DENSITY AND PRESSURE

DENSITY AND PRESSURE

The density of a material is defined as the mass per unit volume.

density  =  r  =  mass/Volume          (the Greek letter rho)

        The units are                      =  kilograms/meter³  =  kg/m³

It’s a measure of how tightly the atoms of a material are packed.   It has nothing to do with the hardness of the material.

Examples:

Material	Density (kg/m3)
air	1.29
ice	917
water	1000
aluminum	2700
lead	11300
gold	19300

        The specific gravity of a material is the ratio of its density to that of water.  For example, the specific gravity of aluminum would be 2.7.  This number is dimensionless.

        The pressure, P, is defined as the ratio of force to area:

        The units of pressure are:  Newtons/meter²  = N/m² = Pascals  =  Pa

Example:

        A hammer supplies a force of 700 N.  The hammer head has an area of 7.1 x 10^-4 m².  What is the pressure?

                                                P  =  F/A

                                                P  =  (700 N/7.1 x 10^-4 m²)

                                                P  =  9.86 x 10⁵  N/m²

                                                P  =  9.86 x 10⁵  Pa

VARIATION OF PRESSURE WITH DEPTH

        If a fluid is at rest, then all points at the same depth must be at the same pressure (otherwise it would be moving!).  However the pressure WILL vary with depth: it will have the weight of the fluid on top of it.

P = pressure,  P_o = pressure due to the air (atmospheric pressure)

        Because the fluid is at rest, the net force will be  _________ ?