Coefficient of Rolling Friction
Can the coefficient of rolling friction be less than 0?
I don't see how. The COF of rolling friction is the loss of angular
momentum with time as a wheel rolls across a horizontal surface. If the COF
were negative, it would seem that the wheel would accelerate in the absence
of a force.
The coefficient of rolling friction is not friction in the strictest sense:
it is a normal force due to the fact that a surface does not remain
perfectly flat. When a wheel/disk/cylinder rolls along a surface, the
surface gives a little, forming a temporary "dent" to accomodate the wheel.
A comparable example is a footprint temporarily forming in a carpet as you
walk across it.
This dent provides a little bit of material in front of the wheel, pushing
back with a slightly tilted normal force. An extreme example is trudging
through deep snow. To move forward, you have to push the snow out of the
way. For the wheel to move forward, it has to push the little bit of
surface down or aside. To have no "rolling friction", the surface must be
completely stiff, not giving at all.
The coefficient of rolling friction cannot be negative. If it were, this
little dent in the ground would help you to move, rather than slow you down.
That is not what happens.
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Update: June 2012