Friction in Rotations: Do We Disregard It?

[clmech]

As far as I’ve gathered, for a system to rotate there has to be some static friction acting upon it and dynamic friction can be zero. But now I’m a bit confused about this as we completely disregarded static friction in some tasks where a system was rotating. So was my original assumption wrong? If so, I don’t quite understand how rotations work then; is there any friction necessary? And if there is, why are we able to disregard it sometimes?

Thank you in advance and have a good day.

 

[andrewkirk]

We don't need friction. Consider a plank in free space that is struck near one end by a rock traveling in a direction perpendicular to the long axis of the plank. The plank will start to rotate as well as translating in the direction of the rock's motion.

An egg-beater is another example. Or if you lift the rear wheel of a bike off the ground with one hand and turn the pedals with the other hand to make the rear wheel spin.

None of those scenarios need friction for rotation to occur. But most scenarios need friction in order to convert rotary motion to translational motion - eg a bicycle with its rear wheel on the ground - the friction between ground and rear tyre converts the rear wheel's rotary motion to translational motion of the bike.

 

[clmech]

We don't need friction. Consider a plank in free space that is struck near one end by a rock traveling in a direction perpendicular to the long axis of the plank. The plank will start to rotate as well as translating in the direction of the rock's motion.

Thank you, that makes sense, can I ask about systems with strings though? For example a pulley or something rotating attached on a string; Is friction necessary then, for the pulley or the object to even be rotating?

 

[russ_watters]

For example a pulley or something rotating attached on a string; Is friction necessary then, for the pulley or the object to even be rotating?

Yes. Do you know which type?

 

[clmech]

Yes. Do you know which type?

Static friction?
Also, if so, then I’m very confused, as we disregarded it in a task about a yo-yo; or, to be precise I wrote some equations including static friction and the teacher wrote back that it was not necessary to include it. It was just a regular yo-yo without dynamic friction and the task was to find the string tension, but I don’t understand why it’s wrong/not needed to include static friction at all in such tasks. We also did this in some pulleys which were rotating that we observed, but I assumed it was just because it wasn’t needed to find the necessary rotational equations, but now I’m not so sure.

 

[russ_watters]

Static friction?

Correct!

Also, if so, then I’m very confused, as we disregarded it in a task about a yo-yo; or, to be precise I wrote some equations including static friction and the teacher wrote back that it was not necessary to include it. It was just a regular yo-yo without dynamic friction and the task was to find the string tension, but I don’t understand why it’s wrong/not needed to include static friction at all in such tasks. We also did this in some pulleys which were rotating that we observed, but I assumed it was just because it wasn’t needed to find the necessary rotational equations, but now I’m not so sure.

I'd need to see the actual problems to know for sure, but I don't see static friction being important for a typical yo-yo problem. They might assume (albeit incorrectly) that the string is anchored, so it doesn't need static friction to hold it to the axle.

 

[kuruman]

Summary:: I need some clarification about how friction in rotations works.

As far as I’ve gathered, for a system to rotate there has to be some static friction acting upon it and dynamic friction can be zero.

Be careful with what you say. Motion does not necessarily imply force, or torque as in this case. An object may be rotating and keep on rotating without any friction acting on it. Newton's First law says so.

What do you mean by "dynamic" friction? Is it another name for kinetic friction?

 

[Lnewqban]

Summary:: I need some clarification about how friction in rotations works.

And if there is, why are we able to disregard it sometimes?

Friction naturaly happens in most real cases, but it uses energy.
That could complicate some problems for which considering the occurrence of friction would be irrelevant.
Therefore, some ideal problems disregard friction.

Nevertheless, friction is necessary for some mechanisms to work, like pulleys and belts, tires and pavement, rope knots, etc.
In problems including those, friction must be considered.

 

[clmech]

Correct!

I'd need to see the actual problems to know for sure, but I don't see static friction being important for a typical yo-yo problem. They might assume (albeit incorrectly) that the string is anchored, so it doesn't need static friction to hold it to the axle.

