In measuring mass and forces in engineering, various systems of units are used. Which system is adopted is a matter of convention and convenience, but the Systeme Internationale or S. I. system is adopted officially in many parts of the world. In this system the unit of mass is the kilogramme, the unit of acceleration is the metre-per- second-per-second, or m/s2. A force, as the second law of motion indicates, is measured in terms of the acceleration of a mass, or Force = Mass x Acceleration. In S. I. units, the unit of force becomes the Newton, one Newton being the force required to accelerate one kilogramme of mass at one m/s2. The mass of a model, on or near the planet Earth, is constantly acted on by the acceleration due to gravity, which has for practical purposes the value of 9.81 m/s2. Hence a model of 1 kg. mass exerts a downward force or weight of 1 x 9.81 Newtons. Metric kitchen scales in common use do not usually read in Newtons, but so long as they are used on Earth, they may be taken as reading kilogrammes directly as units of mass. In aerodynamic figuring, however, the forces must be expressed as Newtons to maintain consistency. Many modellers are accustomed to other systems, such as the British Imperial system, or some variety of it In this, the unit of mass is the slug, of acceleration the foot-per-second-per-second, of force, the pound-force. Scales reading in pounds measure the force exerted by one slug of mass under the influence of Earth’s gravity. The acceleration due to gravity is 32.2 ft/s2.

There are other systems of units. Which is used is a matter of individual preference, but whichever is employed it must be consistent, so that one unit of force always equals one unit of mass multiplied by one unit of acceleration. If this rule is not observed confusion results. A fuller explanation of the rival systems of units with conversion scales may be found in Metrication for the Modeller (M. A.P. Technical Publication, 1972).