Rolling friction and static friction....

[fog37]

Hello,

Static friction implies no relative (maybe just instantaneously) motion between the two objects that are in contact. Rolling friction pertains to rolling objects and develops due to the asymmetric deformation of the surface over which the body rolls (if the deformation was symmetric, the rolling object would regain the potential energy stored in the deformation).

I think a cylindrical object, previously set into rolling on a different surface, could freely roll on "perfect" ice (coeff. of static friction equal to zero) but still suffer from rolling friction, correct? What I am saying is that it is essentially possible to roll without static friction...
That implies that rolling friction and static friction are frictional forces essentially decoupled from each other.

But I guess, in real life, when a wheel rolls on a real surface, it experience both rolling friction and static friction. Is that correct? Or is that true only when the wheel is rolling and accelerating?

thank you!

 

[jbriggs444]

I think a cylindrical object, previously set into rolling on a different surface, could freely roll on "perfect" ice (coeff. of static friction equal to zero) but still suffer from rolling friction, correct? What I am saying is that it is essentially possible to roll without static friction...

Rolling resistance is more usefully viewed as a torque than as a frictional force.

Let us look at the situation a little more closely. We have this rotating wheel which is also translating across a friction-free surface with rotation rate tuned just right so that the surfaces are not in relative motion at the contact patch. For simplicitly, let us assume a perfectly rigid surface and a slightly non-rigid wheel.

The leading edge of the contact patch is being deflected inward. There is a normal force associated with this deflection. The trailing edge of the contact patch is deflecting back outward. There is a normal force here as well. The normal force on the trailing edge is lower than that on the leading edge. The wheel resists deflection more strongly than it rebounds from deflection. [Which is another way of saying that it's a slightly inelastic interaction].

That difference means that the midpoint of the supporting contact force from the surface is slightly forward of where it would otherwise be. The downward force of gravity and the upward support force must be equal and opposite (else the wheel would be rising into the air or tunneling into the ground). But they do not share the same line of action. They form a couple -- a source of torque on the wheel.

With inadequate friction, the wheel will slow down under this torque (which we may now refer to as "rolling resistance") until the wheel ends up sliding across the ice instead of rolling.

With friction, there is a retarding force from static friction that acts to slightly slow the linear motion of the wheel and to partially maintain its rotation rate so that the no slip condition is upheld and the wheel rolls gently to a stop.

 

[sophiecentaur]

IMO, the step from Static to Rolling Friction is too great. You need to consider Slipping Friction first. Rolling Resistance is caused by more than the contact friction forces and involves the distortion of ground, tyres etc, with associated energy loss.

When a car is accelerating, the driving wheels will be slipping to some extent (less than the static friction force) and this provides the forward force to accelerate the car and to overcome the rolling losses. The driving force will (logically) be in the forward direction and the resistance force will be (obvs) in the backwards direction. Trying to hang on the the limited definition of friction as a "Force that makes things slow down" will always give problems. The directions that force arrows are drawn on a diagram need to make sense (again obvs) and a dodgy definition will not ensure this.