"Warp" <war### [at] tagpovrayorg> wrote in message 
news:472379eb@news.povray.org...
>  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.)

This violates one of the fundamental principles of dynamics, the principle 
of conservation of momentum.

When you "pull it back", this will cause the object to start spinning again, 
and you'll find that when you have the projectiles back where they started, 
the spin will be exactly the same as when you started.

The energy you lost in friction/heat will be identical to the energy you 
used to fire the cannons in the first place.

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