i6. The Friction of Solids is caused by the roughness
and unevenhess of the surfaces of contact. In the case of
Sliding Friction the asperities of the one surface interlock with
those of the other, and motion can only take place by the rid-
ing of the one set over the other, by the tearing off of the
projecting parts, or by rubbing them down : in either case the
process gives rise to a resistance which is the greater as the
roughness is greater, add the less as the surfaces are smoother •
an absolutely smooth surface would be frictionless. Rollings
Friction is observed where any surface of revolution, or other
smoothly curved surface, is rolled upon another surface, plane
or curved. Its cause is identical with that of sliding friction
that irregularity of form and of surface which will not permit
motion to occur without irregular variation of the distance
between the centre of gravity of the rolling body and the line
of motion in the common tangent of the two bodies, at the
point or line of contact. Where the surfaces are hard, smooth,
and symmetrically formed, this friction is small ; where they
are soft, rough, or irregular, this form of friction is observed
in greater degree. Absolutely smooth, hard spheres or cylin-
ders, rolling on absolutely hard, smooth surfaces, meet with
no frictional resistance ; bodies having rough surfaces, those
made of compressible material, and those of irregular surface
and form, exhibit greater friction as these defects are exag-
gerated. Both forms of resistance evidently depend upon the
character of the material as well as upon the form of the sur-
faces of contact.
The resistance of knife-edges, as in balances, is a fohn of
rolling friction.
