64 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



EXAMPLE. What is the velocity of a stone which has fallen 

 from rest for a distance of 100 feet ? 



Here v 2 = 2<js 



. = 2 x 32 x 100 



= 6400 



< = 80 feet per second. 



Parallelogram of Forces. A body can only move in one 

 direction at any given moment ; but yet it can be under the 

 influence of, or be acted upon by, any number of forces. The 



question arises How 

 can the direction be 

 determined in which 

 a body, under the in- 

 fluence of several 

 forces at the same 

 moment, will move, if 

 free to move ? 



Or, the question may 

 be regarded in another 

 way. How can we 

 substitute a single 

 force (called the Re- 

 sultant) which will 

 produce the same 

 effect, for all the sepa- 

 rate forces acting to- 

 FIG. 24. Experiment to illustrate the Parallelo- 

 gram of Forces. gether upon a body at 



the same moment ? 



A few experiments will make the question and its answer 

 quite clear as far as substituting a single force for two separate 

 forces is concerned. 



EXPT. 57. Round two pulleys G and K, or very smooth pegs 

 (Fig. 24), pass a tine thread to which two unequal weights 

 are attached. To some convenient place on the thread tie a 

 third weight as shown. Let the masses come to rest. It will 

 be found that a parallelogram may be constructed, the sides 

 and diagonal of which are nearly proportional to the weights 

 used. They would be just proportional if the pulleys were 

 quite smooth. 



EXPT. 58. Attach a scale of inches to the edge of a black- 



