = Ma
where f (force, in this discussion weight) = M (Mass) x a (acceleration, in this case due to gravity). It follows that mass is only proportional to weight under constant local conditions.

A spring scale can only show force, not mass!

Good Luck
johnwm
________________________________________________________
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RE: Units

To johnwm: it appears you are right and wrong at the same time. You are right in that a spring scale measures (balances) forces (aka weights) but its readings are given in mass units since mass and weight are proportional.

If you plan to use a spring balance calibrated at sea level (g=9.8 m/s2) to show the right mass at higher elevations (the Everest as JulianHardy's example) you'd need to recalibrate it to take into account the change in the acceleration of gravity.

In short: spring scales measure weight but report mass.

RE: Units

The distinction appears to be in our understanding of 'proportional'. My understanding is:

Two related quantities x and y are called proportional (or directly proportional) if there exists a constant non-zero number k such that    y = kx In this case, k is called the proportionality constant of the relation.

If that is the case, then acceleration due to earth's gravity is decidedly not constant, hence my earlier comment that mass is not generally proportional to weight

Good Luck
johnwm
________________________________________________________
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RE: Units

Gravity will change depending on where your are on the earth, not just the altitude of the loacl land mass. The earth is not a sphere but an obltae spheroid. This means that gravity is greater at the equator than at the poles.

JMW
www.viscoanalyser.com
eng-tips, by professional engineers for professional engineers
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RE: Units

Johnwm and JulianHardy: acceleration of gravity called "g" at sea level, is -for the sake of our talk in this thread- considered a constant from sea level up to the highest mountain.

The value of g at sea level can be estimated from other "constants" as follows:

g=G.Me/Re2=9.81 m/s2

where

G is the universal gravity constant: 6.67x10-11  N.m2/kg2
Me is mass of Earth ~5.97x1024 kg
Re is the radius of Earh ~ 6.37x106 m

At a mountain's top, say, 8000 m, the value of "g" doesn't change much. See, please,

gmountain's top=G.Me/(Re+8000)2=9.79 m/s2

A change of about 0.25%, negligible when considering the accuracy of the scales that measure the mass of a person.

Following the same reasoning, at a space shuttle altitude of 250 km, the acceleration will drop to ~9.09 m/s2 or about 93% of its "surface" value.

On the Moon whose mass is 7.35x1022 kg, only ~1% that of Earth, but whose radius is only 1.74 x106 m, the value of the acceleration is only 1.62 m/s2, about one-sixth that of the Earth.

Besides, gMars=3.75 m/s2, gSun=3.0x1012 m/s2.

RE: Units

I don't know, I learned to snow ski later in life, (as in not as a teenager with no fear) and the gravity seems pretty powerful at the top of mountains when the skis are pointed downhill.  Friction factor is less up there, too.

rmw

RE: Units

2
JMW "This means that gravity is greater at the equator than at the poles".

Although technically correct, that statement might be misleading to some without qualification, since it depends on ones definition of gravity and gravitational acceleration, which is somewhat ambiguous. The centrifugal effect of the earth's rotation is more than sufficient to overcome the higher equatorial "true" gravity due to the oblateness of the earth's figure, so that the gravitational acceleration as actually measured  at the equator is less  than that at the poles. According to the International Geophysical Congress, the gravitational acceleration as a function of latitude is given by the "international gravity formula" which is:

g(phi) = 978.049*(1 + 0.0052884*(sin(phi))^2 - 0.0000059*(sin(2*phi))^2)

where phi = latitude
and g(phi) is in cm/sec^2

So at the equator, g = 9.780490 m/sec^2
and at the poles,  g = 9.832213 m/sec^2

Just for the record, the variation in g with height above sea level is given approximately by:

g = g(phi) - 0.00000286*h

where h = height in meters and h<=40000 m

RE: Units

Nice pint EM, worth rewarding.

JMW
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RE: Units

There is some confusion all around. Gravity decreases with increasing distance from Earth's center. Thus at the equator, and because of Earth's bulge, the value of g should be smaller, not larger than at the poles.

Centrifugal effects would act against gravity, reducing even more the value of g at the equator. Both facts taken together (plus some gravitational influence from the Sun and the Moon, aka tides) produce the g values quoted by EM.

Variations in the value of g on two points on earth surface may reach values of 0.052 m/s2.

RE: Units

Looks like i am corrected also. It pays to check ones facts first.
I'll let the star stand for EM for introducing the correct equations.

JMW
www.viscoanalyser.com
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Units

Incidentally, the difference in centripetal acceleration a  experienced by an object at the equator and at the poles is 2/3 of the 5.2 cm/s2 by which g at the poles exceeds that at the equator. The remainder is due to the bulge. See,

a = REarthω2

where ω=2πf  is the angular velocity and f ~ 8.6x104 s per revolution (=1 day).

