The systems of dimensions and units used in mechanics are based on Newton's second law of motion, which is force equals mass multiplied by acceleration, or

for consistent systems of units, where F is force, m is mass, and a is acceleration.

In the English unit system, engineers define a pound of force as the force required to accelerate 1 slug of mass at the rate of 1 foot per second per second. One slug of mass has a weight of approximately 32.2 lb when acted upon by the acceleration of gravity present at the surface of the Earth. Thus, Equation (A-1) in English units is

In the International System of Units (or SI metric), engineers define a new ton of force as the force required to accelerate 1 kilogram of mass at the rate of 1 meter per second per second. Thus, Equation (A-1) in the SI metric is

Physicists, however, utilize a version of the SI metric that defines a dyne of force as the force required to accelerate 1 gram of mass at the rate of 1 centimeter per second per second.

Unfortunately, these different systems tend to create confusion. In many parts of the world engineers use the kilogram for both force and mass units. With universal adoption of metric SI, however, this confusion should gradually disappear.

Any system based on length (L), mass (M), and time (T) is absolute because it is independent of the gravitational acceleration g. A system based on...