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.
