# Why is rolling easier than sliding?

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## Main Question or Discussion Point

I learnt that rolling involves the coefficient of static friction unlike sliding that involves the coefficient of kinetic friction. It's known that the coefficient of static friction is always higher than the coefficient of kinetic friction. This should result in rolling to be more difficult than sliding as it involves higher frictional force, which is not the case in real life.

## Answers and Replies

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The static friction happens between wheel and surface.
Between wheel and axis there is dynamic friction.

kuruman
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Without any friction, a cylinder on an incline will slide but not roll. Static friction does not dissipate energy; it converts translational energy of the center of mass to rotational energy about the CM. There is rolling friction (look it up) which dissipates some energy but not as much as kinetic friction.

jbriggs444
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Consider, for instance, a wagon wheel. It is about one meter in diameter and has a hub with a hole in it which is about five centimeters in diameter. Inside the hole sits the axle about which the wheel rotates.

The drag from the rolling wheel relates to kinetic friction at the axle. The drag on a wheel with the brakes locked up relates to kinetic friction at the road. Static friction is not involved.

The reason that a wagon wheel rolls easily is due to mechanical advantage. (And grease). You have the same load on the wheel bearing (1/4 of the wagon weight) as there is on the road. So the force of kinetic friction is nominally the same. But with a 20 to 1 mechanical advantage, rolling is easier than sliding. Grease improves that further. And reduces wear.

It is worth noting that the coefficient of static friction is the maximum frictional force that can exist between two mating surfaces that are not yet slipping. Just because the surfaces are not yet slipping that doesn't mean that the force has to be that high. Two surfaces with no tangential force between them can succeed in not slipping just fine! Like a book sitting on a table.

PeroK
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I learnt that rolling involves the coefficient of static friction unlike sliding that involves the coefficient of kinetic friction. It's known that the coefficient of static friction is always higher than the coefficient of kinetic friction. This should result in rolling to be more difficult than sliding as it involves higher frictional force, which is not the case in real life.
Rolling doesn't involve any friction to continue. Rolling continues through conservation of linear and angular momentum. Imagine a wheel rolling off a surface into space - it will keep rotating.

Static friction is necessary to initiate rolling from a surface (and to brake!). It's the static friction that provides the torque for angular acceleration. But, as the friction is static (no slipping), there is no displacement between the wheel and surface while they are in contact and no work done.

Also, if you slide a wheel along a rough surface, then kinetic friction will a) reduce the kinetic energy of the wheel and b) translate some of its linear motion to rotational motion. The wheel then reaches an equilibrium where it's rolling without slipping and friction is no longer required.

Thanks for your answers. But I am lost indeed in those details you mentioned. Let me rephrase the question.
Assume a single car tire on a horizontal surface in two situations not attached to anything:
1- It's rolling (µs is involved)
2- It's sliding (µk is involved)
since Fsmax=µsN (where Fsmax is the maximum static friction and µs is the coefficient of static friction), Fk=µkN (where Fk is the kinetic friction and µk is the coefficient of kinetic friction), and µs > µk, I can assume that the tire will experience higher frictional force while rolling than while sliding.
This conclusion is totally counterintuitive to me.

Please put my question in context with the following quote of my physics teacher.
"If you lock your wheels driving down the road on dry concrete if they are sliding, or skidding, you will have less friction than if they are rolling. (µs > µk)
This is in theory the idea of antilock breaking systems (ABS) in cars; they cause intermittent lockage of breaks to keep the wheeling rolling intermittently to prevent sliding and thus provide higher friction force (stoppage force) using μs instead of μk."

PeroK
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Thanks for your answers. But I am lost indeed in those details you mentioned. Let me rephrase the question.
Assume a single car tire on a horizontal surface in two situations not attached to anything:
1- It's rolling (µs is involved)
This is wrong. There is no friction involved in rolling. See post #5.

jbriggs444
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since Fsmax=µsN (where Fsmax is the maximum static friction and µs is the coefficient of static friction), Fk=µkN (where Fk is the kinetic friction and µk is the coefficient of kinetic friction), and µs > µk, I can assume that the tire will experience higher frictional force while rolling than while sliding.
That conclusion is overstated. The tire can experience higher frictional force while rolling than sliding.$$F_\text{s} \leq F_\text{smax} = \mu_sN > \mu_kN = F_\text{k}$$The actual force of static friction can be either greater or less than that of kinetic friction.

Modern ABS systems do not actually achieve an increase in friction above locked-up skidding. But they do preserve some measure of control while braking with maximum pedal force. Hence their utility.

Edit: It seems that modern controllers are smarter than the ones I'd understood from yesteryear and can improve traction above kinetic.

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PeroK
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I can assume that the tire will experience higher frictional force while rolling than while sliding.
This conclusion is totally counterintuitive to me.
If you have a bicycle, you can try the following experiment:

1) Lock the brakes and try to push the bike along the ground. It's very difficult. It can take quite a force to get it moving at all. That is static friction.

2) Once it is moving, it's not so hard to keep it going. That's kinetic friction, which is less than static friction.

3) Release the brakes so that the wheels can roll. Now, as you push static friction gets the wheels rotating. But, as the bike moves there is almost no resistance. And, once the bike is moving at constant speed static friction no longer plays a part.

When talking about static friction it's imporant to distinguish case 1) from case 3)!

...
Please put my question in context with the following quote of my physics teacher.
"If you lock your wheels driving down the road on dry concrete if they are sliding, or skidding, you will have less friction than if they are rolling. (µs > µk)
This is in theory the idea of antilock breaking systems (ABS) in cars; they cause intermittent lockage of breaks to keep the wheeling rolling intermittently to prevent sliding and thus provide higher friction force (stoppage force) using μs instead of μk."
That explanation is correct, except that the ABS releases the brake pressure from the locked tire and immediately re-applies the brake.
Each time the tire stops rolling while the car is still moving, the ABS repeats those steps.

