The recoil of a hose when the tap is suddenly opened

most fish are neutrally boyant, hence the approximate density of water.

other non-swimming sea creatures, (snails, crusataceans etc) are denser.

Bye. Jasen

jasen wrote in news:emeo29$53b$ snipped-for-privacy@jasen.is-a-geek.org:

Thanks, captain obvious.

The answer lies in the Conservation of Momentum.

In the frame of reference of the table upon which the bag rests, and assuming the fish and water are initially at rest, then the initial momentum of the bag and its contents is zero. Once the fish starts moving, its forward momentum is be cancelled by the movement of the bag and/or water so that the total momentum remains at zero. If the fish accellerates sufficiently, the resulting backward force might be enough to overcome the static friction with the table, cause the bag to roll backward a little. Or, if the fish accelerates slowly and then collides with the wall, the bag might jump forward as depicted in the movie. Momentum is conserved in each case but energy is transferred to the bag of water. A child sitting in a cardboard box and lurching it forward by aggressive rocking employs a similar technique.

-- Joe Legris