1.3. LAWS OF MOTION
The theory of gas turbine engines is based on the laws and principles of physics discussed in the subparagraphs that follow.
Newton's First Law of Motion. The first law states that a body in a state of rest remains at rest, and a body in motion tends to remain in motion at a constant speed and in a straight line, unless acted upon by some external force.
Newton's Second Law of Motion. The second law states that an imbalance of forces on a body produces or tends to produce an acceleration in the direction of the greater force, and the acceleration is directly proportional to the force and inversely proportional to the mass of the body.
Newton's Third Law of Motion. The third law states that for every action there is an equal and opposite reaction, and the two are directed along the same straight line.
Bernoulli's Principle. This principle states that if the velocity of a gas or liquid is increased its pressure will decrease. The opposite is also true. If the velocity of a gas or liquid is decreased its pressure will increase. This fact relates directly to the law of conservation of energy.
Einstein's Law of Conservation of Energy. This law states that the amount of energy in the universe remains constant. It is not possible to create or destroy energy; however, it may be transformed.
Boyle's Law. This law states that if the temperature of a confined gas is not changed, the pressure will increase in direct relationship to a decrease in volume. The opposite is also true -- the pressure will decrease as the volume is increased. A simple demonstration of how this works may be made with a toy balloon. If you squeeze the balloon, its volume is reduced, and the pressure of air inside the balloon is increased. If you squeeze hard enough, the pressure will burst the balloon.
Charles' Law. This law states that if a gas under constant pressure is so confined that it may expand, an increase in the temperature will cause an increase in volume. If you hold the inflated balloon over a stove, the increase in temperature will cause the air to expand and, if the heat is sufficiently great, the balloon will burst. Thus, the heat of combustion expands the air available within the combustion chamber of a gas turbine engine.
Pressure and Velocity. Air is normally thought of in relation to its temperature, pressure, and volume. Within a gas turbine engine the air is put into motion so now another factor must be considered, velocity. Consider a constant airflow through a duct. As long as the duct cross-sectional area remains unchanged, air will continue to flow at the same rate (disregard frictional loss). If the cross-sectional area of the duct should become smaller (convergent area), the airflow must increase velocity if it is to continue to flow the same number of pounds per second of airflow (Bernoulli's Principle). In order to obtain the necessary velocity energy to accomplish this, the air must give up some pressure and temperature energy (law of conservation of energy). The net result of flow through this restriction would be a decrease in pressure and temperature and an increase in velocity. The opposite would be true if air were to flow from a smaller into a larger duct (divergent area); velocity would then decrease, and pressure and temperature would increase. The throat of an automobile carburetor is a good example of the effect of airflow through a restriction (venturi); even on the hottest day the center portion of the carburetor feels cool. Convergent and divergent areas are used throughout a gas turbine engine to control pressure and velocity of the air-gas stream as it flows through the engine.