The difference between lbm and lbf

I know that to many people this is probably a simple concept but when I recently looked up "the difference between lbm and lbf" on Google, I received some crappy/ambiguous explanations so I decided to write this article and demystify it once and for all.

For those of you who are unaware, SI = System International and its base units are: kg, m, s, K, mol, A, cd. On the other hand, lbm = "pound mass" and lbf = "pound force" are from the AE (American Engineering) unit system.

Now that that's over, from Newton's second law, we know that:



Where a stands for acceleration and is most commonly a = g = 9.81 m/s2 at the Earth's surface. Here we have used SI units.

Now, in AE units:



Where a = g = 32.19 ft/s2 at the Earth's surface.
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The mass of an object, whether in kg or lbm, does not change. Acceleration due to gravity changes depending upon whether the object in question is on the surface of the Earth, on Mars, on the Moon or 3km beneath the ocean.

Weight is measured in N or lbf.
Mass is measured in kg or lbm.
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Hopefully this highlights to you that the lbf is simply the equivalent of the Newton, N, and that the lbm is the equivalent of the kg.

Below is a conversion example.

Convert 5 N into lbf:



Hopefully that clears it up. Please do comment if anything is amiss.

K