# Why is static friction necessary for pure rolling?

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• 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.
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?

tbn032 said:
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.
tbn032 said:
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
tbn032 said:
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.

Lnewqban said:
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
tbn032 said:
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

## 1. Why is static friction necessary for pure rolling?

Static friction is necessary for pure rolling because it provides the necessary force to initiate and maintain the rolling motion. Without static friction, the object would slip and slide instead of rolling, making it difficult to control its motion.

## 2. How does static friction affect the motion of an object during pure rolling?

Static friction acts in the opposite direction of the rolling motion, creating a torque that helps the object maintain its rotational motion. It also provides the necessary force to overcome any resistance or obstacles in the object's path.

## 3. Can pure rolling occur without static friction?

No, pure rolling cannot occur without static friction. Without this force, the object would not be able to maintain its rotational motion and would instead slide or slip, resulting in a different type of motion.

## 4. What factors affect the amount of static friction necessary for pure rolling?

The amount of static friction required for pure rolling depends on the coefficient of friction between the object and the surface it is rolling on, as well as the object's mass and radius. A larger mass or smaller radius may require more static friction to initiate and maintain rolling.

## 5. How does the presence of static friction affect the energy of an object during pure rolling?

Static friction helps to conserve the object's energy during pure rolling by converting potential energy into kinetic energy. This allows the object to maintain its motion without losing energy to sliding or slipping. However, some energy may still be lost to frictional forces.

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