Unbanked Curve Motion: Friction vs Intuition

[JJ__]

This is just a conceptual question. I get that when a car is turning on an unbanked curve, the friction provides the centripetal force. I don't understand why this is though. I thought friction is supposed to oppose the direction of motion. But that would imply that the direction of motion points directly out from the circle. But intuitively it seems like the direction of motion would be tangent to the circle (i.e. perpendicular to the friction)...

 

[Merlin3189]

The same question arises if I walk (or cycle, or drive) across a slope in a direction perpendicular to the slope (or at any angle not parallel to the slope.)
I think the way to look at this is to see that, in order to move in such a way, I need to apply an appropriate force to the surface and friction acts opposite to this force. That force will involve components up the slope and in the direction of motion.
The deceptive element is that simple friction questions involve objects moving parallel to any slope, so that the force we apply is parallel to the motion.
Friction does not always act in the direction of motion. Rather perhaps, the net force, including a contribution from friction, acts in the direction of any acceleration (or negative acceleration, aka deceleration.)

 

[jbriggs444]

I thought friction is supposed to oppose the direction of motion.

Kinetic friction opposes the direction of relative motion. But the surface of the tire and the surface of the road are not in relative motion. The wheels are rolling so that the contact patch does not slip on the pavement. Instead, we are dealing with static friction.

Static friction provides whatever force is needed to prevent relative motion between two surfaces. The contact patch is not slipping. There is no relative motion. The wheels are free to roll forward or backward. Accordingly, little or no static friction is needed parallel to the car's motion to prevent slippage fore and aft. However, unless the tires slip right or left, the car is constrained to move along the curved path where the wheels point. Static friction acts to prevent the tires from slipping right or left away from this path.

Of course, static friction can only provide force up to the limit imposed by the coefficient of static friction.