CHAPTER VI 



APPLICATIONS OF CHRONOPIIOTOGRAPEIY TO 

 MECHANICS 



Summary. — Bodies falling in air— Ballistic experiments— The resist- 

 ance of the air to surfaces variously inclined — Applications of 

 chronophotography to hydrodynamics— Fluid veins ; changes in 

 shape of fluid waves: intrinsic movements of fluid waves — 

 Currents and eddies— Influence of the shape of bodies placed 

 in currents— Oscillations and vibrations— Rolling of ships- 

 Vibrations of metal bridges. 



Bodies falling in Air. — To determine the movement 

 of a falling body is one of the most difficult problems 

 in dynamics. It may be said that Galileo's classical 

 experiment was the origin of all experimental mechanics, 

 for it taught us that a force could be measured by the 

 motion it imparted to a material body. 



Motion which is of uniform acceleration implies the 

 absence of resistance; but when a body falls through 

 the air, the resistance of the latter modifies the law of 

 motion : it increases as the square of the velocity, and 

 finally becomes equal to the force of gravity itself. 

 At that moment the fall becomes uniform, that is 

 to say, the resistance of the air is equal to the weight 

 of the body. 



Chronophotography would be a quick and easy 

 method of measuring the resistance offered by the air 

 to bodies of various forms, and moving with various 

 degrees of velocity ; but experiments of this kind 



