Mass Versus Weight - Overview

Overview

Mass corresponds to the general, everyday notion of how "heavy" something is. However, mass is actually an inertial property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force. Per Sir Isaac Newton’s 326-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Inertia is sensed when a bowling ball is pushed horizontally on a level, smooth surface. This is quite distinct from its weight, which is the downwards gravitational force of the bowling ball one must counter when holding it off the floor. Importantly, the weight of the bowling ball on the Moon would be one-sixth of that on the Earth although its mass remained unchanged. Consequently, wherever the physics of recoil kinetics (mass, velocity, inertia, inelastic and elastic collisions) dominate and the influence of gravity is a negligible factor, the behavior of objects remains consistent even where gravity is relatively weak. For instance, billiard balls on a billiards table would scatter and recoil with the same speeds and energies after a break shot on the Moon as on Earth; they would, however, drop into the pockets much more slowly.

In the physical sciences, the terms "mass" and "weight" are rigidly defined as separate measures in order to enforce clarity and precision. In everyday use, given that most masses on Earth have weight and this relationship is usually highly proportional, "weight" often serves to describe both properties, its meaning being dependent upon context. For example, in commerce, the net weight of retail products actually refers to mass and is properly expressed in pounds (U.S.) or kilograms (see also Pound: Use in commerce). Conversely, the load index rating on automobile tires, which specifies the maximum structural load for a tire in kilograms, refers to weight; that is, the force due to gravity. Before the late twentieth century, this distinction was not as strictly applied, even in technical writing, so that expressions such as "molecular weight" (for molecular mass) are still seen.

Because mass and weight are separate quantities, they have different units of measure. In the International System of Units (SI), the kilogram is the unit of mass, and the newton is the unit of force. The non-SI kilogram-force is also a unit of force typically used in the measure of weight. Similarly, the avoirdupois pound, used in both the Imperial system and U.S. customary units, is a unit of mass and its related unit of force is the pound-force.