APPLICATIONS TO MECHANICS 



87 



In order that the phenomenon may appear more 

 striking, let us unite two bodies of unequal mass by a 

 string and throw them, giving them a twist at the same 

 time. These two bodies (Fig. 57) will rotate round 

 each other like a star and its satellite, but neither one 

 nor the other will follow a parabolic trajectory ; but 



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Fig. 57.— Movement of a sy-tem of two balls bound togetht-r by a siring. 



the centre of gravity of the system, which they together 

 constitute, will move exactly in that path. 



Now, in experiments of this kind, one can show how 

 the resistance of the air modifies the movement of the 

 object. Let us examine, for instance, the trajectory of 

 a round projectile, which is thrown in a horizontal 

 direction. Let us construct a diagram of this move- 



Fig. 5S.— Trajectory of a projectile in respect to two axes (negative image). 



ment (Fig. 58), and let us find the relationship which 

 each position of the object bears to two axes at right 

 angles to one another. If the resistance of the air does 

 not interfere with the horizontal movement forward 

 the latter should be uniformly maintained. Xow, if 

 the last section in the horizontal direction be measured 



