Matter - Physics for Aviation

Matter is the foundation, or the building blocks, for any discussion of physics. According to the dictionary, matter is what all things are made of; whatever occupies space, has mass, and is perceptible to the senses in some way. According to the Law of Conservation, matter cannot be created or destroyed, although it is possible to change its physical state. When liquid gasoline vaporizes and mixes with air, and then burns, it might seem that this piece of matter has disappeared and no longer exists. Although it no longer exists in the state of liquid gasoline, the matter still exists in the form of the gases given off by the burning fuel.

Characteristics of Matter

Mass and Weight

Mass is a measure of the quantity of matter in an object. In other words, how many molecules are in the object, how many atoms are in the object, or to be more specific, how many protons, neutrons, and electrons are in the object. The mass of an object does not change regardless of where you take it in the universe, or with a change of state. The only way to change the mass of an object is to add or take away atoms. Mathematically, mass can be stated as follows:

Mass = Weight ÷ Acceleration due to gravity

The acceleration due to gravity here on earth is 32.2 feet per second per second (32.2 fps/s). An object weighing 32.2 pounds (lb) here on earth is said to have a mass of 1 slug. A slug is a quantity of mass that will accelerate at a rate of 1 ft. /s2 when a force of 1 pound is applied. In other words, under standard atmospheric condition, which is that gravity is equal to 32.2 fps/s, a mass of one slug would be equal to 32.2 lb.

Weight is a measure of the pull from gravity acting on the mass of an object. The more mass an object has, the more it will weigh under the earth’s force of gravity. The only way for an object to be weightless is for gravity to go away, because it is not possible for the mass of an object to disappear. When we view astronauts on the space shuttle, it appears that they are weightless. Even though the shuttle is far from the surface of the earth, the force of gravity has not completely gone away, and the astronauts are not weightless. The astronauts and the space shuttle are actually in a state of free fall, so relative to the shuttle the astronauts appear to be weightless. Mathematically, weight can be stated as follows:

Weight = Mass × Gravity


Attraction is mutual force acting between particles of matter, which tends to draw them together. Sir Isaac Newton called this the “Law of Universal Gravitation.” Newton showed how each particle of matter attracts every other particle, how people are bound to the earth, and how the planets are attracted in the solar system.


Porosity means having pores or spaces where smaller particles may fit when a mixture takes place. This is sometimes referred to as granular—consisting or appearing to consist of small grains or granules.


Impenetrability means that no two objects can occupy the same place at the same time. Thus, two portions of matter cannot at the same time occupy the same space.


The density of a substance is its weight per unit volume. The unit volume selected for use in the English system of measurement is 1 cubic foot (ft3). In the metric system, it is 1 cubic centimeter (cm3). Therefore, density is expressed in pounds per cubic foot (lb⁄ft3) or in grams per cubic centimeter (g⁄cm3).

To find the density of a substance, its weight and volume must be known. Its weight is then divided by its volume to find the weight per unit volume. For example, the liquid which fills a certain container weighs 1,497.6 lb. The container is 4 ft long, 3 ft wide and 2 ft deep. Its volume is 24 ft3 (4 ft. × 3 ft. × 2 ft.). If 24 ft3 of liquid weighs 1,497.6 lb, then 1 ft3 weighs 1,497.6 ÷ 24, or 62.4 lb. Therefore, the density of the liquid is 62.4 lb/ft3. This is the density of water at 4 °C (Centigrade) and is usually used as the standard for comparing densities of other substances. In the metric system, the density of water is 1 g⁄cm3. The standard temperature of 4 °C is used when measuring the density of liquids and solids. Changes in temperature will not change the weight of a substance, but will change the volume of the substance by expansion or contraction, thus changing its weight per unit volume.

The procedure for finding density applies to all substances; however, it is necessary to consider the pressure when finding the density of gases. Pressure is more critical when measuring the density of gases than it is for other substances. The density of a gas increases in direct proportion to the pressure exerted on it. Standard conditions for the measurement of the densities of gases have been established at 0 °C for temperature and a pressure of 76 cm of mercury (Hg), which is the average pressure of the atmosphere at sea level. Density is computed based on these conditions for all gases.

Specific Gravity

It is often necessary to compare the density of one substance with another substance. For this purpose, a standard is needed. Water is the standard that physicists have chosen to use when comparing the densities of all liquids and solids. For gases, air is most commonly used, but hydrogen is also sometimes used as a standard for gases. In physics, the word “specific” implies a ratio. Thus, specific gravity is calculated by comparing the weight of a definite volume of the given substance with the weight of an equal volume of water. The terms “specific weight” or “specific density” are sometimes used to express this ratio.

The following formulas are used to find the specific gravity of liquids and solids.

Formula of liquids and solids specific gravity

The same formulas are used to find the density of gases by substituting air or hydrogen for water.

Specific gravity is not expressed in units, but as pure numbers. For example, if a certain hydraulic fluid has a specific gravity of 0.8, 1 ft3 of the liquid weighs 0.8 times as much as 1 ft3 of water: 62.4 times 0.8, or 49.92 lb.

Specific gravity and density are independent of the size of the sample under consideration and depend only upon the substance of which it is made. See Figure 1 for typical values of specific gravity for various substances.

Matter - Physics for Aviation
Figure 1.Specific gravity of various substances

A device called a hydrometer is used for measuring specific gravity of liquids. This device consists of a tubular glass float contained in a larger glass tube. [Figure 2] The larger glass tube provides the container for the liquid. A rubber suction bulb draws the liquid up into the container. There must be enough liquid raising the float to prevent it from touching the bottom. The float is weighted and has a vertically graduated scale.

Matter - Physics for Aviation
Figure 2. Hydrometer for checking battery specific gravity

To determine specific gravity, the scale is read at the surface of the liquid in which the float is immersed. An indication of 1000 is read when the float is immersed in pure water. When immersed in a liquid of greater density, the float rises, indicating a greater specific gravity. For liquids of lesser density, the float sinks, indicating a lower specific gravity. An example of the use of the hydrometer is to determine the specific gravity of the electrolyte (battery liquid) in an aircraft battery. When a battery is discharged, the calibrated float immersed in the electrolyte will indicate approximately 1150. The indication of a charged battery is between 1275 and 1310. The values 1150, 1275, and 1310 represent 1.150, 1.275, and 1.310. The electrolyte in a discharged battery is 1.15 times denser than water, and in a charged battery 1.275 to 1.31 times denser than water.

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