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I had asked whether the frictional force depends on the surface area in contact but the expets says no...

So why does the rolling friction less than that of sliding friction...?

Sliding friction is the friction which an object experience when it is slid over a surface but the rolling friction is experienced when an object rolls over a surface...

Ex. Sliding friction - Moving a box over a floor

Rolling friction - moving a box over floor by putting small cylinders under it

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So why does the rolling friction less than that of sliding friction...?

Rolling friction is more properly called rolling resistance. Heat is dissipated due to the inelastic deformation of the material of a rolling object, such as the rubber of a tire, when it contacts the road. The material compresses when it contacts the road and decompresses when it leaves the road during each revolution. The squeezing and un squeezing of the material generates internal friction and heat.

If you look at the Engineering Toolbox site you will find that the coefficient of rolling resistance is typically two orders of magnitude less than the coefficient of kinetic (sliding} friction. For that reason rolling resistance generates less heat than kinetic friction.

For more details on rolling resistance, see this article from Wikipedia: https://en.wikipedia.org/wiki/Rolling_resistance.

Static friction does not involve relative motion between the contacting surfaces and is therefore considered non dissipative (non heat producting).

Hope this helps.

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  • $\begingroup$ Made my comment to Steeven before I read your post. Good add. $\endgroup$
    – Bill N
    May 23, 2020 at 15:23
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"Rolling friction" is actually not a friction in the same sense as kinetic/sliding friction and static friction. A better term would be rolling resistance.

A useful answer from a few years ago is here: Is there any friction between wheel and road?

  • Kinetic friction is seen when the wheel slides over the asphalt. This is friction between objects that slide over one another.

  • Static friction is a friction that holds something still. It always works in the direction that prevents two objects from sliding.

  • Rolling resistance is different. Ideally there is no kinetic friction, and static friction only grabs the asphalt and doesn't reduce the speed (on horizontal surface and without wheel torque). Other forces that do work against the wheel rotation are collectively called rolling resistance.

Such other effects can be due to an uneven surface causing normal forces a bit off of the centre, due to soft surfaces or a soft wheel material that absorb the kinetic energy and thus slow down the car etc.

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  • $\begingroup$ +1 for clarifying rolling resistance. This might be included in your "soft surfaces" thought, but there is the damped (semi-elastic) oscillation of the stretching and compressing of the tire material as it rolls around the axle, compressing on its way down to the roadway and stretching on its back up. $\endgroup$
    – Bill N
    May 23, 2020 at 15:21
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    $\begingroup$ @BillN Good point. I've added that. And appended an "etc." to cover for everything else I'm also missing 🙂 $\endgroup$
    – Steeven
    May 23, 2020 at 15:23
  • $\begingroup$ @Steeven, if a better word for rolling friction is rolling resistance then shouldnt same happen for kinetic friction as i think heat is produced in that case too. So I am slightly confused when you say rolling is different than the other 2. I get it is different than Static but what about kinetic.? Thx $\endgroup$
    – user31058
    Jun 11, 2020 at 12:50
  • $\begingroup$ @user31058 Kinetic and static frictions are forces. Rolling friction isn't a force (not one particular force, at least). If we think of the term friction as a force, then rolling friction is therefore a misleading term. $\endgroup$
    – Steeven
    Jun 11, 2020 at 13:00
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Rolling friction - moving a box over floor by putting small cylinders under it

Suppose there is friction enough between the contact surfaces cylinders - floor and cylinders - box to avoid slippage. But besides that, the rolls are very well turned and aligned, and the surfaces are flat without any bumps. And box, cylinders and floor are made from hard material, (steel for example).

In this case, the resistance to an applied force has nothing to do with friction. It is only a reaction force (Newton's 3rd law) while the system accelerates. It has 2 sources:

$$1) ma$$ related to the mass and linear acceleration of the box and rolls, $$2) nI\frac{\alpha}{r}$$ related to the angular acceleration of the rolls ($\alpha$), where $I$ is the moment of inertia, $r$ the radius and $n$ the number of rolls.

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