70 



THE PENDULUM. 



FIG. 31. 



139. Motion of the Pendulum. When the 



supporting thread or bar is vertical, the centre of gravity 

 is in the lowest possible position, 

 and the pendulum remains at 

 rest, for the force of gravity tends 

 to draw it downward producing 

 pressure at the point of support, 

 but no motion. But when the 

 pendulum is drawn from its ver- 

 tical position, the force of grav- 

 ity, MG, is resolved ( 91) into 

 two components, one of which, 

 MC, produces pressure at the 

 point of support, while the other, 

 MH, acts at right angles to it, 

 producing motion. Gravity there- 

 fore draws it to a vertical position, when inertia carries it 

 beyond until it is stopped and drawn back again by grav- 

 ity. It thus swings to and fro in an arc, MNO. 



140. Definitions. The motion from one extremity 

 of this arc to the other is called a vibration or oscillation. 

 The time occupied in moving over this arc is called the 

 time of vibration or oscillation. The angle measured by 

 this arc is called the amplitude of vibration. The trip 

 from M to is a vibration; the angle MAO is the 

 amplitude of vibration. 



141. Centre of Oscillation. A short pendulum 

 vibrates more rapidly than a long pendulum ; this is a 

 familiar fact. It is evident, then, that in every pendulum 

 (not simple) the parts nearest the centre of suspension tend 

 to move faster than those further away, and force them to 



