# Homework Help: Static friction: car braking

1. Feb 8, 2006

### scavok

The coefficient of static friction between the tires of a car and a horizontal road is s=0.56. If the net force on the car is the force of static friction exerted by the road, what is the magnitude of the maximum acceleration of the car when it is braked?

I really don't understand what is going on in this question. How can I solve this without the mass, velocity, or acceleration of the car? Why is it talking about static friction instead of kinetic friction if the car is moving? How can the net force be the static friction exerted by the road if the car is moving?

vvv Thanks, that's what I needed.

Last edited: Feb 8, 2006
2. Feb 8, 2006

### andrevdh

When the tyres of a car are rolling over the road surface there are no relative motion between the contact point of the tyre and the road - only static friction acts on the tyre. If the car is turning the wheels this friction acts towards the front on the tyres thus pushing the car forwards (action-reaction pairs). When the car is braking this frictional force will act towards the rear of the car, thus slowing it down. As long as the car is not skidding we will be dealing with static friction for the braking force on the car. Static friction can increase only up to a certain limit, so this condition will only do so much before the tyres start to slip over the road when applying the brakes too hard, in which case we are dealing with kinetic friction - which is generally smaller than static friction anyway, unless the rubber starts to melt!