Trouble understanding what rolling friction is

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Discussion Overview

The discussion revolves around the concept of rolling friction, also referred to as rolling resistance, particularly in the context of a sphere rolling on a flat surface. Participants explore the nature of frictional forces involved in rolling motion, the conditions under which these forces apply, and the distinctions between static and dynamic friction in rolling scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how friction can act on contact points of a sphere that are at zero velocity during pure rolling.
  • Another participant argues that if the sphere is rolling without acceleration, the frictional force should be zero, referencing Newton's second law.
  • A different viewpoint suggests that the sphere is not a perfect rigid body and deforms, leading to a shift in normal forces and raising confusion about the application of friction at contact points.
  • Several participants seek clarification on the definition of rolling friction, with one noting that it is technically called rolling resistance and that friction applies only if the sphere is accelerating or sliding.
  • One participant introduces the concept of rolling static friction, providing an analogy to mechanical interactions in a rack and pinion mechanism, while also discussing the role of surface deformation in resistance to rolling.
  • Another participant seeks a clearer definition of "rolling static friction," indicating uncertainty about the term and suggesting that static friction can exist between an unmoving wheel and the ground.
  • One participant elaborates on their interpretation of rolling static friction as the non-slipping contact of a rolling body when torque is present, distinguishing it from rolling dynamic friction.

Areas of Agreement / Disagreement

Participants express differing views on the nature and application of rolling friction, with no consensus reached on definitions or the conditions under which friction acts during rolling motion.

Contextual Notes

Participants highlight various assumptions regarding the rigidity of the sphere, the conditions for frictional forces, and the definitions of rolling static and dynamic friction, which remain unresolved.

tbn032
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In pure rolling the of the sphere contact points of the sphere are at zero velocity,how is friction opposite to the motion of sphere being applied to these points?how the frictional force f is bieng applied to the sphere?
20220819_105439.jpg
 
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If the sphere is simply rolling on a flat surface, then the frictional force must be zero because there is no acceleration. F = m a
 
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The sphere which is rolling is not a perfect rigid body, and thus it deforms. Thus the normal forces is shifted to the right from center of mass.In my book, it is written that 3 forces are acting on the sphere.
A)weight mg
B)friction at contact
C)the normal force
I am confused about the friction at contact force. How is it being applied?
 
can you please explain what exactly is rolling friction?
 
What is the book title and author?
 
Concepts of physics 1,HC verma
 
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Rolling static friction can exist when there is no rotation or movement.
For example, the driving wheels of a locomotive, trying hard to start movement of heavy cars, but without slipping on the rails.

Not a very scientific approach, but I like explaining rolling static friction like being partially similar to the mechanical teeth meshing that happens in a rack and pinion mechanism.
At microscopic level, that is aproximately the interaction that happens among two rough surfaces, in addition to molecular attraction.

The deformation of the surfaces that the book explains, will contribute to the resistance to rolling, but I believe that it is not related to pure static friction.
There is not much at all of that deformation among the steel wheels and hard steel of the rails in my previous example, yet, friction makes the heavy cars move finally.

Rack-pinion-Gear-wheel-cogwheel.gif
 
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Lnewqban said:
Rolling static friction
Do you have a definition in mind for "rolling static friction". I do not know what you mean by that phrase.

Certainly there can be static friction between an unmoving wheel and the ground upon which it rests.
 
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jbriggs444 said:
Do you have a definition in mind for "rolling static friction". I do not know what you mean by that phrase.

Certainly there can be static friction between an unmoving wheel and the ground upon which it rests.
I see it as the instantaneous non-slipping contact of a rolling body against another surface, flat or not, when a torque is present.
Once the limit of adherence if exceeded, it becomes “rolling dynamic friction” in my mind, if some rolling of the body remains.
 

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