Rolling Bowling Ball with backspin

  • Context: Undergrad 
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Discussion Overview

The discussion revolves around the behavior of a bowling ball that is rolling with backspin on a surface with friction. Participants explore the implications of the ball's motion, particularly when its center of mass velocity (Vcm) becomes momentarily zero while still spinning. The conversation includes theoretical considerations and experimental observations.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Experimental/applied

Main Points Raised

  • One participant describes a scenario where a ball with clockwise backspin moves to the left and experiences kinetic friction that slows both its Vcm and spin.
  • Another participant suggests that when Vcm becomes zero, kinetic friction may reverse the Vcm and increase it until the ball rolls without slipping.
  • A different viewpoint posits that the ball might immediately start pure rolling to the right and return to its starting position when Vcm is zero, questioning the plausibility of this scenario.
  • Some participants argue that finite accelerations rule out the possibility of an infinite impulse being applied to change the Vcm instantaneously.
  • An experimental suggestion is made involving a "Hula Hoop" to demonstrate similar principles of motion and backspin, indicating that practical experimentation can provide insights into the theoretical discussion.

Areas of Agreement / Disagreement

Participants express differing views on the outcome when Vcm is zero, with some supporting the idea of a reversal of Vcm due to friction, while others argue against the feasibility of instantaneous changes in motion. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants note that the assumptions regarding the nature of friction and the conditions under which the ball transitions from slipping to rolling are not fully defined, leading to uncertainty in the conclusions drawn.

FallenApple
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Ok so a ball starts moving to the left while the rotation is clockwise, hence the backspin. On this surface there is friction. So clearly as the ball moves to the left, there is Kinetic friction to the right slowing down the Vcm and the spin as well.

So assuming that the initial backspin is large enough such that that eventually the Vcm becomes momentarily 0 while the ball is still spinning clockwise.

At this point, what will happen? I've heard two things.

1: the kinetic friction pointing to the right will reverse the Vcm and increase it until it starts rolling without slipping.

2: A the moment the Vcm is 0, the ball immediately starts pure roll to the right and returns to where it started.Which one is more plausible?

I'm thinking #1 is because the backspin will cause friction to the right even though the Vcm is 0. Hence causing a short time interval where there is still slipping.

But #2 is somewhat plausible as well. If the material of the friction is low, maybe the kinetic friction will not exist at that instant when Vcm is 0, because the backspinight be weak to cause friction.
 
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Accelerations will always be finite, that alone is sufficient to rule out the wrong scenario.
 
mfb said:
Accelerations will always be finite, that alone is sufficient to rule out the wrong scenario.
Right, if the ball starts rolling to the right with pure roll at that instant then that implies an infinite impulse was given to jump the Vcm to the one needed. Which is impossible
 
Experiment can be entertaining. Take a "Hula Hoop" (a length of plastic tubing perhaps 2 cm in diameter bent and joined end-to-end to form a circular hoop perhaps 1 meter in diameter). Hold it underhand and throw it horizontally forward at a gentle pace. Just prior to the point of release use a flicking or snapping motion of the wrist to impart a goodly amount of backspin.

Watch as the hoop travels away from you, stops and ultimately returns to your hand, staying upright throughout.
 

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