Newton's first law states that a body at rest tends to remain at rest. A body in motion tends to remain in motion. This law can be demonstrated easily in everyday use. For example, a parked automobile will remain motionless until some force causes it to move-a body at rest tends to remain at rest. The second portion of the lawa body in motion tends to remain in motion-can be demonstrated only in a theoretical sense. The same car placed in motion would remain in motion (1) if all air resistance were removed, (2) if no friction were in the bearings, and (3) if the surface were perfectly level.

NEWTON'S SECOND LAW

Newton's second law states that an imbalance of force on a body tends to produce an acceleration in the direction of the force. The acceleration, if any, is directly proportional to the force. It is inversely proportional to the mass of the body. This law can be explained by throwing a common softball, The force required to accelerate the ball to a rate of 50 ft/sec'would have to be doubled to obtain an acceleration rate of 100 ft/sec'. However, if the mass of the ball were doubled, the original acceleration rate would be cut in half. You would have 50 ft/sec'reduced to 25 ft/sec'.

NEWTON'S THIRD LAW

Newton's third law states that for every action there is an equal and opposite reaction. You have demonstrated this law if you have ever jumped from a boat up to a dock or a beach. The boat moved opposite to the direction you jumped. The recoil from firing a shotgun is another example of action-reaction. Figure 2-4 depicts these examples.

In an airplane, the greater the mass of air handled by the engine, the more it is accelerated by the engine. The force built up to thrust the plane forward is also greater. In a gas turbine,

Figure 2-4.-Newton's third law of motion.

the thrust velocity can be absorbed by the turbine rotor and converted to mechanical energy. This is done by the addition of more and progressively larger power turbine wheels.

SPEED, VELOCITY, AND ACCELERATION

SPEED is defined as the distance covered per unit of time, such as a car traveling at 60 mph. VELOCITY is speed in a certain direction, such as a car traveling due north at 60 mph. ACCELERATION is the rate at which velocity increases. If, for example, the propeller shaft rate of rotation increases from stop to 100 rpm in 20 minutes, the acceleration is 5 rpm per minute. In other words, the velocity has increased 5 revolutions per minute, during each minute, for a total period of 20 minutes. A body moving at a constant speed has no acceleration. When the velocity of an object increases by the same amount each second or minute, you have uniform acceleration. Uniform deceleration is when the decrease in velocity is the same each second or minute.