National Physical Laboratory

What are the differences between mass, weight, force and load? (FAQ - Mass & Density)

Mass is a measure of the amount of material in an object, weight is the gravitational force acting on a body (although for trading purposes it is taken to mean the same as mass), force is a measure of the interaction between bodies and load usually means the force exerted on a surface or body.

Mass

Mass is a measure of the amount of material in an object, being directly related to the number and type of atoms present in the object. Mass does not change with a body's position, movement or alteration of its shape unless material is added or removed. The unit of mass in the SI system is the kilogram (abbreviation kg) which is defined to be equal to the mass of the international prototype of the kilogram held at the Bureau International des Poids et Mesures (BIPM) near Paris. Mass can also be defined as the inertial resistance to acceleration.

Weight

In the trading of goods, weight is taken to mean the same as mass, and is measured in kilograms. Scientifically, however, it is normal to state that the weight of a body is the gravitational force acting on it and hence it should be measured in newtons (abbreviation N), and that this force depends on the local acceleration due to gravity. To add to the confusion, a weight (or weightpiece) is a calibrated mass normally made from a dense metal.

So, unfortunately, weight has three meanings and care should always be taken to appreciate which one is meant in a particular context.

Force

Force is a measure of the interaction between bodies. It takes a number of forms including short-range atomic forces, electromagnetic, and gravitational forces. Force is a vector quantity, with both direction and magnitude.

Load

Load is a term frequently used in engineering to mean the force exerted on a surface or body.

Last Updated: 25 Mar 2010
Created: 9 Aug 2007

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