Hmmmm, my book says sand with less pressure at whatever speed you care and
you'll get less heat. Coefficient of friction being basically the same, (might
actually be less at low pressure) it all depends on pressure, not speed.

George, what you say is true, but only for limited ranges of pressure and
speeds. Few things in nature are linear. My book, The Machinist's Handbook,
says this about friction:

For low pressures the friction is directly proportional to the normal pressure
between the two surfaces (Just as you say), but goes on to say:

At very low velocities the friction is independent of the velocity (similar to
what you say)

For high velocities the friction (and coeficient of friction) decreases (the
opposite of what you say)

The friction is independent of the areas in contact.

What this means for heat buildup is that pressing with the same force with three
fingers instead of one yields the same friction (and stock removal) but it
generates less heat per surface area since it is spread out over a larger
contact area. Going faster reduces the friction (and therefore heat buildup),
leading to longer abrasive life, and at the same time the increased speed will
have a cooling effect upon the surfaces, too.

However, increased speed will result in increased injury in the event of an
accident.

Dan