# Static Friction and Centripetal Force

## Main Question or Discussion Point

Hi,

I am very confused. I have searched everywhere online and have drawn free-body diagrams, but I am still confused as to why static fricition, not kinetic friction, provides the centripetal force in a car moving in a circle.

In addition, assuming that the centripetal force of a car moving around a circle is static friction, then you can find the maxium velocity that you can travel without going tangent to the circle, but can you travel at a slower velocity than this maxium velocity? Your centripetal force would change and no longer be equal to the static friction. How does this work?

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If it was not static friction, then the car would be sliding around. Dont forget, static friction is an inequality, so you can go up to a maximum value, but once you go over that velocity the friction will no longer be static and the car will begin to drift or slide.

So when a car is normally driving on the highway is static friction the force that is opposing the motion of the car since it is not "sliding?" I always thought that this was kinetic friction. To me, the car is moving so kinetic friction seems to be what would oppose the motion here.

Also why do the wheels of the car turning not provide the centripetal force?

The tires are spinning but nothing is sliding. When you travel in a circle, your tires are spinning in the direction of motion, but the friction is in direction of the centre of the circle. So the tires would have to slide perpendicular to the motion to have kinetic friction. if that happened, you wouldn't be travelling in a circle anymore.

When race cars are racing there is no kinetic friction, but when some guys spin out of control, thats kinetic friction.

Doc Al
Mentor
So when a car is normally driving on the highway is static friction the force that is opposing the motion of the car since it is not "sliding?" I always thought that this was kinetic friction. To me, the car is moving so kinetic friction seems to be what would oppose the motion here.
What matters is whether the surfaces in contact are in relative motion--sliding or slipping. While the car is certainly moving, the instantaneous contact patch of the tires on the road is not. So, no slipping and thus static friction. (Friction opposes slipping between surfaces, not 'motion' in general.)

When you hear tires screeching, that's kinetic friction. When no screeching, it's static (for the most part). The car might be moving relative to the pavement, but the contact surfaces aren't.

Is it static because the tire and the wheel are only in contact for a split second, and then the two surfaces are no longer in contact and of course cannot be moving relative to each other if they are not in contact?

And when a car turns its wheel to go around a curve, static friction provides the centripetal force. Does turning the wheels in a curve simply allow for static friction to provide the centripetal force?

Because I having been trying to figure out why a cars wheels and engine do not simply provide the centripetal force. But I think it is like I said above that the turning of the wheels allows for the static friction to provide centripetal force. Is this right?

Does turning the wheels in a curve simply allow for static friction to provide the centripetal force?
well turning the wheels causes the instantaneous velocity of the car to be in a different direction than the wheels are facing.