ft-lbm vs ft-lbf

[tmacier]

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

 

[Sschell]

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.

 

[navyasw02]

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

 

[civilized_naah]

"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 !!!

 

[tmacier]

Thank you for the speedy response-

Tim

 

[navyasw02]

"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 !!!

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.

 

[Sschell]

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

 

[mizzoueng]

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.

 

[Sschell]

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

 

[jldavis2]

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.

 

[Guest]

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