Dumb question here...

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JoeysVee said:
Why is the numerical value for gc, 32.17 but g is 32.088????
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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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JoeysVee said:
Why is the numerical value for gc, 32.17 but g is 32.088????
Click to expand...
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).
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Make sense?

 
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.

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

 
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.

 
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.
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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?

 
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.

 
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!
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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.

 
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.
Click to expand...
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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