148. The area of the wooden slide is 9" × 9", but we would have
found that the friction under a given load was practically the same
whatever were the area of the slide, so long as its material remained
unaltered. This follows as a consequence of the approximate law that
the friction is proportional to the pressure. Suppose that the weight
were 100 lbs., and the area of the slide 100 inches, there would
then be a pressure of 1 lb. per square inch over the surface of the
slide, and therefore the friction to be overcome on each square inch
would be 0·27 lb., or for the whole slide 27 lbs. If, however, the
slide had only an area of 50 square inches, the load would produce a
pressure of 2 lbs., per square inch; the friction would therefore be
2 × 0·27 = 0·54 lb. for each square inch, and the total friction would
be 50 × 0·54 = 27 lbs., the same as before: hence the total friction
is independent of the extent of surface. This would remain equally
true even though the weight were not, as we have supposed, uniformly
distributed over the surface of the slide.