Unit weight, also known as specific weight, is the amount of weight of an object per unit volume without any other substances being present in that object. Unlike other measurements, unit weight can vary based upon temperature, pressure, and gravity.
Mass Vs. Weight
Mass is a measurement of the amount of matter in an object, whereas weight depends on the gravitational force applied to that object (weight = m*g). In the United States, the force due to gravity on an object's mass is measured in pounds.
Density is defined as an object's mass per unit volume (density = m/V) and depends on temperature as volume can expand or decrease as temperatures vary. At 4 degrees Celsius, for example, water has a density of 1000 kilograms per cubic meter or 62.4 pounds per cubic foot.
Unit weight is equal to the force of gravity multiplied by the object's density.
The Unit Weight of Water
The unit weight, or specific weight, of water on earth depends on the temperature. At 32 degrees Fahrenheit, for instance, water has a unit weight of 62.4 pounds per cubic foot. This weight decreases as temperature increases with water having a unit weight of 62.24 pounds per cubic foot at 60 degrees Fahrenheit and 62.09 pounds per cubic foot at 100 degrees Fahrenheit.
Unit weight is used in fluid mechanics to express gravitational forces as they act on a volume of a fluid. This aids in mechanical engineering, helping to determine the limits of a certain building material.