The friction, more generally called damping, is the most difficult parameter of the three
to model. It needs to be a combination of all the damping effects in the system. These
may be of many forms. Coulomb friction results from two dry or lubricated surfaces
rubbing together. The contact surfaces between cam and follower and between the follower
and its sliding joint can experience coulomb friction. It is generally considered to
be independent of velocity magnitude but has a different, larger value when velocity is
zero (static friction force Fst or stiction) than when there is relative motion between the
parts (dynamic friction Fd). Figure 1O-5a shows a plot of coulomb friction force versus
relative velocity v at the contact surfaces. Note that friction always opposes motion, so