We can say that the volume of the object is directly proportional to its mass. As the volume increases the mass of the object increases in direct proportion. The gradient of the graph equals the density of the material.
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Density, mass of a unit volume of a material substance. The formula for density is d = M/V, where d is density, M is mass, and V is volume. Density is commonly expressed in units of grams per cubic centimetre.
(Density is calculated by dividing an object's mass by its volume.)