ESTIMATING THE AMOUNT OF FRICTION. 106 



er piece or block of the same substance upon this 

 surface ; then raise the plane until it becomes just 

 steep enough for the block to slide down by its weight. 

 Now, by measuring the degree of slope, we know at 

 once the amoimt of friction. Suppose, for example, the 

 two surfaces be smoothly-planed wood : it will be found 

 that the plane must be elevated about half as high as 

 its length ; therefore we know, by the properties of the 

 inchned plane, heretofore explained, that it requires a 

 force equal to one half the weight of the wooden block 

 to slide it over a smooth wooden surface. Some kinds 

 of wood have more firiction than others, but this is about 

 the average. 



From the result of this experiment we may learn 

 that to shde any object of wood across a floor requires 

 an amount of strength equal to one half the weight of 

 the object. A heavy box, for instance, weighing two 

 hundred pounds, can not be moved without a force 

 equal to one hundred pounds. It also shows the im- 

 propriety of placing a heavy load upon a sled in winter 

 for crossing a bare wooden bridge or a dry barn floor, 

 the friction between cast-iron sleigh-shoes and rough 

 sanded plank being nearly equal to one third of the 

 whole weight.* Hence a load of one ton (including 

 the sled) would require a draught equal to more than 

 six hundred pounds, which is too much for an ordina- 

 ry single team. On bare unfrozen ground the friction 

 would be still greater. On a plank bridge, with run- 

 ners wholly of wood, it would be equal to half the load. 

 All these facts may be readily proved by actually 



* On clean hard wood, with polished metallic shoes, the friction 

 would be much less, or a fourth, or fifth. 



