# Direction of static friction for an object on a car that is in circular motion

## Main Question or Discussion Point

Can someone please explain why static friction points in the same direction as the centripetal force for an object (like a box) on the flatbed of the truck that is travelling in uniform circular motion?

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Doc Al
Mentor
Well, the friction is what's applying the force to accelerate the object. So if the object is centripetally accelerating, then the friction is applying a centripetal force.

But I thought friction was supposed to resist motion, not accelerate objects.

Doc Al
Mentor
But I thought friction was supposed to resist motion, not accelerate objects.
Why can't it do both? Friction resists slipping between surfaces. If there were no friction, the object would tend to slide outward. Friction prevents that slipping and in the process exerts an inward accelerating force which keeps the object moving in a circle.

rcgldr
Homework Helper
But I thought friction was supposed to resist motion.
To reword Doc Al's post, friction resists relative motion between surfaces in contact with each other. The surfaces may be accelerating or experiencing an external force (such as gravity on an inclined surface), but if there is sufficient friction, there won't be any relative motion between the surfaces.