The spool can be made to roll
forward
or backward depending on the angle at which the rope is pulled.
The spool rolls when the rope
exerts
a torque about the point of contact between the spool and the
table.
There is no torque if the rope is pulled at the critical angle, as
shown
in the diagram. At greater angles, the spool rolls backward; at
smaller angles it rolls forward.
NOTES
A variation is to arrange a weight
and pulley so that the hanging weight supplies a force at the critical
angle.
Either way the spool is moved, it will roll back to the initial
configuration.
