# Dumb question here...



## JoeysVee (Jul 18, 2009)

Why is the numerical value for gc, 32.17 but g is 32.088????


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## wilheldp_PE (Jul 18, 2009)

Rounding error?


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## JoeysVee (Jul 18, 2009)

no...the merm list both of these exactly like I typed them


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## wilheldp_PE (Jul 18, 2009)

It was a joke...I don't know any way to round .088 and get to .17.


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## benbo (Jul 18, 2009)

JoeysVee said:


> Why is the numerical value for gc, 32.17 but g is 32.088????


gc is a constant unit conversion factor. It has something to do with lbs and slugs - you can google it.

g is the gravitational acceleration and it varies. 32.088 f/s^2 is the value at the equator


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## IlPadrino (Jul 19, 2009)

JoeysVee said:


> Why is the numerical value for gc, 32.17 but g is 32.088????


Remember the units are different...

g = acceleration due to gravity (ft/sec2) = 32.088 ft/sec2 at the equator and 32.258 ft/sec2 at the poles (value depends on location!)

gc = gravitational constant = 32.17 ft-lbm/lbf-sec2

As benbo explained, gc is used to convert lbf to lbm:



> The English system uses the pound-force (lbf) as the unit of weight. Knowing that acceleration has the units of ft/sec2 and using Newton’s second law, we can determine that the units of mass are lbf-sec2/ft. For simplification, 1 lbf-sec2/ft is called a slug. The basic unit of mass in the English system is the slug. However, the slug is an almost meaningless unit for the average individual. The unit of mass generally used is the pound-mass (lbm). In order to allow lbm to be used as a unit of mass, we must divide Newton’s second law by the gravitational constant (gc).


Make sense?


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## JoeysVee (Jul 19, 2009)

Yeah, I know they have different units and I know gc is the gravitational constant, but I always thought they had the same numerical value.

ie. when g is in the numerator and gc is in the denominator of an equation, most people just cancel them out and as long as consistent units are used you're ok. But I just noticed last night they have different numerical values so in the truest sense they shouldn't cancel each other out. Even though the numerical values are close, I would think the difference may screw up a small percentage of problems.


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## benbo (Jul 19, 2009)

JoeysVee said:


> Yeah, I know they have different units and I know gc is the gravitational constant, but I always thought they had the same numerical value.
> ie. when g is in the numerator and gc is in the denominator of an equation, most people just cancel them out and as long as consistent units are used you're ok. But I just noticed last night they have different numerical values so in the truest sense they shouldn't cancel each other out. Even though the numerical values are close, I would think the difference may screw up a small percentage of problems.


In my experience, in engineering and physics people almost always cancel things out that are that close. And with that little difference, I'm sure there is someplace on earth where they are equal because g varies all over the earth. As a matter of fact, if you interpolate JRs extreme values for the poles and the equator, it may be 32.17 at the test site (lol). It's sort of like a problem where they give you the weight of somebody. If, in that problem, the person flew in a jet 1000 miles, there are very few problems where you would not just cancel out the two weights.

MAybe in real life there is some place that this matters, but I cannot imagine a problem on the PE exam where this would matter, since the answers they give are usually even fartehr off than that.


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## JoeysVee (Jul 19, 2009)

Yeah...good points. I just didn't realize they were different. Thanks!


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## maryannette (Jul 19, 2009)

Not a dumb question.


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## IlPadrino (Jul 20, 2009)

JoeysVee said:


> Even though the numerical values are close, I would think the difference may screw up a small percentage of problems.


No, I don't think so... I've never seen a problem that had two answers within 1% of each other! Not even close to 1%...


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## JoeysVee (Jul 20, 2009)

Your right. I don't think it will matter on the PE. But I do think it could screw up a few problems (none on the PE). That small difference could have an effect on some equations.

I just thought they were the same numerical value...I was surprised they were different.


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## IlPadrino (Jul 20, 2009)

JoeysVee said:


> Your right. I don't think it will matter on the PE. But I do think it could screw up a few problems (none on the PE). That small difference could have an effect on some equations.
> I just thought they were the same numerical value...I was surprised they were different.


Engineers don't typically deal with 1% difference... on exams or in real life. Can you think of an example off the top of your head?


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## JoeysVee (Jul 20, 2009)

1% can be the difference in life or death in some cases. Yes I can think of examples....I'm in the nuclear business. I got too much studying to do so no time to argue. I'll agree to disagree.


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## Capt Worley PE (Jul 20, 2009)

You must be down at SRS. I heard they were hiring something like 2K people.


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## RIP - VTEnviro (Jul 20, 2009)

IlPadrino said:


> Engineers don't typically deal with 1% difference... on exams or in real life. Can you think of an example off the top of your head?


Oh, this mile long road was supposed to be at a 5% slope? We constructed it at 6%. It's only 53' higher than its supposed to be at the end. Oh well!


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## benbo (Jul 20, 2009)

VTEnviro said:


> Oh, this mile long road was supposed to be at a 5% slope? We constructed it at 6%. It's only 53' higher than its supposed to be at the end. Oh well!


The way I interpet it, the difference between 5% and 6% would be 6-5/5 = 20%.

