Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Rolling, Static, and Kinetic Friction



To all involved, especially Leigh and David:
To Leigh: the formula T = mu N for static friction could be
acceptable, but knowing that here T means the MAXIMUM FRICTION FORCE
possible, to maintain no-sliding conditions.
To David: The problem of calculating the force of friction is a
slippery one! It can ALWAYS be expressed as T = mu N, by adjusting mu
conveniently. First: we expect it to be independent of N. This is
probably quite correct. Second: it depends critically on the kinds of
surfaces (smoothness, lubrication etc.), so it will depend from point
to point when the object is sliding. Third: it may depend on the velocity
of sliding, and the usual approximation of having one value mu0 at rest
at another (constant) value mu when sliding is just an approximation.
In this approximation, mu0 CANNOT be smaller than mu, as I will show.
Assume an object resting on a plane of inclination alpha.Let alpha
start being zero and increase gradually. At some critical angle,
the friction at rest cannot overcome the tendency to slide, hence it starts
sliding. But the very moment it slides, the friction coefficient
increases and stops the motion. This poor object doesn't know what
to do: it is neither allowed to rest nor to move!
Regards Emilio