# Why is static friction providing centripetal force?

This could be tagged as a duplicate, but I couldn't understand why static friction causes centripetal acceleration in a car taking a circular turn which is moving. Mustn't it be kinetic /sliding friction as the object is moving?

Regarding things that are rolling such as wheels of a car, remember one key thing: kinetic friction is not about moving, but about sliding.

Even though a wheel is moving, it isn't sliding over the surface. There is no kinetic friction going on. Only static friction which holds the contact point still while it is in contact.

Since there is no kinetic friction happening when the car is driving in a circle, only static friction is left to cause the centripetal acceleration.

Now, as @JohnForkosh mentions in a comment, another way to answer your question is that the driving direction and the centripetal (radial) direction are perpendicular and thus completely seperate.

There can easily be sliding (kinetic friction) in one direction but stationarity (static friction) in the other. And this is the case here. Even if the car was sliding in the driving direction, it is still not sliding in the radial direction (it is not moving further away from the circle centre).

• If it isn't kinetic friction can't it be rolling friction? – susan J Jan 13 '18 at 13:54
• @susanJ Ideally only kinetic or static friction takes place. Rolling friction is a term that covers all non-ideal influences (such as energy loss due to a soft surface or a soft wheel or energy loss in axels etc.) So yes, rolling friction might always be present in realistic situations - but even so, that is not the cause of centripetal acceleration in the circle. – Steeven Jan 14 '18 at 7:55