# Friction force in rotational motion

• daivinhtran
In summary, for an object rolling without slipping down an incline, the frictional force fs is less than or equal to its maximum value, which is μsFn. This is because if fs were greater than μsFn, the object would slip and not roll. In rotational motion, static friction occurs when the point of contact is at rest and it acts against the relative motion between the contact points. If there is slipping, the friction becomes kinetic instead of static. The static friction is necessary to cause a change in the velocity profile and can be visualized as a rotating object with gears on a profiled plane. If the relative speed between the point of contact and the ground increases, the friction becomes dynamic. In summary, the
daivinhtran
My textbook says, "for an object rolling without slipping down an incline, the frictional force fs is less than or equal to its maximum value. fs < μsFn

Why is that? What happen it's greater than??

When do we have static friction in rotational motion? (for rolling object)

Then in an example problem about rolling with slipping, it says "there is slipping so the friction is kinetic (not static).? Same question. ==> Why?

Last edited:
For the first question,
friction as in all conditions has a limit.(well u know it). If it weren't that way the world wouldn't function into dynamics(only rotation). down a plane, friction has limit μ*mg*cosθ.if it weren't the way it were, there wouldn't be any slipping. For the moment it might look like it's good. But think like everything stuck to everything.

For second Q,
As you can see that the point of contact in rolling without slipping is at rest.friction acts against the relative motion b/w contact points. That's the work of friction (static).
If you wan't to cause a change in the velocity profile, you have to go against static friction.
For visualization , think a rotating object with spurs (gears) on a profiled (as in the gear) plane.
If it were rolling without slipping, the gear tooth will exactly match into the profiled plane. So there isn't any relative motion b/w the object's point of contact and the plane. If you like to alter the motion you would have to move uphill (That is the static friction in microscopic scale)
causing a relative speed at that instant. If it gains velocity (with slipping) it has inertia . So it turns to dynamic friction. So ,all matters is the relative speed b/w point of contact and the ground (not the body's velocity with the ground)

rahulpark said:
For the first question,
friction as in all conditions has a limit.(well u know it). If it weren't that way the world wouldn't function into dynamics(only rotation). down a plane, friction has limit μ*mg*cosθ.if it weren't the way it were, there wouldn't be any slipping. For the moment it might look like it's good. But think like everything stuck to everything.

For second Q,
As you can see that the point of contact in rolling without slipping is at rest.friction acts against the relative motion b/w contact points. That's the work of friction (static).
If you wan't to cause a change in the velocity profile, you have to go against static friction.
For visualization , think a rotating object with spurs (gears) on a profiled (as in the gear) plane.
If it were rolling without slipping, the gear tooth will exactly match into the profiled plane. So there isn't any relative motion b/w the object's point of contact and the plane. If you like to alter the motion you would have to move uphill (That is the static friction in microscopic scale)
causing a relative speed at that instant. If it gains velocity (with slipping) it has inertia . So it turns to dynamic friction. So ,all matters is the relative speed b/w point of contact and the ground (not the body's velocity with the ground)

So what happen if fs > μsFn?

daivinhtran said:
So what happen if fs > μsFn?

It can not happen. The static friction can not be grater than μsFn.

When you pull an object, resting on the ground, with force F, and F<μsFn the object stays in rest. If the object can roll, it will roll.

If you pull an object with force F>μsFn it will slip.

ehild

daivinhtran said:
So what happen if fs > μsFn?
what does fs in ur statement mean? frictional force or applied force

## 1. What is friction force in rotational motion?

Friction force in rotational motion is the resistance force that occurs when two surfaces are in contact and moving relative to each other in a circular motion. It acts in the opposite direction of the motion and is caused by the microscopic irregularities on the surfaces.

## 2. How is friction force related to rotational motion?

Friction force is an essential factor in rotational motion as it affects the speed, direction, and stability of the rotating object. It can slow down or stop the motion of the object and can also cause it to change direction or slip.

## 3. What factors affect the magnitude of friction force in rotational motion?

The magnitude of friction force in rotational motion depends on the coefficient of friction between the two surfaces, the normal force between them, and the velocity of the rotating object. Additionally, the surface roughness, temperature, and lubrication can also affect the friction force.

## 4. How can friction force be reduced in rotational motion?

Friction force can be reduced in rotational motion by using a smoother surface, adding a lubricant between the surfaces, reducing the normal force between them, and increasing the rotational velocity. These measures can help to decrease the coefficient of friction and thus reduce the friction force.

## 5. What are some real-life examples of friction force in rotational motion?

Some examples of friction force in rotational motion include the wheels of a car rotating on the road, the movement of gears in a machine, and the rotation of a bicycle pedal. Other examples include the spinning of a top, the rotation of a drill bit, and the motion of a record player.

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