All About Engines

I^2 All About Engines direction, of the velocity of the substance. These two facts are characteristic of the impulse turbine, and because the work done is due to a change of velocity, impulse turbines are often referred to as “ velocity ” turbines. They are so important that it is worth while repeating them in another way. There is no change of pressure from the time the steam impinges upon the vane to the time it leaves it. If the vane is fixed the velocity of the steam changes only in direction ; if the vane moves, the velocity changes both in direc- tion and in magnitude—it becomes lower. The calculation of the force exerted upon a vane is a problem for the engineer, but a general idea can be obtained by the use of no more mechanics than many boys already know. An indication of the method may be given. To start a body moving or to alter its velocity requires force, and as, according to Newton’s Third Law of Motion, to every action there is an equal and opposite reaction, a moving body exerts force when- ever its velocity is altered. The product of the mass and the velocity of a, body is called its moMicntuwi, and the force exerted by a. moving' body is measured by the change which its momentum undergoes in unit time, or the rate of change of momentum. Thus, if m lb. of steam are moving with a velocity of v feet per second, its momentum is m x v units. If, however, the change of momentum is measured in this way, the force will be given in poundals, and to obtain its value in pounds it must be divided by