. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . nis being turned into work and the force transmitted through thedynamometer is less than the amount of the resistance whichis actually being overcome. On the other hand, when thevelocity is increasing, the dynamometer indicates a largerforce than that required to overcome the resistances, but theexcess force is being stored up in the train as kinetic has a similar effect, and the force indicated by the dy-namometer may be greater or less than that required at theg
. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . nis being turned into work and the force transmitted through thedynamometer is less than the amount of the resistance whichis actually being overcome. On the other hand, when thevelocity is increasing, the dynamometer indicates a largerforce than that required to overcome the resistances, but theexcess force is being stored up in the train as kinetic has a similar effect, and the force indicated by the dy-namometer may be greater or less than that required at thegiven velocity on a level by the force which is derived from,or is turned into, potential energy. Therefore the resistanceindicated by the dynamometer of a train will not be that on alevel track at uniform velocity, unless the track is actuallylevel and the velocity really uniform. Dynamometer tests under other circumstances are there-fore of no value unless it is possible to determine the truevelocity at any instant and its rate of change, and also to de-termine the grade. Of course, the grade is easily found. An. 20 ;30 40 50 Velocity in miles per hour {To face page 4IS.) GO § 350. TRAIN RESISTANCE. 419 allowance for an increase or decrease of kinetic or potentialenergy must therefore be made before it is possible to knowhow much force is being spent on the ordinary resistances. 349. Gravity or drop tests. Dynamometer tests requirethe use of a dynamometer which is capable of measuring aforce of several thousands of pounds, and w^hich thereforecannot determine such values with a close percentage of accu-T^cy, especially if the force is small. A drop test utilizes theforce of gravity w^hich may be measured wdth mathematicalaccuracy. The general method is to select a stretch of trackAvhich has a uniform grade of about and which is prefer-ably straight for two or three miles. On such a grade carswdth running gear in good condition may be started by a velocity will g
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