140. In a subsequent lecture we shall employ as an inclined plane the
plank we have been examining, and we shall require to use the knowledge
of its friction which we are now acquiring. The weights which we shall
then employ range from 7 lbs. to 56 lbs. Assuming the ordinary law of
friction, we have found that 0·27 is the best value of its coefficient
when the loads range between 14 lbs. and 112 lbs. Suppose we only
consider loads up to 56 lbs., we find that the coefficient 0·288 will
best represent the experiments within this range, though for 112 lbs.
it would give an error of nearly 3 lbs. The results calculated by the
formula _F_ = 0·288 _R_ are shown in Table V., where the greatest
difference is 0·7 lb.