Density and Specific Gravity [PDF]

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Density and Specific Gravity By: Vincent Amazarray

Density: • •

Density of a material is defined as its mass per unit volume. (Mass is a property) It is also defined as its weight per unit volume, although, this quantity is more properly called specific weight.

Specific weight: • The specific weight (also known as the unit weight) is the weight per unit volume of a material. Formula of density:

• D = density • W = weight • V = volume >> the units of density are kg/m3 org/cm3 Sample Problems: 1.) You have a different rock with a volume of 30cm3 and a mass of 60g. What is its density? Given: volume = 30cm3 mass = 60g density = ???

Solution:

2.) You have a rock with a volume of 15cm3 and a mass of 45 g. What is its density? Given: v = 15cm3 m = 45g d = ??? Solution:

Specific gravity: •

Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance.

Formula:

or

Sample Problems: 1.) You have a sample of granite with density 2.8 g/cm3. The density of water is 1.0 g/cm3. What is the specific gravity of your granite? Given:

density of the substance = 2.8 g/cm3 density of the water = 1.0 g/cm3 specific gravity = ??? Solution: Specific gravity is the density of the substance divided by the density of water, so

Note that the units cancel, so this answer has no units. We say "the number is unitless.“

2.) You have a sample of granite with density 174.8 lbs/ft3. The density of water is 62.4 lbs/ft3. What is the specific gravity of the granite now? Given: density of the substance = 174.8 lbs/ft3 density of the water = 62.4 lbs/ft3 specific gravity = ??? Solution: Again, the specific gravity is the density of the substance divided by the density of water, so

This shows that the specific gravity does not change when measurements are made in different units, so long as the density of the object and the density of water are in the same units.