Units
US customary system
The US customary system finds its roots in the British imperial system. Traditionally, this system was based on the measurement of the human body. Today the US customary units are based on SI units. The units that are relevant to the subject of dynamics are given in the following table.
| US customary unit | Symbol | What it measures |
|---|---|---|
| Foot | ft | Distance |
| Slug | slug | Mass |
| Second | s | Time |
| Pound-mass (avoirdupois pound) | lbm | Mass |
| Pound-force | lbf = slug ft/s2 | Force |
| British Thermal Units | BTU | Energy |
| Horsepower | hp | Power |
In the US customary system, there are two units for mass (i.e. slug and pound-mass). A source of confusion comes from the difference between pound-mass and pound-force. One slug equals 32.1740 avoirdupois pounds. At standard conditions, a body weighing 1 pound-force has a mass of 1/32.1740 slug or 1 pound-mass.