Pure rolling with zero friction

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Discussion Overview

The discussion revolves around the dynamics of a sphere undergoing pure rolling motion on a surface with friction. Participants explore the conditions under which friction acts, the implications of pure rolling on the motion of the sphere, and the effects of external forces such as gravity on inclined planes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that once a sphere starts pure rolling, the friction acting on it becomes zero, while others argue that friction cannot be zero in certain conditions, such as on an incline.
  • One participant suggests that static friction acts on the object during pure rolling and can change its value based on conditions.
  • There is a discussion about the effects of external forces, such as the component of gravitational force acting on an inclined plane, which necessitates the presence of friction to maintain pure rolling.
  • Some participants question whether the acceleration of the center of mass becomes zero after pure rolling starts, indicating uncertainty about the dynamics involved.
  • One participant emphasizes the importance of solving the problem using force and moment balances rather than speculating on the outcomes.

Areas of Agreement / Disagreement

Participants express differing views on the role of friction in pure rolling motion, particularly regarding whether it can be zero and under what conditions. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Participants mention various assumptions, such as ideal conditions versus real-world scenarios, and the dependence on the specific setup (e.g., inclined planes). There are also references to non-uniform pure rolling, which remains undefined in the discussion.

AdityaDev
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when a sphere starts moving with initial velocity ##v_0## and zero angular velocity in a plane surface having friction, then first it will start rotating till it starts pure rolling. that is, its velocity of centre of mass will decrease due to backward friction and angular velocity will increase till ##v_{com} = R\omega##. But after it starts pure rolling, will friction become zero?
but if friction is zero then will the ball keep tolling and never stop?
com is Centre Of Mass
 
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I think that the friction acting on it will be 0 from the moment it starts pure rolling. In case of pure rolling static friction acts on the object. It can change its value according to condition.

The reason it stops it that if we don't consider ideal condition then there will be slight deformation of the surface at the bottom of the sphere. And the normal force shifts to the right. So torque due to ##N## decelerates the angular velocity. So it stops after some times.
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If an object isn't accelerating the net force acting on it must be zero.
 
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CWatters said:
If an object isn't accelerating the net force acting on it must be zero.
but In case of non uniform pure rolling where ##a=R\alpha## acceleration is non zero.
 
Satvik Pandey said:
I think that the friction acting on it will be 0 from the moment it starts pure rolling. In case of pure rolling static friction acts on the object. It can change its value according to condition.
If you have the same sphere rolling in an inclined plane starting with ##v_0## and zero angular velocity, then initially, friction will be backward and will make the sphere start pure rolling. But then friction can't be zero after it starts pure rolling right? Because we have an ##mgsin\theta## acting at centre of mass and friction has to act in opposite direction to make the acceleration of bottom most point zero.
Also after it starts pure rolling, will the acceleration of centre of mass become zero?
What do you mean by non uniform pure rolling?
 
AdityaDev said:
If you have the same sphere rolling in an inclined plane starting with ##v_0## and zero angular velocity, then initially, friction will be backward and will make the sphere start pure rolling. But then friction can't be zero after it starts pure rolling right? Because we have an ##mgsin\theta## acting at centre of mass and friction has to act in opposite direction to make the acceleration of bottom most point zero.
Also after it starts pure rolling, will the acceleration of centre of mass become zero?
What do you mean by non uniform pure rolling?

Initially when the CoM of the sphere is given velocity v and if it is left on a rough surface then at first kinetic friction acts on the bottom of the sphere but when ##v## becomes equal to ##R\omega## (i.e when pure rolling starts) after that static friction acts on it. And static friction can change its value according to conditions. So it will be zero because no external force acts on it other than friction acts on it. In case of incline it will not be zero because a component of ##mgsin\theta## acts as you have mentioned.
 
Why don't you just solve the original problem already for the case if ideal kinetic friction, using force and moment balances, and see what the answer comes out to be, rather than continuing to speculate.

Chet
 

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