AIRSHIP AERODYNAMICS  TM 1-320  7  the atmosphere, with density and specific gravity calculations, and  with the action of gravitational forces.  These matters are discussed  in  TM  1-325.  b.  Whenever a solid object moves through a fluid it encounters a  resistance to its motion.  This resistance may be considered from two  points of view.  (1)  Momentum theory.-(a)  By Newton’s first law, a body at  rest  or in motion will remain at rest or continue to travel at constant  velocity unless some force is exerted to change its condition.  To  enable the solid to maintain its motion relative to the fluid, the  molecules of the fl uid must be deflected to make room for t he passage  of the solid.  So to deflect the fluid or air a force must be applied.  I n the case of the airship this force is that furnished by the propeller  thrust.  (b ) It  can be proved mathematically that if air were incompressible  and nonviscous, that is, incapable of offering resistance to shear be› tween the particles, the thrust of air particles opposing the motion  of the solid would exactly equal the thrust of the air assisting the  motion.  H ence there would be no resistance to the motion.  How› ever, in th& atmosphere this ideal condition does not exist and the  resistance is proportional to the total kinetic energy of the deflected  particles of air.  15  FIGURE 4.-U. S. Nnvy rigid  Los  .d.t~geles.  •  •  ’ 