168. I have here a wooden pulley 3"·5 in diameter; the hole is lined
with brass, and the pulley turns very freely on an iron spindle. I
place the rope and hooks upon the groove. Brass rubbing on iron has
but little friction, and when 7 lbs. is placed on each hook, 0·5 lb.
added to either will make it descend and raise up the other. Let 14
lbs. be placed on each hook, 0·5 lb. is no longer sufficient; 1 lb.
is required: hence when the weight is doubled the friction is also
doubled. Repeating the experiment with 21 lbs. and 28 lbs. on each
side, the corresponding weights necessary to overcome friction are 1·5
lbs. and 2 lbs. In the four experiments the weights used are in the
proportion 1, 2, 3, 4; and the forces necessary to overcome friction,
0·5 lb., 1 lb., 1·5 lbs., and 2 lbs., are in the same proportion. Hence
the friction is proportional to the load.