( 221 



CHAPTER M. 



FLEXIBLE IN EXTENSIBLE 8TRIXG8. 



162. A STRING ifl said to be perfectly flexible when any 

 force, however small, which is applied otherwise than along 

 the direction of the string will change its form. For short- 

 ness, we use the word JfariMc as equivalent to perfictly 

 flexible* Sometimes the word chain is used as synonymous 

 with ttring. 



If a (I* ing be kept in equilibrium by two forces, 



one at each end, we assume as scl: that those forces 



be equal and act in opposite directions, so that the 



: assumes the form of a straight line in the direction of 



the forces. In this case the tension of the string is measured 



by the force applied at one end. 



Let ABC represent a string kept in equilibrium by a 



force T at one end A and an equal force T , 



at the othi-r end C acting in opposite direc- A * 



tions alone the line A C. Since any portion AD of the string* 

 mi it follows that a force T must act on AD 

 II towards C in order to balance the force a 



;i force T must a // to- 



wards .4 i hat #C may be in equilibrium. Tin* result 



is expressed by saying that the tension of the string it the same 

 throughout, 



contrary be expressed, a string is supposed 

 inextensiMe and the boundary of a transverse section ot 

 supposed to be a curve, every chord of which is indefinitely 

 small. 



When a flexible string is acted on by other forces 

 besides one at each end it may in equilibrium assume a 

 curvilinear form. If at any point of t - L we suppose 



