THE MEASUREMENT OF ROTARY MOTION 59 



If the forward motion of a body, either on earth or in the 

 heavens, is great compared with the force of attraction, the 

 body passes beyond the effective action of the latter force, 

 and, in the case of the heavenly bodies, may never return. 

 Some comets act in this manner. See Section 57, Comets. 



References : 



1. 1803 : 19-20. Pendulum Motion Analyzed. 



a. 1801 : 60. Curvilinear Motion. 



b. 1804 : 24-25. Rectilinear and Curvilinear Motion. 



c. 1805 : 63. Motion of a Pendulum. 



d. 1807 : 93-97. Curvilinear Motion. 



e. 1808 : 49. Curvilinear Motion. 

 /. 1809 : 113. Curvilinear Motion. 



EXPERIMENT FOR THE HOME 



Swing a stone at the end of a weak string. Note the pull 

 which is necessary to make the stone continue in a circular 

 path. 



Make the stone revolve faster until the string breaks. In 

 this case the forward motion is too great for the force acting 

 at right angles to it to be affected by the pull of the string. 

 Note that the stone moves off in a straight path. This is due 

 to the fact that the effect of the string ceases as soon as the 

 string breaks. Gravitation decreases gradually, and, even in 

 the case of very rapidly moving comets, produces a slightly 

 curved path. 



42. THE MEASUREMENT OF ROTARY MOTION 



The motion produced by turning a body on an axis is called 

 rotary motion. If a certain point on the outside of the body 

 moves until it returns to its original position, we say that the 



