Rope swinging with periodic radius changes

  • Context: Graduate 
  • Thread starter Thread starter David Carroll
  • Start date Start date
  • Tags Tags
    Periodic Radius Rope
Click For Summary

Discussion Overview

The discussion revolves around the dynamics of an object tied to a rope that is swung in a manner where the radius of the rope periodically changes, specifically exploring the implications of this setup on the object's trajectory and acceleration. The focus includes theoretical considerations of motion, acceleration at corners, and the feasibility of achieving a square path through mechanical means.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that swinging a rope with an object tied to it could theoretically create a square path if the radius changes periodically.
  • Another participant claims that achieving a perfect square path would result in infinite acceleration at the corners, necessitating infinite force.
  • A question is raised regarding the feasibility of the proposed experiment, with a focus on whether it is indeed possible to create such a trajectory.
  • It is argued that a centrally-directed force would make it impossible to have corners in the path of non-zero size, suggesting that the trajectory would instead be a line directed toward the center.
  • One participant proposes that if the radius contraction were continuous, the velocity at the corners could not be zero, questioning the mechanics of a motorized reel that would control the rope's length.
  • Another participant notes that if the force is central, angular momentum is conserved, implying that the velocity cannot be zero at the corners.
  • A later reply discusses the mechanics of the reel, suggesting that while the reel's velocity could be zero, the overall system's angular momentum would still be maintained.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of achieving a square trajectory and the implications of acceleration at the corners. There is no consensus on whether the proposed mechanism could work as intended.

Contextual Notes

The discussion includes assumptions about the nature of forces involved, the mechanics of the reel, and the implications of angular momentum, which remain unresolved and are dependent on specific definitions and conditions.

David Carroll
Messages
181
Reaction score
13
If one simply swings a rope with an object tied to the end of it, the object describes a circle. But if one were to create a contraption that caused the radius of the rope to periodically decrease 4 times every revolution, one could cause the path of the object to describe a square. My question is, why is the object's acceleration not infinite once it makes the 90 degree angle?
 
Physics news on Phys.org
For a perfect square and with a finite velocity of the object, acceleration "is" infinite at the corners. And you would need an infinite force.
 
So the above experiment would be impossible?
 
For a centrally-directed force such as a rope, yes it would be impossible to make the corners of the path perfectly square. The loophole that mfb left open ("finite velocity") could allow for a finite force if the velocity at the corners were zero. But with a centrally-directed force, the resulting trajectory cannot be a square of non-zero size. It would have to be a line headed straight toward the center.

The remaining loophole, a square of zero size, is probably not what you had in mind.
 
If the period of radius contraction were perfectly continuous, then the velocity at the corners couldn't be zero, could it? In other words, the centrally-directed force is a reel which reels in the rope slightly 4 times per revolution in such a way to create a trajectory of a square for a ball tied to the other end of the rope. And if the reel were connected to a perfectly timed motor, then the reeling would be smooth and continuous. But if that's the case, how could the velocity of the ball be zero at the corners? Wouldn't a motorized reel using a constant force result in constant velocity for the ball at the end of the rope?

Oh, wait a minute...when the motorized reel has extended the rope to its upper limit, the reel itself has reached zero velocity because it cannot go from reeling out to reeling in zero time. Otherwise the reel itself would have infinite acceleration. I just answered my own question!
 
If your force is central, then angular momentum is conserved and not zero, so the velocity can never get zero.
 
The velocity of the reel would be zero. The reel has an axis that is perpendicular to another contraption that is spinning that reel. That contraption would have a constant angular momentum. But the axis of the reel itself, qua reel, would change momentum once it ceased to be reeling out and started reeling in.
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 16 ·
Replies
16
Views
3K
  • · Replies 8 ·
Replies
8
Views
2K
Replies
1
Views
2K
  • · Replies 6 ·
Replies
6
Views
8K
Replies
11
Views
6K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 4 ·
Replies
4
Views
3K
Replies
2
Views
1K