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Name:  Petros
Status:  student
Age:  19
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Can the coefficient of rolling friction be less than 0?


Replies:
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.

Vince Calder


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.

Kenneth Mellendorf



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