That achieves two important things: better deceleration and keeping steering (in case of front wheels).
What seems confusing to you?

This is wrong. There is no friction involved in rolling. See post #5.
How then can you explain the following quote?
"If you lock your wheels driving down the road on dry concrete if they are sliding, or skidding, you will have less friction than if they are rolling. (µs > µk)
This is in theory the idea of antilock breaking systems (ABS) in cars; they cause intermittent lockage of breaks to keep the wheeling rolling intermittently to prevent sliding and thus provide higher friction force (stoppage force) using μs instead of μk."

That explanation is correct, except that the ABS releases the brake pressure from the locked tire and immediately re-applies the brake.
Each time the tire stops rolling while the car is still moving, the ABS repeats those steps.

That achieves two important things: better deceleration and keeping steering (in case of front wheels).
What seems confusing to you?
If the explanation is correct, it's a fact that the static friction of rolling wheels is higher than the kinetic friction of sliding wheels, which is not experienced when I roll or slide a wheel on its own as it's much easier to roll the wheel probably because less friction is involved.

PeroK
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How then can you explain the following quote?
Braking is very different from rolling. The brakes induce the static friction to activate. Until the brakes are applied there is no friction (static or otherwise) involved in rolling - that's why it's so efficient and you can coast along without power for so long.

PS I didn't notice that you were talking about braking until the "additional info" in post #6.

jbriggs444
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If the explanation is correct, it's a fact that the static friction of rolling wheels is higher than the kinetic friction of sliding wheels, which is not experienced when I roll or slide a wheel on its own as it's much easier to roll the wheel probably because less friction is involved.
It's because you are not applying the brakes. Static friction (of tires on road) can be greater than kinetic friction (of tires on road) if you are applying the brakes very hard, just short of breaking loose and going into a skid.

A.T.
I learnt that rolling involves the coefficient of static friction unlike sliding that involves the coefficient of kinetic friction. It's known that the coefficient of static friction is always higher than the coefficient of kinetic friction. This should result in rolling to be more difficult than sliding as it involves higher frictional force, which is not the case in real life.
The losses in rolling are determined by the coefficient of rolling resistance:
https://en.wikipedia.org/wiki/Rolling_resistance#Rolling_resistance_coefficient

The coefficient of static friction determines whether you get rolling or sliding, but has nothing to do with the efficiency of rolling.

If the explanation is correct, it's a fact that the static friction of rolling wheels is higher than the kinetic friction of sliding wheels,...
That is a fact.

... which is not experienced when I roll or slide a wheel on its own as it's much easier to roll the wheel probably because less friction is involved.
That is an error of perception.
You cannot push a tire (bracing yourself on something solid) and make it roll if there is no friction at the contact patch: you need that resisting force to create the torque that will induce the tire rotation.

When you perceive that your tire easily rolls, what you have is a succession of contact patches that grip hard against the surface, one little area after the other.
When each little section of contact patch finishes its job, it is lifted and separated from the surface by the geometry of the wheel, while the following one is beginning to land and establishes new grip.
None of those little areas slide or skid under normal conditions of pure rolling.

For that reason, a car can happily roll forward while is cornering hard: the persistent and strong static friction of each little area of successive contact patches prevents the car from sliding out of the curve.
If the cornering force is strong enough to overwhelm the static friction, a slide starts (dynamic sideways friction) and does not stop unless some other condition changes, because its value is always smaller than the value of static friction.

Think of a rack and pinion mechanism: you have smooth rolling with tenacious grip (think huge static friction) between wheel and linear gear:

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"There is no friction involved in rolling."

Maybe not once a wheel has already begun to roll on a perfectly level ideal frictionless track.

But certainly friction is necessary for rolling something uphill, even slightly uphill.

PeroK
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"There is no friction involved in rolling."

Maybe not once a wheel has already begun to roll on a perfectly level ideal frictionless track.

But certainly friction is necessary for rolling something uphill, even slightly uphill.
This is wrong.

You can rotate a wheel uphill by any torque. The same way you can rotate a wheel that is not touching any surface.

The track does not need to be frictionless for there to be no friction.

A bacon and egg breakfast 'involves' a pig and a chicken. Like sliding and rolling, their involvement isn't the same.

This is wrong.

You can rotate a wheel uphill by any torque. The same way you can rotate a wheel that is not touching any surface.

The track does not need to be frictionless for there to be no friction.
The more details you introduce the more I get confused. :(
Does a rolling car wheel on its own experience more friction than the same wheel sliding? If yes, why is it easier to roll the wheel than to slide it? [Please keep the answer as simple as possible]

PeroK
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The more details you introduce the more I get confused. :(
Does a rolling car wheel on its own experience more friction than the same wheel sliding? If yes, why is it easier to roll the wheel than to slide it? [Please keep the answer as simple as possible]
I would ignore this digression. Concentrate on rolling and braking.

I would ignore this digression. Concentrate on rolling and braking.
Does a rolling car wheel on its own experience more friction than the same wheel sliding? If yes, why is it easier to roll the wheel than to slide it? [Please keep the answer as simple as possible]

PeroK
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Does a rolling car wheel on its own experience more friction than the same wheel sliding? If yes, why is it easier to roll the wheel than to slide it? [Please keep the answer as simple as possible]
What part of "friction is not involved in rolling" do you not understand?

What part of "friction is not involved in rolling" do you not understand?
Actually, rolling involves static friction. That's why I am confused.

Giancoli-Physics-Principles-with-Applications-7th (2014) Page 204

PeroK