Understanding Friction on a Car

[Shark 774]

Hi,

I'm struggling to understand the different friction forces acting on a car. What is rolling friction and what is its effect? What is static friction and how can it provide forward motion, but also oppose forward motion when the car breaks??

Thanks.

 

[Forestman]

Hey Shark,

This might be of help

http://en.wikipedia.org/wiki/Friction

 

[jack action]

In addition, from http://hpwizard.com/car-performance.html" :

The http://en.wikipedia.org/wiki/Rolling_resistance" (F_r = fr F_v) is a force that acts in a way similar to the friction force (F_f = µ F_v). The only difference is that the rolling resistance direction depends on the rolling direction of the tire and the friction force direction depends on the torque application. So sometimes the rolling resistance acts against the friction force (acceleration) and sometimes it works with the friction force (braking).

Since the rolling resistance and the friction force are always in the same plane, you could say that the result is a tire with «net» friction coefficient (= μ ± fr) that is greater or smaller depending if the vehicle is accelerating or braking.

The rolling resistance is very small and can be omitted in most cases. The net effect is usually a variation of 1-2% of the original tire friction coefficient. It is more important in off-road situations where that variation can go up to 30% (tire on sand for example).