Originally posted by doodinthemoodWait...you mean BOTH ends of the thread are hanging and you pull one and then the other?
Yes, the spool rotates clockwise, and winds itself up.
Originally posted by mtthwLet me see if I understand the situation and can explain it more precisely.
It does if you understand Mephisto's explanation. Think of the forces involved.
Originally posted by AThousandYoungBut try this alternative way of thinking.
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.
Originally posted by mtthw"External" to the spool? Then there's gravity and friction as well as the pulled thread.
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.
Originally posted by AThousandYoungFriction is the only external force that moves you and your bike when you ride it (on a flat surface).
[bHowever, friction is never so strong as to propel something, as it's claimed to do in this case. [/b]
Originally posted by Mephisto2Bicycles have "internal" force though. Of course if you put an engine on the spool it will do what you want.
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).
Originally posted by AThousandYoungFriction 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.
"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.