Re: Rolling, Static, and Kinetic Friction

[Jim Green <Jim.Green@Snow.edu>]

> "Rolling" friction will be present from friction in the bearings, or in
> the case of your Tonka cars between the wheel and the axle or the axle and
> the car depending upon which is rotating with respect to which, and to a
> lesser degree from air friction.  

Don't most people use "rolling friction" to describe the difficulty caused
by a compressible tire -- the fact that the horizontal tangent to the tire
is not along the roadway -- such that the tire is always "climbing out of a
hole" ???

> Now static friction will occur between the tires and the road (or incline)
> since the tire is instantaneously at rest with respect to the road
> surface.  


> So going downhill, kinetic friction is uphill and static friction is
> downhill.

In the quoted post "kinetic friction" has been defined as the rotational
friction on the axal by the bearing surface.  How could it's direction be
"uphill"?

For a real car on a level roadway the "static friction" is usually the force
which makes the car accelerate.  But if the car is being pushed, the "static
friction" (of a rigid tire) is reduced and might vanish or even impede the
motion if the pushing makes the tire skid -- likewise for a car accelerating
downhill under gravity only.

If the toy car is accelerating downhill under gravity and with rigid
rotating tires, there is no "static friction" -- unless of course the toy
car has a miniature supercharged engine in it.  

Just babbling here in the corner because its too hot to work in the garden.




Jim.Green@Snow.edu                             

Victory goes to the player who makes the
next-to-last mistake.  Savielly Grigorievitvh

Hey, let's have some new cliches.