MECHANICS 257 



a unit of time. A body falling under the action of gravity is 

 a case of uniformly accelerated motion, the acceleration being 

 equal to 32.16 ft. per sec. and being usually denoted by g. 

 Let / = number of seconds that the body falls; 



v velocity, in feet per second, at the end of the time t; 

 h = distance that the body falls during the time t. 

 Then, 



2h , / 



v = g t = = V2/z = 8.02 V*. 



Vt gP V* 



*---- -.015547* 



CENTRIFUGAL FORCE 



Let F = centrifugal force, in pounds; 



W= weight of revolving body, in pounds; 

 r = distance from the axis of motion to the center of 



gravity of the body, in feet; 

 v = velocity, in feet per second. 

 Then, 



Wv* 

 gr 



When the track on a bridge is curved, the moving cars 

 exert on the bridge a lateral thrust, equal to F, that has to 

 be taken by the lateral bracing of the bridge. In applying the 

 preceding formula, W is to be taken as the maximum weight 

 of the live load for which the chords of the bridge are designed ; 



5,730 

 v is usually expressed in miles per hour and r= , D being 



the degree of curvature. The formula then becomes 

 F=. 00001167s 2 DW 



For curves of 4 or under, v is usually taken as 60 mi. per hr., 

 and usually for D exceeding 4, v = 60 2D. 



W is to be assumed as acting 5 ft. above the base of the 

 rail. The overturning moment due to the force F is therefore 



