by Guest » Wed Mar 22, 2006 2:21 am
The pound can be either a unit of mass, or a unit of force, specifically the force of standard gravity on a one pound mass (note this relationship would not hold on the moon.)
Thus, it can be hard to determine exactly what is being discussed. Not all engineers do this, but many using Imperial units insist on labelling lbf and lbm for pounds-force and pounds-mass respectively to ensure there is no misunderstanding of what is meant.
In a torque, pound-mass makes little sense so pound-force is meant. There is really no difference between a ft-lb and a ft-lbf except clarity. I assume ft-lbs is a messy abbreviation formed from the plural.
In the equation f = ma, using pounds for both mass and force is a major drawback of Imperial. At least three concepts are used by different groups of engineers:
1) Pound for force only, introduce the artifical unit "slug" for mass (about 32 lb.) so 1 slug accelerated at 1 ft/sec^2 requires a force of 1 lb. Invented by British, but more common in US.
2) Pound for mass only, the artifical unit "poundal" for force. 1 poundal is force to accelerate one pound mass at 1 ft/sec^2. The UK uses this more than US, or I should say, used to use.
The poundal is about 1/32th of a pound-force.
3) Pound mass, and pound force. The f=ma equation is modified to include standard gravity which should be g-sub-zero but I can't do it here. f = m*(a/g). Accelerating one pound at 1 g requires one pound of force. Aerospace seems to like this one.
The fourth solution is to go metric and many US industries have (the auto industry for example). The three groups of engineers who use the first three solutions confuse the hell out of each other. (I am a member of the fourth group)
Many other standard equations need to be patched for pound as force and mass. I was trained in metric and frankly don't know how to do them all. I was entirely baffled by an "English" form of Bernoulli's equation for example.
The abbreviation lb for pound comes from the Latin word libra, a weight more or less a pound.