# direction of friction on an object rolling with constant speed

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.

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.

• Can a body roll without friction? – gpuguy Jun 6 '15 at 13:39

Newton's first law states that any object undergoing a constant velocity or is at rest will continuously be at that state of motion ( either at rest or constant velocity ) Using this law, and coming to your situation if an object is moving at a constant velocity and if it encounters a frictional force it will indubitably decrease in speed and gradually fall to zero. The friction force is a property of a material to oppose motion so if it is a rough surface. There will be some friction. But since the situation we are considering is wheels. The friction ( coefficient of friction) is very very small, so small it can be considered or modeled as if the friction force is zero. Imagine that if the surface and the wheel have 0 coefficient of friction (ideal). Do you think that the wheel will roll?

• The object's contact point is not moving with constant velocity. only its speed is constant- the direction is changing continuously – gpuguy Jun 6 '15 at 13:11