Why is there a limiting value of friction (to avoid slipping) when driving in a circle

Question

When thinking about this in reality and using equations I come to different conclusions.

Using F = mv^2/r

1. In reality I know that if the speed is too fast the car will not be able to go around the circle without skidding off.

2. But when I think using equations, I think that the friction is acting as the centripetal force( the force keeping the car going round in a circle) so if there if more speed there is more friction hence it keeps going round in a circle.

Obviously 2. is wrong and what I do not understand is why. Why is there a limiting value of F that cannot be eclipsed or else the car will slip 'n' skid.

Thanks (I'm doing A levels - so an explanation equivalent to my level will be appreciated)

Answer 1

Obviously 2. is wrong and what I do not understand is why. Why is there a limiting value of F that cannot be eclipsed or else the car will slip 'n' skid.

Because it is static friction that provides the centripetal force and there is an upper limit to the amount of static friction force and thus the centripetal force. The maximum static friction force on an given wheel is:

$$f_{f-max}=\mu_{s}N$$

Where $\mu_s$ is the coefficient of static friction between the wheel and the surface and $N$ is the normal force acting on the wheel, which is the fraction of the total car weight supported by wheel.

The static friction force matches the required centripetal force but only up to the maximum possible static friction force. If the centripetal force required for circular motion of the car causes the maximum static friction to be exceeded, friction changes from static to kinetic and skidding occurs.

Hope this helps.