[ODE] Contact Constraints: First Order Slip?

[Thomas Harte]

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Can anyone explain to me the purpose of the first order slip variables within the 
dSurfaceParameters structure contained within the dContact structre, used when 
establishing a contact constraint?

My understanding of slip is that it is the difference between the linear velocity a 
particle wants to travel at by virtue of the body it is attached to, and the speed at 
which it is travelling over a surface. In the general case, the coefficient of friction 
is a function of slip - although often it isn't related to slip at all.

E.g. if we have a box scraping along a surface, the slip is simply the linear velocity 
of the body. However, if we are talking about a car wheel, then the slip is the 
difference between the linear velocity of the outside of the tyre and the speed at 
which the wheel is moving along the ground. This is important for rubber tyres 
because the coefficient of friction for rubber is most certainly a function of slip, 
and a fairly interesting one at that.

Anyway, I therefore don't see why I must express 'first order slip', surely ODE 
already has enough information to calculate it internally? I notice the paramter is 
described next to the relevant contact flag simply as 'slip', and only later by the 
corresponding dReal as 'first order slip'. It therefore strikes me that it is probably 
not asking me for the first order differential of slip (e.g. if the motor attached to 
my car wheel is changing speed) in order to help with numerical accuracy regarding 
the discrete model of time. But perhaps that is exactly what it is asking me for?

One thing I can understand from the documentation : "this needs more 
description".

-Thomas

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