# What does maximum normalized frictional force mean in this tutorial?

I tried following this tutorial: https://nccastaff.bournemouth.ac.uk/jmacey/MastersProjects/MSc12/Srisuchat/Thesis.pdf, but in page 19 I ran onto a problem - I don't understand the step 4. It is also not explained before. Could somebody explain what is it and what values may be used?

An important concept in physics is that "friction never does mechanical work"; it only retards motion by generating heat.

Since mechanical work is calculated as the dot product of the force acting on a body with the displacement of said body (or more specifically, the integral over time of said dot product), this principle means that the total frictional force must always be such as to minimize the amount of mechanical work done by all forces acting on the body. Thus it is always necessary to check and ensure that this is true, else one will have an unrealistic simulation.

The maximum normalized frictional force (at any point of friction) referred to is obtained by multiplying the normal force at that point by the corresponding "coefficient of friction* (static or kinetic as appropriate) for that point of friction.

The normal force is that exerted by the ground/track at that point just sufficient to both prevent the vehicle from dissolving into the ground/track, and from spontaneously starting to rotate off of the track. The latter can only occur when other forces acting away from the ground/track have already reduced the corresponding normal force to zero, usually from aerodynamic lift of some sort.

Update:

The Normal Force and the Frictional Force are respectively the perpendicular and parallel components of the reaction force from a surface when another body comes in contact. Neither can do mechanical work. The usual model is that the Frictional Force varies linearly as the Normal Force, with the coefficient of friction being the ratio. The coefficient of static friction (applicable to initiating movement or during rolling without slipping) is usually significantly greater than the coefficient of kinetic friction (applicable otherwise). Both are a property of the combination of materials at the point of contact and of the roughness of both - there is no theoretical model I am aware of that will predict these values, so you must simply look up what typical values are for the circumstances of interest.

• Q1 = what is usual coefficient of friction in this case? And if I get you right, it is not what usually "normalize" means than, right? Q2 = is the normal force mentioned in that link? – Adrians Netlis Nov 25 '15 at 5:59
• @AdriansNetlis: Correct. See my update. – Pieter Geerkens Nov 25 '15 at 6:33
• OK! I mean the values for car. Also - my English knowledge is general and programing only, so I don't understand a lot of physics stuff(my native language is Latvian, which is very different). – Adrians Netlis Nov 25 '15 at 16:12