Mass is the quantity of matter contained in a body. When we speak of a pound of lead, the word pound expresses a definite quantity of matter. Commercially, weight always stands for mass. A merchant estimates his stock in pounds and usually understands by those weights nothing more than the quantity of matter possessed. The unit of mass is the quantity of matter in a standard pound.

The weight of a body is the measure of the force attracting it towards the center of the earth. Figure 7 illustrates the principle of ordinary weighing scales.