ω=2π/(8.6 x104 s)~0.73 x 10-4/s

With REarth~6.4x108cm

a = 6.4x108cm x (0.73 x 10-4/s)2~3.4 cm/s2

BTW, the torque the Sun and the moon exert on the equatorial bulge is considered responsible for the tilt of our spinning planet. As a result Earth's rotation axis precesses, taking about 26,000 years to describe a complete circle. Thus the North Star (to which Earth's axis points) won't always be Polaris.

RE: Units

Well, there is no question that the effect of the sun's and moon's gravity on the equatorial bulge, coupled with the fact that the axis is tilted, causes the 26000 year precession. But I am not sure why you would say that the torque exerted by the sun and moon is responsible for the tilt in the first place. In fact, I have seen it argued that were it not for the presence of the moon, the tilt would wander out of control and could become extreme. I am skeptical of this, since the tilt of most other planets like venus and mars which lack large moons is not extreme.

RE: Units

I have a book from college for both Statics and Strengths of Materials that referred to "thousands of pounds" as "kips" (kilo-pounds, I guess), and "thousands of pounds per sqare inch" as "ksi" (kilo-pounds per square inch?).

I have never seen this type of notation since...

Another source of confusion I've run into has been the usage of lbf (pounds of force) and lbm (pounds mass?). I had a long winded difference in opinion with a colleague once who insisted that he was correct in using lbm as a mass quantity in his equation. The unit he was looking for was the slug, which is (on earth)= [weight]/[32.2 ft/s^2]. I can't remember where he got this value in lbm, but I do remember how adamant he was, and how blantantly wrong the result was when he used this value.

-Mark

RE: Units

Actually, before you jump on me, J. Kasting of Penn State does say that the obliquity of Mercury Venus and Mars has varied chaotically in the past, although how he knows this I don't know. Without the Moon, he says the Earth's obliquity would vary from 0 to 85 degrees in just tens of millions of years. But to me it's rather suspicious that the obliquity of Mercury Venus and Mars are all relatively small at the moment - you would think on the basis of chance that at least one of them would be likely to be currently extreme. All planets as far as Saturn are currently within a 27 degree tilt. I have seen one study that says Mars obliquity varies between 15 and 35 degrees, but nothing like 85 degrees. Another controversial subject.

RE: Units

justkeepgiviner: Nobody ever uses slugs in practice to my knowlege. There is nothing wrong with using the lbm unit, which in the UK used to be defined by a platinum mass, but is defined today in terms of the kilogram. Your definition of slug is incorrect - it has nothing to do with weight and is totally independent of whether you are on the earth or not. The correct definition is "that mass which is accelerated to 1 ft/sec^2 by a force of 1 pound", and is a derived unit in the force based ft lbf sec system. The lbm strictly belongs to either the ft lbm sec or the ft lbm lbf sec systems, which are mass based.

RE: Units

Youre right about nobody using the slug. I forgot to mention that in the end we solved the problem by converting the all of the original values to SI units :)
-Mark

RE: Units

RE: Inclination of Axes of Planets

EnglishMuffin

My references give the following planetary inclinations:

Sun: 7 degrees ( I know the Sun is not a planet, before anyone corrects me!)
Mercury: 2 degrees
Venus: 178 degrees (i.e. Venus spins backwards - I think this could qualify for the title "extreme tilt".)
Earth: 23.4 degrees
Mars: 25.2 degrees
Jupiter: 3.1 degrees
Saturn: 26.76 degrees
Uranus: 97.9 degrees (lying on its side)
Neptune: 29.6 degrees
Pluto: 12.5 degrees (roughly on its side)

I don't have any information on how much the planetary inclinations have varied in the past.

RE: Units

Yes - the current tilt information is relatively easy to find. I'm just questioning 25362's statement that the tilt of the earth's axis is due to the sun and moon, since by all accounts the moon's presence is actually supposed to reduce the tilt and its variations if anything. However, I did find this http://www.aas.org/publications/baas/v35n4/dda2003/16.htm, which claims the tilt variations are caused by spin-orbit resonances. Nothing to do with units of course - we are all likely to get red flagged at this rate. Interesting though.

RE: Units

EnglishMuffin is for the nth+1 time right again. I didn't express myself correctly. I should have said that the gravitational forces of the Sun and moon on the equatorial bulge apparently exert a torque on our tilting spinning planet. And as a result the rotation axis precesses. As for the origin of the tilt I know nothing.

RE: Units

I didn't find any information on Sedna, assuming that scientist agree it is a planet and not a "Kuiper belt object" (one internet search and I'm an expert already).

Actually, what is the definition of a planet? (my seguey back to a language related topic), we seem to be a long way from the original loose description of a "wanderer".

Is the Moon a planet in a shared orbit, or a satellite or a what?

Anmyway, at 93million miles out, we are a long way off topic again.

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

RE: Units

Some people think we should call the earth-moon system a double planet. But these folks have to contend with the following conundrum: When is a moon not a moon? Answer : When it's our moon!

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