@lewm, N is not necessarily gravity. It is the force applied perpendicular to "ground" which is not necessarily horizontal to the earth's surface. You are right, there is no unit of surface in the equation and I am not sure what the work around is but it is probably going to be a change in "u."
The same force on a knife tip is going to generate more friction than the same force on a marble. "u" for the knife tip is going to be much higher than "u" for the marble. Whatever.
@larryi@, all this agrees with Frank Schroder's approach (which Peter Ledermann adopted) Letting the arm drift slowly inward on a blank area is going to require less anti skating force than holding it still because friction in the groove is lower than friction on the blank surface.
The same force on a knife tip is going to generate more friction than the same force on a marble. "u" for the knife tip is going to be much higher than "u" for the marble. Whatever.
@larryi@, all this agrees with Frank Schroder's approach (which Peter Ledermann adopted) Letting the arm drift slowly inward on a blank area is going to require less anti skating force than holding it still because friction in the groove is lower than friction on the blank surface.