# Supose a car, going at a constant velocity

## Main Question or Discussion Point

1. Supose a car, going at a constant velocity, decelerates to a stop by pressing the brakes. Is the friction used to stop the car kinetic or static friction? PLEASE explain why. My teacher said the answer was static friction, but I don't understand this. Since the car is already moving and the surface of the wheel is sliding past the ground, isn't kinetic friction causing the car to come to a stop.

2. When a car is going around a circular curve, is the friction used to accelerate the car towards the center of the curve static or kinetic friction. Again, my teacher said the answer was static friction, but I don't understand this. Since the car is already moving and the surface of the wheel is sliding past the ground, isn't kinetic friction causing the car to accelerate towards the center?

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UltrafastPED
Gold Member
Is the bottom of the tire sliding on the roadway? That would be kinetic friction.

If you carefully consider the motion of the tire you will find that at the point of contact ("where the rubber meets the road") the tire and the roadway are in static contact.

The brake pads are subject to kinetic friction.

"Rolling resistance" or "rolling friction" would be a better term than static friction; see
http://en.wikipedia.org/wiki/Rolling_resistance