A ball is rolling freely on a flat surface

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Discussion Overview

The discussion revolves around the physics of a ball rolling on a flat surface, particularly focusing on the effects of friction and motion when transitioning between surfaces with different frictional properties. Participants analyze concept questions from a physics exam that challenge their understanding of rolling motion, friction, and the implications of moving onto a frictionless surface.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that a ball cannot roll on a friction-free surface and can only slide, questioning the implications of "rolling freely."
  • Others propose that if a ball is rolling on a surface with friction and then moves to a frictionless surface, it should continue to rotate and translate without stopping.
  • A participant references a website claiming that friction for uniform rolling at constant velocity is zero, prompting questions about the validity of this statement.
  • Some participants argue that the correct answers to the exam questions differ from what was provided, suggesting that the questions may be misleading or incorrect.
  • One participant emphasizes the importance of understanding the independence of rotational and translational motion in the context of conservation laws.
  • There is a suggestion that the common experience of skidding in low-friction conditions, such as on ice, informs their understanding of the physics involved.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the correct answers to the concept questions, with no consensus reached on the interpretations of "rolling freely" and the role of friction in the scenarios presented.

Contextual Notes

Participants highlight the ambiguity in the phrasing of the questions and the assumptions regarding friction and motion, indicating that the discussion is influenced by differing interpretations of the terms used.

Who May Find This Useful

This discussion may be of interest to students preparing for physics exams, educators seeking to understand common misconceptions about rolling motion and friction, and anyone interested in the dynamics of motion in physics.

  • #31
If angular momentum wrt to the COM stays the same, the ball's rate of rotation omega stays the same. Since v=omega*r, linear velocity must also remain the same.
If angular momentum wrt to the COM stays the same, the ball's rate of rotation omega stays the same and if the ball is rolling uniformly, i.e., if v=r*omega throughout its motion-this we cannot take for granted- , then yes, the linear velocity remains the same.
 
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  • #32
donotremember said:
Therefore the consensus will be:

The correct answer to the first question is (d), NOT (b)

The correct answer to the second question is (a), NOT (c)

Do we all agree?

I would agree with this. Professors...I've seen a lot of professors be outright wrong in my days. It's a very frustrating situation, because the professor and class end up laughing at YOU, because, after all, how can the professor be wrong?

One professor made the claim that to go fast doesn't require power...just gearing (I'm talking on the Earth in the real world). When I brought up the question of why an aircraft carrier needs a million horsepower, his answer was that acceleration needs power, and an aircraft carrier needs to constantly accelerate to point itself towards the wind.

I was like...ooh.

k.

And yes, he confirmed that the USS Nimitz can go 30 knots through the Atlantic using only a small electric motor and the right gearing :P

Plently of other examples in my college days...
 

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