I have few (more or less) general questions: 1. I know that: "The coefficients of friction are nearly independent of the area of contact between the surfaces". I understand this, but I know from everyday life, when car tire tread is worn off, braking distance increases and maximum acceleration & traction decreases (it's just my observation and assumption) which is caused by friction force. My explanation for this problem is following: I know tire has grooves/patterns, so not whole tire area is touched to the ground and when it's worn off grooves disappear, contact area increases and friction decreases. Is nearly independent causing such a big change in everyday life, or am I far away from real answer? 2. As I understand friction coefficient depends on 2 surfaces (for ex. asphalt/rubber friction coef. is m1 and ice/rubber is m2). Firstly, is there any parameter that characterizes smoothness/friction (or something like that, I think you'll understand what I mean) of single surface? Secondly, if I know asphalt/rubber (m1) and ice/rubber (m2) friction coefficient, can I calculate asphalt/ice friction coefficient (and how)?