# Car turning on a curved road and friction

1. Jun 10, 2013

### andyrk

Friction opposes motion. When a car is turning around a curved road, the friction between the tyres and the roads provides the necessary centripetal force. That is, the frictional force would be in the same direction the driver is turning in. So would motion of the wheels be opposite to friction? This is not the case since the wheels turn where friction acts.. So why is this happening?

Last edited: Jun 10, 2013
2. Jun 10, 2013

### Dick

In the case of a rolling wheel friction does not oppose motion. It opposes the acceleration tending to make the wheel slide over the road instead of roll. That would be the centripetal acceleration only if the car has constant velocity.

3. Jun 11, 2013

### rcgldr

Dynamic / kinetic / sliding friction opposes relative motion between two surfaces. Rolling resistance opposes rolling motion, but this mostly due to loss of energy between deformation and restoration that occurs at the contact patch of a tire.

Static friction can be in any direction. In the case of a turning car, the static friction is a newton third law pair of forces, the tire pushing outwards on the pavement, and the pavement pushing inwards on the tire. In a steady turn, the force from the pavement is centripetal.

4. Jun 11, 2013

### andyrk

That means that the wheels want to go outward on the straight track they were following earlier before coming into the circular path or highway. So they want to move away from the circular path they are following and come back to the original straight path. So for this transition of going from the circular path to the curved path they have a tendency to move radially outward until the straight path is met again. To counteract this friction acts radially inwards. So this is the necessary centripetal force. Right?

5. Jun 11, 2013

### rcgldr

The static friction is between the tires and the pavement. As I mentioned before, that static friction results in a Newton third law pair of forces, the tire exerts an outward force onto the pavement, the pavement exerts an inwards (centripetal) force onto the tires.

Both forces are techincally reaction forces, the force from the tires is related to the acceleration of the car (times its mass), and the force from the pavement is related to the tiny amount of acceleration of the earth (times its mass).

6. Jun 11, 2013

### haruspex

Not quite. Friction between two surfaces opposes relative motion of the two surfaces - always. In the absence of friction, the car would continue in a straight line, the tyres sliding sideways on the road. Friction acts to oppose the slide.
Similarly, for a car accelerating on the flat, without friction the wheels would spin, with the part of the tyres in contact with the road moving 'backwards' (in relation to the orientation of the car). Therefore friction pushes the car forwards.