The specific volume of a system in a gravitational field may vary from point to point. The specific volume increases as the elevation increases. Therefore the definition of specific volume involves the specific volume of a substance at a point in a system. The specific gravity of a substance is the ratio of its weight to the weight of an equal volume of water. The amount of mass in a unit volume of the substance.
Density is also the reciprocal of the specific volume.
The specific weight of a substance is the force with which a gravitational body such as the earth attracts a unit volume of the substance. It is expressed as pounds per cubic foot or pounds per gallon. It can also be determined by multiplying the density of the fluid by the force of gravity. The viscosity of a fluid is associated with the resistance to the sliding motion of one fluid layer over another. If the layers move relatively easy with respect to each other, the shear force is small, giving rise to low viscosity fluids.