Be careful with what you say. Motion does not necessarily imply force, or torque as in this case. An object may be rotating and keep on rotating without any friction acting on it. Newton's First law says so.

What do you mean by "dynamic" friction? Is it another name for kinetic friction?

Yes I suppose I meant kinetic friction (Sorry, I just translated it directly from my language, I should’ve double checked the english terms). I’m not sure if dynamic is another name for kinetic friction, but that’s what I was thinking about.

Thank you for the clarification on the first part.

 

[clmech]

Yes I suppose I meant kinetic friction (Sorry, I just translated it directly from my language, I should’ve double checked the english terms). I’m not sure if dynamic is another name for kinetic friction, but that’s what I was thinking about.

Thank you for the clarification on the first part.

I accidentally replied to both answers here, sorry about that.

 

[clmech]

Correct!

I'd need to see the actual problems to know for sure, but I don't see static friction being important for a typical yo-yo problem. They might assume (albeit incorrectly) that the string is anchored, so it doesn't need static friction to hold it to the axle.

So, to clarify, in a typical yo-yo problem we usually disregard static friction?

 

[hutchphd]

Friction naturaly happens in most real cases, but it uses energy.

That is not at all correct. For instance a belt on a pulley uses static friction to drive the pulley yet no energy is "used". Same with a wheel on the ground. Please be more careful.
Perhaps we should commit to using the term stiction instead of static friction and sliption instead of dynamic friction. Or something

 

[Lnewqban]

That is not at all correct. For instance a belt on a pulley uses static friction to drive the pulley yet no energy is "used". Same with a wheel on the ground. Please be more careful.

Are those two mechanisms 100% efficient regarding energy?
I am as careful as I can be, always.

 

[russ_watters]

So, to clarify, in a typical yo-yo problem we usually disregard static friction?

I'm only guessing yes -- I'd need to see the actual problem to know for sure.

 

[A.T.]

Are those two mechanisms 100% efficient regarding energy?

Static friction doesn't dissipate energy. But real world mechanisms do not involve only static friction.

 

[A.T.]

So, to clarify, in a typical yo-yo problem we usually disregard static friction?

Maybe you should clarify what a "typical yo-yo problem" is. If the string is attached to the axle via a loop that allows free spinning at the bottom, then you do need friction. Otherwise the yo-yo would not start spinning when you drop it, and would not windup the string when it climbs.

But problems are often simplified, and complex aspects are included only implicitly.

 

[clmech]

I'm only guessing yes -- I'd need to see the actual problem to know for sure.

Yes, understood, unfortunately I don’t have the text of the task anymore, but this helps. Thank you.

 

[clmech]

Maybe you should clarify what a "typical yo-yo problem" is. If the string is attached to the axle via a loop that allows free spinning at the bottom, then you do need friction. Otherwise the yo-yo would not start spinning when you drop it, and would not windup the string when it climbs.

But problems are often simplified, and complex aspects are included only implicitly.

I was only quoting a previous reply to my question when I used the ‘typical’ word. If I still had the text of that task I’d post it, but unfortunately all of this is being derived from my memory right now. Though there was spinning involved for sure, that’s why I guessed friction was necessary. I understand it’s hard to say anything about this problem without having the actual text though, I only used it as an example of what was confusing me about friction. But if the answer is that static friction is necessary to allow rotations for objects on strings, I’m almost positive I just read too much into the problem when it should’ve been simplified.

 

[jbriggs444]

But if the answer is that static friction is necessary to allow rotations for objects on strings

Static friction is not required for objects to rotate on strings.

Consider the case of a string wound around a frictionless spool with one end of the string firmly attached to the spool. [By contrast, the axle of a yoyo has friction with the string but no firm attachment]. The only tangential force on the string from the spool is at the point of attachment. No static friction there. The radial force acts throughout the length of the string. No static friction there either. Nonetheless, there is a net force and a net torque from the string on the spool that can cause motion and rotation.

 

[A.T.]

But if the answer is that static friction is necessary to allow rotations for objects on strings,

It's not in general. It depends on the specific case. And even if static friction is involved, is still depends on the the question, whether you have to model it as such.