COF is a proportional measurement.Isn’t CoF a measurement?
Think about the tires on your car and an asphalt highway.
Tires wear,.... now make the rubber harder on your tires. The COF changes.
Change the asphalt highway to sand. The COF changes again.
To talk about 2 pool balls made of the same material,(ie: phenolic resin) moving at 12 inches per second ; the COF is practically nil.
Take a piece of lead and collide it head-on with a piece of tungsten steel at the speed of light,.... NOW you have a COF worth talking about!