132 THE WHIRLING/TABLE. 



the moment when the body is at/ J., the direction of i 

 motion is that of the arrow b ; aid, if not constrained by 

 the string, the body would move in virtue of its inertia 

 towards B. The string, which compels it to move to D, 

 must thus clearly be puUed outwards by the body 

 which tends to move towards B. Arrived at D, the 

 direction of motion is that of the arrow c, but the body 

 is again constrained by the string to maintain the same 

 distance from (7, and cannot move in a straight line as it 

 would do if moving in virtue of its inertia alone. If the 

 string breaks, or the hand lets go, as in the case of a 

 ball thrown by means of a sling, the motion becomes 

 at once rectilinear. 1 



Experiments on centrifugal force may be performed 

 by means of the apparatus shown in fig. 95, usually 



G 



FIG. 95 (an. proj. | real size). 



called a ' whirling -table/ It serves also for experiments 

 on sound and light, and should be purchased from an 

 instrument-maker. On a board B B, which rests upon a 

 rectangular rim or frame Z Z, a circular plate of iron, 



1 The line which in this case gives the direction of motion at any instant, 

 is called a tanyent to the circle. It is a straight line which has only one 

 point common with the circle. Every tangent is perpendicular to the radius 

 which is drawn from that point (the ' point of contact ') to the centre of the 

 circle. Thus h is perpendicular to A C, c to D C. 



