Why does a coin take 2 full rotations around another coin?

  • Context: High School 
  • Thread starter Thread starter Devin-M
  • Start date Start date
  • Tags Tags
    Rotation
Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of a coin making two full rotations around another coin when rolled along its circumference. Participants explore the mechanics behind this observation, questioning the relationship between rotation and revolution, and the implications of the path taken by the moving coin. The scope includes conceptual reasoning and mathematical reasoning related to circular motion and geometry.

Discussion Character

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

Main Points Raised

  • Some participants express confusion about why a moving coin requires two full rotations instead of one, questioning if the path length is effectively longer than the circumference suggests.
  • Others propose that one rotation accounts for the revolution around the stationary coin, while the second rotation is due to the coin's own rotation as it travels along the curved path.
  • A participant introduces a mathematical perspective, suggesting that the angular velocity of the orbiting coin must account for both its linear motion and its rotation.
  • Some participants emphasize that the path taken by the coin is curved, which necessitates additional rotation beyond what would occur in a straight line.
  • There are references to visual aids and previous discussions that may help clarify the concepts involved.
  • Some participants argue that the distance traveled by the outer portion of the coin is longer than that of the inner portion, leading to further confusion about the relationship between distance and rotation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the explanation for the two rotations. Multiple competing views remain regarding the mechanics of the motion and the implications of the path length.

Contextual Notes

Some participants mention the need for visual aids to better understand the concepts, and there are references to previous threads that may provide additional context. The discussion includes various assumptions about the relationship between the sizes of the coins and the nature of their motion.

Who May Find This Useful

This discussion may be of interest to those studying mechanics, geometry, or anyone curious about the dynamics of circular motion and the nuances of rotational movement.

  • #61
Mister T said:
Note that the moon makes one rotation when it makes one revolution around Earth, because the same side of the moon always faces Earth.
Exactly. When you take pure sliding along a straight line (no rotation), and merely roll that entire scenario so the line becomes a circle (preserving the contact point of the coin with the line), the you already have 1 rotation, like the tidally locked Moon has.
 
Physics news on Phys.org
  • #62
Devin-M said:
Yes, I'm simply musing out loud how difficult it is to intuit the offered answer.
I was out of the loop when this thread ran, originally. This quoted statement only goes to show how useful Maths is. Intuition is a false friend but (the appropriate) Maths will not let you down.
 
  • #63
Devin-M said:
This one baffles me, I still can’t get my head around it (no pun intended).
Ahh. May I ask you that what does"Why does a coin take 2 full rotations around another coin? "mean?
I cannot get the point .
Does coins really take 2 full rotations around another coin then it will be stationary??
 
  • #64
painter said:
Ahh. May I ask you that what does"Why does a coin take 2 full rotations around another coin? "mean?
I cannot get the point .
Does coins really take 2 full rotations around another coin then it will be stationary??
Take two coins. Place them flat on a table, one above the other:
https://en.wikipedia.org/wiki/Coin_rotation_paradox said:
1715179903862.png
Roll the top coin around the bottom coin without slipping so that it completes a complete circle. Watch how many times the picture on the moving coin rotates [relative to the fixed orientation of the table].
 
  • #65
painter said:
Ahh. May I ask you that what does"Why does a coin take 2 full rotations around another coin? "mean?
I cannot get the point .
Does coins really take 2 full rotations around another coin then it will be stationary??
Have you actually read this entire thread?
 
  • Like
Likes   Reactions: russ_watters, Vanadium 50, phinds and 1 other person
  • #66
painter said:
Does coins really take 2 full rotations around another coin then it will be stationary??
You have to apply a force to make the coin move. Anytime you stop applying the force the coin stops moving (due to friction).

When you move the coin you have to rotate it yourself in such a way that it doesn't slip against the coin in the center.
 
  • #67
Mister T said:
in such a way that it doesn't slip against the coin in the center.
Use gears instead; no significant friction involved.
 
  • #68
sophiecentaur said:
Use gears instead; no significant friction involved.
Some coins have ridges that act like little gear teeth. But I meant friction between the coins and the surface they rest on.
 
  • #69
Mister T said:
You have to apply a force to make the coin move. Anytime you stop applying the force the coin stops moving (due to friction).
Mister T said:
Some coins have ridges that act like little gear teeth. But I meant friction between the coins and the surface they rest on.
This is over-thinking the issue. The coins do not slip as they move against each other. How this happens is irrelevant and a distraction.
 
  • Like
Likes   Reactions: phinds
  • #70
Before duplicating the entire thread, maybe we should give @painter an opportunity to read it.
 
  • Like
  • Love
Likes   Reactions: SammyS, Averagesupernova and DaveC426913
  • #71
I love it when you guys go all in with something like this. :)
 

Similar threads

  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 37 ·
2
Replies
37
Views
4K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 9 ·
Replies
9
Views
2K
Replies
6
Views
6K
  • · Replies 6 ·
Replies
6
Views
4K
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K