Rolling without slipping down an inclined plane

[PeroK]

"For many dynamics problems, rolling without slipping means there is a friction force acting on the wheel at the contact point P. This friction force prevents slipping. In this instance the friction is known as static friction since there is no relative sliding between the wheel and surface at the contact point P."~https://www.real-world-physics-problems.com/rolling-without-slipping.html
I'll go away now.

Just to add a point:

If the wheel is accelerating or decelerating (in the case of rolling without slipping), then a non zero static friction force acts. If the wheel is rolling at constant speed, on a horizontal surface, then no friction acts. To see this, simply consider the wheel moving with constant angular momentum about its centre. No external force is needed to maintain this.

 

[dyn]

Some of the above posts seem to mirror my confusion. When a ball/disc slides down an incline plane , the point of contact is moving relative to the surface so the external torque that causes the ball/disc to start rotating is kinetic friction. At the instant the rolling without slipping condition is met the kinetic friction instantaneously changes to static friction. That seems to be correct but it also appears a bit strange.
Also if the ball/disc is accelerating for a long time , then static friction force might not be enough to keep up with the linear speed so the ball/disc could start sliding again ?

 

[jbriggs444]

Some of the above posts seem to mirror my confusion. When a ball/disc slides down an incline plane , the point of contact is moving relative to the surface so the external torque that causes the ball/disc to start rotating is kinetic friction.

Yes. If one begins with the ball already moving but not turning fast enough [or too fast], it must be sliding and the relevant force is kinetic friction.

At the instant the rolling without slipping condition is met the kinetic friction instantaneously changes to static friction. That seems to be correct but it also appears a bit strange.

Yes, it is correct.

Also if the ball/disc is accelerating for a long time , then static friction force might not be enough to keep up with the linear speed so the ball/disc could start sliding again ?

The coefficient of static friction is generally larger than the coefficient of kinetic friction. For an inclined plane with contant slope, if kinetic friction is more than enough to keep up with the acceleration then static friction is also sure to be more than enough.

 

[Ehyeh Asher Ehyeh]

I believe the convention is to use the term ‘Static Friction’ for a rolling situation like this. It doesn’t matter how you parse the terms as long as you understand so you can solve the problem.
This is an idealized situation where the angular and linear accelerations are constant, so don’t worry about reality because that would be ‘engineering’ and not physics.
It was a fun post anyway, so thanks for that much!

 

[jbriggs444]

I believe the convention is to use the term ‘Static Friction’ for a rolling situation like this.

It is more than convention.

Microscopically, the mating surfaces are not moving relative to one another. They can settle into place and form a somewhat more solid "grip" than surfaces that are sliding across one another. That is precisely the difference between static and kinetic friction.

The models of static and kinetic friction in terms of $\mu_k$ or $\mu_s$ times normal force are engineering approximations. Not basic physical principles.

 

[Ehyeh Asher Ehyeh]

The Nomenclature is not recognized by the disc or the inclined plane. Get it?

 

[Dale]

The Nomenclature is not recognized by the disc or the inclined plane. Get it?

But the formulas are “recognized by the disk” in the sense that one of them correctly describes the behavior of the disk and the other does not. The nomenclature serves to identify which formulas to use.

 

[Ehyeh Asher Ehyeh]

The disc and plane are indifferent to your use of language. The behaviours do not depend on Semantics. That is unless you are talking Voodoo?

 

[Dale]

When a ball/disc slides down an incline plane , the point of contact is moving relative to the surface so the external torque that causes the ball/disc to start rotating is kinetic friction.

IF the ball starts out sliding this is correct. However, note that it is possible (in fact common) for the ball to start rotating in the static friction condition from the beginning. In such cases there would be no “instant the rolling without slipping condition is met” because it would always be met.

An example would be a ball at the top of a rough ramp held in place at the top for a time and then released to roll under gravity. Such a ball would be in the no-slip condition from the beginning.

 

[Dale]

The disc and plane are indifferent to your use of language. The behaviours do not depend on Semantics. That is unless you are talking Voodoo?

I am not talking about semantics at all. Why are you?

 

[dyn]

Physics is great ! You ask a seemingly simple question and it always ends up getting so much more complex. Thanks everyone for replying to this thread

 

[Ehyeh Asher Ehyeh]

The Quantum Froth of physics questions...good one!