quasi-static rolling friction

[JohnDenker]

AFAICT the important topic of _quasi-static rolling friction_ is not mentioned in chapter 10 (work and friction), and it's not indexed under friction.

There is "some" discussion of rolling motion chapter 12 (torque) on page 297.

For example, when a rubber tire rolls along a road, at any given moment a particular patch of rubber is in contact with the road, and under normal conditions this patch is _not moving_ relative to the road, to a good approximation. Under other conditions the tire can skid relative to the road.

This has immense significance in real-world situations. Applications include tires, billiard balls, bowling balls, roller bearings, and many others.

Students who have been taught only two situations (static friction and kinetic friction) are very likely to misunderstand rolling friction.

On page 297 it says

The force of static friction to which wheels are subject slows them down.

I don't understand that at all. Ideally, static friction is non-dissipative. For a ball rolling in a bowl, half the time the quasi-static rolling friction is slowing the ball down, and the other half of the time it is speeding the ball up.

Also it says:

the force of static friction can be greater than the force of kinetic friction.

That conflicts with page 252 which says the static friction is always greater than the kinetic friction.

Suggestions:

• It would be a big help to mention this in chapter 10, with a link to the relevant part of chapter 12.
• It would be a big help to index this under F for friction (not just R for rolling). See #181 for a catalog of index-related issues.

[JohnDenker]

Also: Somewhere (possibly chapter 10, possibly chapter 12) it would be good to discuss skidding, i.e. rolling with nontrivial slip. It's not discussed AFAICT. I don't see it indexed under F for friction, S for skidding, or R for rolling.

Also: Somewhere it would be good to discuss the _sideways_ forces on a tire in a turn.

You may think this is a trivial and obvious corollary to what's been said, but some students are confused by it. Among other things, some of them are baffled by the fact that the tire spins freely when force is applied in one direction but strongly resists when force is applied in the perpendicular direction. This starts with the design of the axle and bearings, but it has tremendous consequences for the static and/or kinetic friction in a turn.

For this reason, it is somewhat illogical to introduce quasi-static rolling friction in the chapter on torque. Logically it ought to be introduced in the chapter on friction. Then it can be applied to torque situations as well as non-torque situations. OTOH, as always, questions of sequencing are not very important. The benefits of the spiral approach outweigh the benefits of strictly logical sequencing. On the third hand, the point remains: no matter when and where the topic of quasi-static rolling friction comes up, both the rolling and non-rolling directions should be considered.