# ft-lbm vs ft-lbf



## tmacier (Sep 12, 2010)

Working thru NCEES Problem # 125 and they appear to convert:

1hp = 33,000 ft-lbm/min

I recall many caution flags being thrown during schooling not to mix lbm and lbf, and the only things I can find show:

1hp = 33,000 ft-lbf/min

Could someone elaborate on what I am missing here?

Thanks in advance for your time.

Tim


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## Sschell (Sep 12, 2010)

power = force x velocity... so lbf is correct...

of course, we are on earth, so g ~ gc; thus lbm(g/gc) ~ lbf... but you certaintly need to keep track of your units, and the book not showing this step is poor form.


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## navyasw02 (Sep 12, 2010)

sschell_PE said:


> power = force x velocity... so lbf is correct...
> of course, we are on earth, so g ~ gc; thus lbm(g/gc) ~ lbf... but you certaintly need to keep track of your units, and the book not showing this step is poor form.


"Mama says English units are the devil."

-Bobby Boucher, PE


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## civilized_naah (Sep 12, 2010)

navyasw02 said:


> "Mama says English units are the devil."
> -Bobby Boucher, PE


Of course, what really gets me is that we still call them ENGLISH UNITS, when the English switched to SI more than 30 years ago !!!


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## tmacier (Sep 12, 2010)

Thank you for the speedy response-

Tim


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## navyasw02 (Sep 12, 2010)

civilized_naah said:


> navyasw02 said:
> 
> 
> > "Mama says English units are the devil."
> ...


Familiarity with english units has amounted to about half of the mistakes I've made on practice problems. Almost all of my undergrad and grad school courses used SI.


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## Sschell (Sep 13, 2010)

^me too, I learned everything in SI... then I got a job and had to get uesd to doing it all in American  units.


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## mizzoueng (Sep 13, 2010)

I thought that during the test lbf = lbm. Isn't that one of the caviats they show in the front of the MERM? I don't have my copy on me or I would check.


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## Sschell (Sep 13, 2010)

it may be, but it is still important to track your units, switching freely between f and m can cause confusion in some cases.


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## jldavis2 (Sep 13, 2010)

sschell_PE said:


> it may be, but it is still important to track your units, switching freely between f and m can cause confusion in some cases.


yeah there's a whole section devoted to it in PPI's ref manuals from what I remember. I found that as tedious as it is, tracking my units all the way through during prep made me think about what units are logical, and therefore actually understand the concepts better.


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## Guest (Aug 31, 2020)

So thatвЂ™s where the trouble lies! : http://yetiboots.site


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