A 1% difference in a 5% slope would be .01*5 = .05%

That's the way I interpet a 1% error. Eitther way it might still be significant, I guess.


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## Flyer_PE (Jul 20, 2009)

IlPadrino said:


> Engineers don't typically deal with 1% difference... on exams or in real life. Can you think of an example off the top of your head?


When it comes to relay settings, I can think of a lot.


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## FLBuff PE (Jul 20, 2009)

Ooooo....engineer fight! opcorn:


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## RIP - VTEnviro (Jul 20, 2009)

benbo said:


> The way I interpet it, the difference between 5% and 6% would be 6-5/5 = 20%.
> A 1% difference in a 5% slope would be .01*5 = .05%
> 
> That's the way I interpet a 1% error. Eitther way it might still be significant, I guess.


That's a good point.

I tend to deal with site plans. So you see roads and slopes and pipes all to be built to some % grade. I'm used to thinking of it as the contractor's grade is 1 or 2% higher or lower as being off by 1%.

What you're describing is what I'm used to hearing called 'tolerances'. 1/4" per every 10' or something like that.


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## RIP - VTEnviro (Jul 20, 2009)

FLBuff PE said:


> Ooooo....engineer fight! opcorn:


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## IlPadrino (Jul 20, 2009)

Flyer_PE said:


> When it comes to relay settings, I can think of a lot.


That sounds an awful lot like "programming"... and you know what us Real Engineers think about software developers!


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## IlPadrino (Jul 20, 2009)

JoeysVee said:


> 1% can be the difference in life or death in some cases. Yes I can think of examples....I'm in the nuclear business. I got too much studying to do so no time to argue. I'll agree to disagree.


Fair enough... keep your head down!


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## IlPadrino (Jul 20, 2009)

VTEnviro said:


> That's a good point.
> I tend to deal with site plans. So you see roads and slopes and pipes all to be built to some % grade. I'm used to thinking of it as the contractor's grade is 1 or 2% higher or lower as being off by 1%.
> 
> What you're describing is what I'm used to hearing called 'tolerances'. 1/4" per every 10' or something like that.


Yeah... we're talking about the difference between using 32.17 vs 32.088 in an equation. I didn't word it very well. It's like the difference between a design spec saying to use a 5% slope and you used a 5.05% slope. No one's going to die over this!


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## Flyer_PE (Jul 20, 2009)

IlPadrino said:


> That sounds an awful lot like "programming"... and you know what us Real Engineers think about software developers!


Actually, the calculation I'm working on right now is for a plant that has just damn near no operating margin. There are parts of this thing where the level of detail is moronic but the client set the rules and is paying the bills.....


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## RIP - VTEnviro (Jul 20, 2009)

IlPadrino said:


> That sounds an awful lot like "programming"... and you know what us Real Engineers think about software developers!


I know a 'software engineer'. I love waiving my PE-ness in front of him.


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## MGX (Jul 20, 2009)

Flyer_PE said:


> Actually, the calculation I'm working on right now is for a plant that has just damn near no operating margin. There are parts of this thing where the level of detail is moronic but the client set the rules and is paying the bills.....


My favorite clients!



VTEnviro said:


> I know a 'software engineer'. I love waiving my PE-ness in front of him.


I commend you!


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## ferryg (Jul 21, 2009)

IlPadrino said:


> No, I don't think so... I've never seen a problem that had two answers within 1% of each other! Not even close to 1%...



Yes, you CERTAINLY don't need to worry about anything like this on the exam. Questions never vary by this level of precision.

As far as real world scenarios go, I suppose that's what factors of safety are for.


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## ferryg (Jul 21, 2009)

VTEnviro said:


> I know a 'software engineer'. I love waiving my PE-ness in front of him.



I've never understood that, as I thought in a lot of jurisdictions it was not legal practice to refer to one as an "engineer" unless they had the professional credentials to back it up.

I realize the term "software engineer" is common and widespread, but as an EIT, we were NEVER allowed to refer to ourselves as engineers. We were designers...or some other pseudo-term...but NEVER engineer. That was one of the perks of passing...getting your business card changed from designer to engineer! :spboba:


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## ElCid03 (Aug 4, 2009)

ferryg said:


> I've never understood that, as I thought in a lot of jurisdictions it was not legal practice to refer to one as an "engineer" unless they had the professional credentials to back it up.
> I realize the term "software engineer" is common and widespread, but as an EIT, we were NEVER allowed to refer to ourselves as engineers. We were designers...or some other pseudo-term...but NEVER engineer. That was one of the perks of passing...getting your business card changed from designer to engineer! :spboba:



I could not agree more. I know it sounds petty, but it took me ten years of very hard work to become a licensed engineer. It is a noble profession that is thousands of years old, and only those who meet the standards should be allowed to use the title.


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## mattsffrd (Aug 7, 2009)

ElCid03 said:


> I could not agree more. I know it sounds petty, but it took me ten years of very hard work to become a licensed engineer. It is a noble profession that is thousands of years old, and only those who meet the standards should be allowed to use the title.


wait...does that mean janitors can no longer call themselves "sanitation engineers?"


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