# Physicist + Cotton reel

doodinthemood
Posers and Puzzles 04 Jan '08 21:12
1. 04 Jan '08 21:12
Keep him entertained for hours! This is quite a nice puzzle actually. It goes like this:

If you wind up the cotton reel but leave a bit dangling off, then place it on a table so the cotton goes clockwise and eventually comes off the top of the reel and goes off to the right. If you pull that thread, it's obvious that the reel will roll to the right and unravel.

But.

If you place it so the cotton goes round the reel anticlockwise, comes of the bottom of the reel and goes right. If you pull that thread, what will happen?

kinda diagrams:
First cotton reel: O--- pull and it goes to the right and unravels.
Second cotton reel: O___ pull and what happens?
2. AThousandYoung
All My Soldiers...
05 Jan '08 11:39
The spool will rotate counterclockwise. Is that all you're asking?
3. 05 Jan '08 15:08
The spool will rotate clockwise, winding more wire onto it as you keep pulling the spool towards you. You exerce a force above the contact point of the spool to the table, meaning that you are trying to rotate around that contact point, just like a bicycle wheel.
4. 05 Jan '08 15:48
Somebody get me a reel and some thread quick!
5. 08 Jan '08 19:19
Yes, the spool rotates clockwise, and winds itself up.
6. AThousandYoung
All My Soldiers...
08 Jan '08 23:331 edit
Originally posted by doodinthemood
Yes, the spool rotates clockwise, and winds itself up.
Wait...you mean BOTH ends of the thread are hanging and you pull one and then the other?

Because if you keep pulling on the same end it's not going to wind itself up no matter how you arrange things.
7. 09 Jan '08 07:20
no, one end of the thread is hanging. Underneath the reel. You pull it and it does wind up.
8. AThousandYoung
All My Soldiers...
09 Jan '08 13:51
Originally posted by doodinthemood
no, one end of the thread is hanging. Underneath the reel. You pull it and it does wind up.
That makes no sense.
9. 09 Jan '08 13:59
Originally posted by AThousandYoung
That makes no sense.
It does if you understand Mephisto's explanation. Think of the forces involved.
10. AThousandYoung
All My Soldiers...
09 Jan '08 14:221 edit
Originally posted by mtthw
It does if you understand Mephisto's explanation. Think of the forces involved.
Let me see if I understand the situation and can explain it more precisely.

There is a spool, or a cylinder with a groove for thread to fit. It is lying on a table such that it can roll around. There's cotton thread wound around it in a counter clockwise direction from our perspective (side view). The end of the thread comes loose of the spool at the point where the spool meets the table and extends out to the right along the table surface.

Pull the end of the thread, and there will be a force on the spool along the thread and along the table surface which does not pass through the center of mass, which is at a height equal to the radius of the spool. This force will tend to make the spool spin counterclockwise.

However there will be friction between the spool and the table which will resist this spinning. I think you have some idea that this friction will be enough to make the spool roll forward, but that's not what will happen.
11. 09 Jan '08 14:50
Originally posted by AThousandYoung
Let me see if I understand the situation and can explain it more precisely.

There is a spool, or a cylinder with a groove for thread to fit. It is lying on a table such that it can roll around. There's cotton thread wound around it in a counter clockwise direction from our perspective (side view). The end of the thread comes loose of the spool a ...[text shortened]... this friction will be enough to make the spool roll forward, but that's not what will happen.
But try this alternative way of thinking.

The only external force on the spool is in the direction it is being pulled. Therefore it must travel in this direction. But as long as the friction is enough to stop slipping, it can only do this by winding up the string.

Tricky, isn't it?

One slightly subtle point that Mephisto alludes to that might help is this. in a realistic reel the thread will meet the spool slightly above the table (the ends of the spool will be slightly raised). This means that the force you apply and the friction together will create a couple.
12. AThousandYoung
All My Soldiers...
09 Jan '08 21:58
Originally posted by mtthw
But try this alternative way of thinking.

The only external force on the spool is in the direction it is being pulled. Therefore it must travel in this direction. But as long as the friction is enough to stop slipping, it can only do this by winding up the string.

Tricky, isn't it?

One slightly subtle point that Mephisto alludes to that might ...[text shortened]... y raised). This means that the force you apply and the friction together will create a couple.
"External" to the spool? Then there's gravity and friction as well as the pulled thread.

The spool does not have to travel anywhere necessarily. It might spin in place or even start spinning fast enough to actively unravel.

However, friction is never so strong as to propel something, as it's claimed to do in this case. All the friction will do is help counteract any movement that would otherwise occur.
13. 09 Jan '08 22:131 edit
Originally posted by AThousandYoung
[bHowever, friction is never so strong as to propel something, as it's claimed to do in this case. [/b]
Friction is the only external force that moves you and your bike when you ride it (on a flat surface).

Edit: funny enough it is also friction that prevents you and your bike to continue moving when you stop generating power = pushing the pedals (internal force in the you+bike system).
14. AThousandYoung
All My Soldiers...
09 Jan '08 22:201 edit
Originally posted by Mephisto2
Friction is the only external force that moves you and your bike when you ride it (on a flat surface).

Edit: funny enough it is also friction that prevents you and your bike to continue moving when you stop generating power = pushing the pedals (internal force in the you+bike system).
Bicycles have "internal" force though. Of course if you put an engine on the spool it will do what you want.
15. 09 Jan '08 22:59
Originally posted by AThousandYoung
"External" to the spool? Then there's gravity and friction as well as the pulled thread.

The spool does not have to travel anywhere necessarily. It might spin in place or even start spinning fast enough to actively unravel.

However, friction is never so strong as to propel something, as it's claimed to do in this case. All the friction will do is help counteract any movement that would otherwise occur.
Friction isn't propelling anything. Pulling the string is doing the propelling, so the spool moves in that direction. Friction just stops it spinning in place.

I know I'm on pretty safe ground here, because I've just done it with a cotton-reel on the table in front of me. Trust me, the spool winds up ðŸ™‚