In the SI (metric) system of units, kilograms are used for mass, and newtons for weight. There is no confusion as the units are different.
However, in the US system, pounds are used for both mass and weight. Confusing? You didn’t think it was going to be easy did you?
To differentiate between the two, I have developed the following table.
| Property | Mass | Weight |
|---|---|---|
| SI Unit | Kilograms (kg) | Newtons (N) = kg·m/s2 |
| US Unit | Pounds-Mass (lb·m) | Pounds-Force (lb·f) = lbm·ft/s2 |
| Formula | W = mg | |
| Mass is a constant property of an object | Weight varies with the gravitational field |
It’s not as complicated as you think. Every time you see the unit of pounds, it may or may not have the acceleration of gravity included in it: W = mg. Unfortunately, you simply have to know whether you are dealing in mass or weight. On the bright side, it’s usually fairly obvious (and the units have to work).
g = 9.81 m/s2 = 32.09 ft/s2.
Daniel says
November 21, 2019 at 8:03 pmThat was a great answer. I loved how you encouraged the person with “it’s not as complicated as you think”. You a should have added..”just wait” There is the hundredweight (Cwt). There is the metric which is 50kg. The other tons are always 20 (Cwt). The US hundredweight is 100 lb (avdp) or 44.5 kg and the British (or imperial) Hundredweight is 112 lb (avdp). Which obviously eventually gives rise to the short ton of 2000 lb (US) and the long ton 2240 lb (UK) and obviously the metric ton which is actually the tonne which is 2204 lb (avdp). The French with the SI had to confuse everything!
Shariq Bin Aleem says
December 22, 2019 at 12:23 amPlease correct me if i am wrong, 1 lbf= 1 slug * 1 ft/s2 (in place of 1 lbf=1 lbm * 1 ft/s2), as 1 slug = 32 lbm