effects of temperature, altitude, and density of air on aircraft  performance are covered in the following paragraphs.  Density is weight per unit of volume. Since air is a mixture  of gases, it can be compressed. If the air in one container is  under half as much pressure as an equal amount of air in an  identical container, the air under the greater pressure weighs  twice as much as that in the container under lower pressure.  The air under greater pressure is twice as dense as that in  the other container. For the equal weight of air, that which  is under the greater pressure occupies only half the volume  of that under half the pressure. Motion is the act or process of changing place or position.  An object may be in motion with respect to one object and  motionless with respect to another. For example, a person  sitting quietly in an aircraft flying at 200 knots is at rest or  motionless with respect to the aircraft; however, the person  and the aircraft are in motion with respect to the air and to  the earth. The density of gases is governed by the following rules: Thus, air at high altitudes is less dense than air at low  altitudes, and a mass of hot air is less dense than a mass of  cool air.  Changes in density affect the aerodynamic performance of  aircraft with the same horsepower. An aircraft can fly faster at  a high altitude where the density is low than at a low altitude  where the density is greater. This is because air offers less  resistance to the aircraft when it contains a smaller number  of air particles per unit of volume.  Air has no force or power, except pressure, unless it is in  motion. When it is moving, however, its force becomes  apparent. A moving object in motionless air has a force  exerted on it as a result of its own motion. It makes no  difference in the effect then, whether an object is moving  with respect to the air or the air is moving with respect to  the object. The flow of air around an object caused by the  movement of either the air or the object, or both, is called  the relative wind.  The terms “speed” and “velocity” are often used  interchangeably, but they do not have the same meaning.  Speed is the rate of motion in relation to time, and velocity is  the rate of motion in a particular direction in relation to time.  Humidity is the amount of water vapor in the air. The  maximum amount of water vapor that air can hold varies  with the temperature. The higher the temperature of the air,  the more water vapor it can absorb.  An aircraft starts from New York City and flies 10 hours at  an average speed of 260 miles per hour (mph). At the end of  this time, the aircraft may be over the Atlantic Ocean, Pacific  Ocean, Gulf of Mexico, or, if its flight were in a circular path,  it may even be back over New York City. If this same aircraft  flew at a velocity of 260 mph in a southwestward direction,  it would arrive in Los Angeles in about 10 hours. Only the  rate of motion is indicated in the first example and denotes  the speed of the aircraft. In the last example, the particular  direction is included with the rate of motion, thus, denoting  the velocity of the aircraft.  Acceleration is defined as the rate of change of velocity.  An aircraft increasing in velocity is an example of positive  acceleration, while another aircraft reducing its velocity is an  example of negative acceleration, or deceleration. Assuming that the temperature and pressure remain the same,  the density of the air varies inversely with the humidity. On  damp days, the air density is less than on dry days. For this  reason, an aircraft requires a longer runway for takeoff on  damp days than it does on dry days.  By itself, water vapor weighs approximately five-eighths  as much as an equal amount of perfectly dry air. Therefore,  when air contains water vapor, it is not as heavy as dry air  containing no moisture. The fundamental laws governing the action of air about a  wing are known as Newton’s laws of motion.  Newton’s first law is normally referred to as the law of  inertia. It simply means that a body at rest does not move  unless force is applied to it. If a body is moving at uniform  Newton’s Laws of Motion Humidity Velocity and Acceleration Aerodynamics and the Laws of Physics Density The law of conservation of energy states that energy may  neither be created nor destroyed. 1.  Density varies in direct proportion with the pressure. 2.  Density varies inversely with the temperature. 1.  Absolute humidity is the weight of water vapor in a  unit volume of air. 2.  Relative humidity is the ratio, in percent, of the  moisture actually in the air to the moisture it would  hold if it were saturated at the same temperature and  pressure.  2-3