48 FORCE AND MASS. 



1. General Mechanics, and Mechanics of Solid Bodies. 



9. Force and Mass. It has been shown in the pre- 

 ceding article that no change in the state of bodies can 

 take place without special causes. The causes which 

 produce or change motion are called forces. When we 

 observe that unsupported bodies move towards the 

 earth, we say that the earth exerts an attracting force 

 upon bodies, and call this force gravitation. The 

 weight of bodies is a measure of the force of gravitation, 

 and other forces are estimated by comparison with it. 



The laws which regulate the relations between 

 bodies, the forces that act upon them r and the motions 

 produced by these forces in the bodies, may be investi- 

 gated by the, apparatus represented in fig. 43. 



A massive wheel of brass, R, of 100^ weight, with a 

 thin axis of steel, a a, is easily moveable between the 

 points s s. The posterior point is fixed, but the anterior 

 may be adjusted in the proper position by the screw S, 

 and then clamped by the nut G. The axis must 

 revolve with the greatest ease. If it rattles while in 

 motion, the nut G must be undamped by turning it to 

 the left, until it is about 1 millimetre distant from the 

 brass frame , to which it is clamped ; the screw S must 

 then be properly adjusted and held in its correct posi- 

 tion by the hand while the nut G is again firmly 

 clamped. A fine silken cord passes over the wheel and 

 carries two weights P P, at its extremities. Each 

 weight is 70 gr , and is provided with two small hooks on 

 opposite sides. Besides these, four larger weights of 





