# Friction and Rolling Motion

Is friction always zero when pure rolling has begun? Or is it static? Is there a difference between saying static friction at the point of contact and zero friction at the point of contact at the time when pure rolling has begun? That is of course for a rough surface on which the object is rolling.

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ehild
Homework Helper
The friction is static during pure rolling, and the force of static friction Fs≤μN.

Fs=0 during free rolling on horizontal surface, but different from zero when other external force acts, for example, when rolling on an incline.

ehild

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gneill
Mentor
Static fiction is there for a rolling object, but does no work unless the object is being accelerated, and then static friction comes into play (think about situations such as a car accelerating or braking, or being shoved sideways while the car travels straight ahead). The magnitude of the static friction force depends upon external forces acting to try to accelerate the object.

There's another type of friction called rolling friction which is separate from the kinetic or static versions. It's a friction that manifests due to deformation of the surfaces in contact. Think about the way a car tire flexes out-of-round as it rolls. The flexing consumes energy (which is why car tires become warm as you drive). The surface on which an object rolls may also be deformed by the weight of the object. In such a case the object is effectively always rolling out of a small depression as it moves along.

Static fiction is there for a rolling object, but does no work unless the object is being accelerated, and then static friction comes into play.
Doesn't Kinetic Friction come into play then?

ehild
Homework Helper
Doesn't Kinetic Friction come into play then?
Kinetic friction comes into play during slipping. Pure rolling excludes slipping.

ehild