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

[A.T.]

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.

 

[sophiecentaur]

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.

 

[painter]

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??

 

[jbriggs444]

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:

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].

 

[Averagesupernova]

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?

 

[Herman Trivilino]

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.

 

[sophiecentaur]

in such a way that it doesn't slip against the coin in the center.

Use gears instead; no significant friction involved.

 

[Herman Trivilino]

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.

 

[DaveC426913]

You have to apply a force to make the coin move. Anytime you stop applying the force the coin stops moving (due to friction).

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.

 

[Vanadium 50]

Before duplicating the entire thread, maybe we should give @painter an opportunity to read it.

 

[sbrothy]

I love it when you guys go all in with something like this. :)