54 ELEMENTS OF LABORATORY WORK 



Two equal masses are connected by a silk thread which 

 passes over a pulley turning on friction-wheels. A small 

 additional mass is made to rest on the mass near the scale. 

 This causes an acceleration to be given to the masses, the 

 heavier descending. By trial, the space passed over in one 

 second may be noted, and the moveable stage placed there. 

 Then the commencement and end of the fall will agree with 

 two consecutive ticks of the seconds-clock. The space passed 

 over during two seconds will be found to be four times that 

 passed over during one second, and during three seconds, nine 

 times, and so on. In order to give a long drop the pulley 

 should be fixed as high as possible. 



These observed phenomena, together with those of the last 

 section, may be most simply explained on the supposition that 

 the acceleration which a body acquires by its proximity to the 

 earth is independent of any motion which it may already 

 have. A body, displaced 16 feet from the earth and allowed 

 to fall, has at the moment it is stopped, i.e. at the end of 

 one second, a speed of 32 feet per second, but if it has had a 

 displacement of 64 feet, it will take two seconds to return, and 

 will, at the end of the first second, have a speed of 32 feet 

 per second, with which it commences the last second. Conse- 

 quently it has at the end of two seconds a speed of 32 plus 

 32, i.e. 64, and at the end of t seconds a velocity of 32 t. In 

 using Atwood's machine, the displacement of a body from the 

 earth's surface corresponds with the distance between the 

 starting-point and the stage which checks the fall. 



We may here state Newton's First Law of Motion, which 

 is bound up with the above explanation : 'Every body 

 perseveres in its state of rest or of moving uniformly in a 

 straight line, except so far as it is made to change that state 

 by external force.' 



39. Meaning of the Term Force,' When a body falls to 

 the earth there is a change in the material system comprising 

 the earth and that body. Each must be considered essential 

 to the change that is, it is a mutual change. 



The inevitableness of the return of a body to the earth 

 gives rise to the statement that it is caused by the attraction 



