Re: Rolling, Static, and Kinetic Friction

["Edwin R. Schweber" <EdSchweb@ix.netcom.com>]

Lowell Herr wrote (in part):
> Assume a toy car is rolling down an incline plane with all wheels rolling
> freely (no wheels are sliding) ... What is the direction of that friction? 
> Is it down the incline in the direction the car is moving or opposite the
> direction the car is moving?
> Lowell:

   I can see two reasons why friction should be up the hill. First
imagine an incline sloping down from left to right as you face it.The
wheels of a car coasting down that hill would be rotating in a
clockwise direction with an ever increasing speed. That would
imply an uphill torque and the only force that could maintain that 
torque would be friction.

   Also consider the problem from a conservation of energy perspective.
Since the wheels are rotating they are are picking up some of
the lost potential energy that would have gone into the translational 
kinetic energy. Therefore the translational KE and the translational
velocity of the car would be less than if there were no rotation. Now
switch to a second law perspective. The only way the translational 
speed could be less if there were an upward force in addition to the 
downward component of gravity. Once again that force would appear to be 
friction.

Ed Schweber (email:edschweb@ix.netcom.com)
Physics Teacher
Solomon Schechter Day School
West Orange, New Jersey