Warp wrote:
>   Ok, here's an easy way of stopping the rotation of an object in a closed
> system:
> 
>   Construct a cannon on the surface of the object so that it will shoot
> in the direction of the tangent, towards the rotation direction. Shoot
> a heavy-enough object fast enough so that the recoil of this shot will
> stop the rotation of the main object. (If you are worried about possible
> effects of having just one such cannon on the surface of the object then
> just put another identical cannon at the opposite side of the object,
> aimed towards the opposite direction, ie. still at the direction of
> rotation at that side, and fire both cannons simulatenously.)
> 
>   The projectile is attached to a cable connected to the object at a
> point where the pull caused by the object when it reaches the cable
> length causes an even force on the center of mass of the object. (Such
> a point must exist because there are two extremes: Attaching the cable
> on the object at the point where the cannon is would effectively negate
> the whole operation. However, attaching the cable on the opposite side
> of the object would actually make it start rotating in the opposite
> direction compared to the original one. A balance point must thus exist
> somewhere in between.)
> 
>   When the projectile has thus been stopped, just pull it back to the
> surface of the object. The rotation of the whole system will have stopped
> without permanently ejecting any material.
> 
>   (Where did all the angular momentum go? Well, stopping the projectile(s)
> will require energy. The majority of this energy will be dissipated as
> heat from the cable.)
> 
Sorry but you are just wrong.  Reminds me of people who have in their 
mind a design for a perpetual motion machine.  They concoct some system 
of levers and weights or magnets that they think will work.  However if 
you do a complete accounting of all of the forces and effects it simply 
does not work.

In this case you assert that there is a point at which the cables can be 
attached that will prevent the stopping action from imparting any spin 
to the parent body.

Would you like to draw a diagram that shows where this point is ?

The projectile will only affect the rotation of the body if it is fired 
at some off-centre tangent.  You will find that any action that brings 
it back to rest with respect to the parent body will cause an equal but 
opposite force also at an off-centre tangent.  This will always negate 
the spin.

No system of cables, pulleys, magnets, gyroscopes, friction, 
electricity, radiation, carbon nonotubes, superconductors or whatever 
can alter that.

As I said if you can some up with some way to convert between energy and 
net angular momentum then you've got a big future ahead!

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