friction
Although there are many questions similar to this, but none of these specifically talk about the situation with constant speed.
A wheel is rolling on a horizontal plane (having some friction) with constant speed.
What will be the direction of force of friction on the contact point of the wheel ?
One of the books by Resnik Haliday says there would not be any friction since the "contact point will not have any tenancy to slide". I am unable to understand this point.
When the wheel is rolling without slipping, rolling resistance brings the wheel to rest. Here friction is not the right answer let me explain you how.
This friction is not the answer why it comes to rest because friction comes into play to oppose any slippage. The concept of friction most books provide are deficient. The term "rolling friction" is also a misnomer.
The correct explanation for it is rolling resistance for which the word "rolling friction" is often used creating confusions.
Let me explain you rolling resistance and how consideration of friction gives wrong conclusions (I have provided one link explaining rolling motion and correct concept of friction at the end of this answer):
Consider a wheel rolling smoothly. what is the direction of friction force? We might think it must be opposite to the direction of motion thats why it will stop after some time. But, this friction force is providing a torque also making its angular velocity to increase. So, we might think we took the direction of friction force wrongly. So, we take the direction of friction force to be in the same direction of motion which again gives wrong result. Here is the paradox!
Here comes the role of normal reaction. A perfect rigid body doesn't exist. Rolling dynamics in the real world of non-rigid elastic materials is a complex interplay of contact forces due to deformation, and consideration of friction alone can lead to contradictory and unrealistic conclusions. The contact between the wheel and the surface on which it is moving is a surface. This surface is formed due to deformed shape of either the wheel or the surface on which it is moving or both. Because the deformations are not symmetric since the wheel is moving forward, on that surface the reaction forces on the forward portion of the surface is more than that on the backward portion, giving a torque to the wheel in counter-clockwise direction.
Wheel rolling to the right, with surface deformation. The deformation is greatly exaggerated. Normal force components across the deformed region are not uniform in size. They are greater on the forward side, producing a counterclockwise net torque.
This results in slowing down the speed and eventually comes to rest. There are other two cases when the wheel deforms but the surface doesn't and the wheel doesn't deform but the surface does. Please read the content in the link provided.
In the 2nd case if friction is not present the wheel will simply slide forward on the surface. This will no longer be rolling.
Please read this for more details: http://lockhaven.edu/~dsimanek/scenario/rolling.htm
"Rolling friction" is a misnomer. Never be confused by it. Its not friction. Its ROLLING RESISTANCE-the correct name. It comes into play because objects are not perfect rigid in real life. It comes into play because of the deformation of shape of objects when in contact. When an object rolls on another WITHOUT slipping, there is a surface contact between them.
It is the normal reaction that is responsible look at the diagram. A body rolling like this (in the diagram) is deformed and the normal forces on the front parts of the surface are always more (whether road is hard or soft) and results in a net backward force that will gradually stop the rolling body. The deformation depends on the nature of the two bodies, depending on their rigidity. Rolling resistance is usually less than static friction. Both are expressed in the same form where the coefficients are called coefficient of rolling resistance and coefficient of static friction. Experimentally, coefficient of rolling resistance is found to be smaller than coefficient of static friction for bodies with same mass. Coefficients are determined experimentally.
Best Answer
What Haliday describes is in an "Ideal" situation, where the wheel and surface are perfectly solid and there are no other forces like air resistance. Then the only force between the wheel and surface is an upward force equal to the weight of wheel (assuming we have gravity)
Actually if there is any friction, then energy will be lost and the wheel will not move in constant speed and loose speed over time.
This looks strange to us because in real life we never get to that ideal situation, but we get close. For example a marble ball on a marble smooth surface in vacuum. In practical engineering, we always have a rolling resistance which occurs from lack of that solidness. That is low on a metal-on-metal wheel like rails , but higher on cars. It gets higher when tyre pressure is lower which causes more fuel consumption.
The rolling resistance is a complex thing when looked at from theoretical physics point of view. It is cause by molecule level friction that results in loss of energy when part of the wheel compressed under the weight rolls out of the contact point.