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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 

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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. 

