Density and Pressure

Definitions:

• The density ( ) of an object of total mass M and volume V is given by:

  $$\rho {M \over V}$$ (5)

  The units are usually taken to be kg/m ³.
• The specific gravity of a substance is the ratio of the density of the substance to the density of water ( $\rho_{{\:\rm water}}^{}$ = 1 x 10³ kg/m ³).
• Pressure: When an object is immersed in a fluid or gas, it experiences a uniform inward force over its entire surface, due essentially to the weight of the fluid sitting on top of it. The pressure on the object is defined to be the force per unit area:

  $${F\over A}$$ (6)

  The units of pressure are normally given in Pascals ( Pa ) where:

  1 Pa = 1 N/m ². (7)

Note: At the earth's surface, the pressure felt by an object due to the weight of the atmosphere above it is:

1 atmosphere = 1.01 x 10⁵   Pa. (8)