Specific Gravity (SG) is a case of relative density. It defines the ratio of the density of a given substance to the density of water (H2O). Substances with a specific gravity greater than one are heavier than water, and those with less than one are lighter than water.
- Relative Density is the ratio of the densities of two materials. The specific gravity is similar but not the same. In specific gravity, the reference is water.
- Gravity is the attractive force between the earth and all other objects When you release a stone from your hand, the stone falls downward. When you drive a car uphill, part of the engine power is used to raise the car against gravity. Car coasts down a hill with the engine not running and the brakes off. The gravity pulls the car downward.
- A vacuum is the absence of air. When a piston moves down in a cylinder, the piston creates a partial vacuum. Atmospheric pressure then pushes air into the vacuum.