# Why is static friction necessary for pure rolling?

• B
• tbn032
In summary, static friction is not necessary for pure rolling, but once achieved, it is a more accurate description of the interaction between surfaces.f

#### tbn032

Suppose a cylinder is launched on a horizontal frictional surface such that it has initial translational velocity v and zero angular velocity .the kinetic friction would be applied between the contact points of the cylinder and the surface, opposite to the direction of the translational motion. This kinetic frictional force will simultaneously apply torque on the cylinder (which will increase its angular velocity) and decrease the translational velocity till the cylinder satisfy the condition for pure rolling(V=RW).when the condition for pure rolling is satisfied, the relative velocity between the contact points and surface would be zero and this there would be static friction between the contact points and the surface.

My confusion is that why is static friction necessary for pure rolling instead of kinetic friction. Static friction is applied when the object satisfies the condition for pure rolling(V=RW). The static friction does not increase or decrease, both translational and angular velocity. Kinetic friction on the other hand ensures that the object follows the condition for pure rolling on a horizontal frictional surface.

According to my understanding, if an object is launched on a frictionless surface such that it initially satisfies the condition for pure rolling, then the object would continue to be in pure rolling motion even in the absence of static friction(static friction=0 because µ=0), thus the static friction is not necessary for pure rolling. But if an object is launched on a frictionless surface such that it initially does not satisfy the condition for pure rolling, then the motion of the object would not be transformed into pure rolling due to absence of kinetic friction. Can you explain that why is the static friction is called to be necessary for pure rolling of an object instead of kinetic friction?

Suppose a cylinder is launched on a horizontal frictional surface such that it has initial translational velocity v and zero angular velocity .the kinetic friction would be applied between the contact points of the cylinder and the surface, opposite to the direction of the translational motion. This kinetic frictional force will simultaneously apply torque on the cylinder (which will increase its angular velocity) and decrease the translational velocity till the cylinder satisfy the condition for pure rolling(V=RW).
Yes.
when the condition for pure rolling is satisfied, the relative velocity between the contact points and surface would be zero and this there would be static friction between the contact points and the surface.
There is no need for friction when pure rolling is achieved.

• Kashmir and topsquark
Can you explain that why is the static friction is called to be necessary for pure rolling of an object instead of kinetic friction?
If the object is starting from zero velocity respect to the surface, static friction should accelerate it quicker than kinetic friction.
Think of a car doing a burn out: acceleration is greater without smoke.

If the object is starting from zero velocity respect to the surface, static friction should accelerate it quicker than kinetic friction.
Think of a car doing a burn out: acceleration is greater without smoke.
I don't see how this is relevant.

• topsquark
Can you explain that why is the static friction is called to be necessary for pure rolling of an object instead of kinetic friction?
It is not "necessary". But once rolling without slipping has been achieved, the surfaces will (ideally) have zero relative motion within the contact patch. Static friction is then a more apt description of the interaction between the two surfaces. If the frictional force across the interface subsequently exceeds the maximum imposed by the coefficient of static friction then the two surfaces will "break loose" and slipping will resume.

• Lnewqban