A substance's mass per unit volume.
Measure of the amount of matter a substance contains.
The amount of space taken up by a substance.
Weights on a Balance
Two weights balanced on a beam on a pivot at its center, positioned at equal distances from the pivot, have equal mass.
0 = W1
If they differ in weight, the heaver weight has to be closer to the pivot. Also the weights multiplied by their distances from the fulcrum are equal.
1 \times d1 = W0 \times d0