# Some Definitions

Density is a measure of how packed a substance is. Its symbol is the Greek letter rho, ρ, and it is defined by
 P = m/ ...(1)

where m is the mass of a piece of fluid and V is its volume. Specific gravity is the ratio of the density of something to the density of water:

 ...(2)

(This is a misnomer, by the way, since "specific gravity" has nothing to do with gravity.) The density of almost all biological tissue is approximately the same as the density of water (remember this):

 ...(3)
Pressure is like a push (that is, force), except we also consider the area over which the push is extended. If you are barefoot and step on the sidewalk, the sidewalk pushes up on you with about 1000 N. If you step on an upright tack, it pushes up on you with the same 1000 N. Why then does your face look so different when it's a tack that you stepped on? In this case, pressure is the important concept. Pressure is defined as

 P = F/A ...(4)

where F is a force, and A is the area over which it acts. Consider this definition carefully, so that you do not confuse force and pressure. The point of a tack has a much smaller area than the bottom of your foot, so the pressure on that point is quite large.

The units for pressure are N/m2, and these have a name:

 ...(5)

Pressure can also be measured in pounds per square inch, or psi. The pressure of the atmosphere at sea level varies, but its average is given by

 1 atm = 1.01 x 105 Pa = 14.7 psi ...(6)