# B Can the fish move the ball?

#### metastable

@jbriggs444 would you infer that if the water has even slight compressibility (as it does, see: http://sites.bsyse.wsu.edu/joan/teaching/bsyse558/W2/Lec5Stu.pdf ), it can at least temporarily shift the outer glass ball slightly by pushing directly off the glass, regardless of whether the ball is floating in space, water or resting on a ground with or without friction between the ball and the contact point with ground?
Yes, making the fluid compressible de-couples the motion of the fish from the motion of the fluid sufficiently to allow brief mismatches between their respective momenta.

#### Dale

Mentor
So are we 100% sure the fish’s normal swimming ability doesn’t rely entirely on water’s slight compressibility?
I am 100% sure. The compressibility of water is completely irrelevant for swimming. The speeds involved in swimming are less than Mach 0.001 so any compressibility is absolutely irrelevant. It doesn't mean that the water is not compressible to some slight degree, but merely that the compressibility is not relevant to swimming. If you were to take a fish out of water and put it in an incompressible fluid of the same density and viscosity then it would swim exactly the same (unless it died from toxic effects).

If it can’t be proven a fish can move its nose forward through 100% incompressible fluid, the realism of any scenario modeling a fish swimming in such a fluid is called into question.
All modeling of swimming and most modeling of boats is done with the incompressible assumption. They go forward just fine, both in the models and in reality.

What force (if not differential pressure) causes the water molecules in front of the fish to move behind the fish as it swims in 100% incompressible fluid?
First, you are incorrect that an incompressible fluid can not have differential pressure. It should be obvious thinking about the vertical differential pressure in hydrostatic equilibrium. But there will also be differential pressures any time the fluid or any portion thereof is accelerating.

Second, you are neglecting shear stresses which occur regardless of compressibility. A lot of swimming, if not most, is based on shear forces rather than compressive forces.

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#### mgkii

...Or what if it starts swimming from the top of the ball, aiming directly towards a point just beside where the ball makes contact with the ground, building up kinetic energy as it swims downwards, and then it collides with the point next to the ball's point of contact with the ground it was aiming towards?
That's one smart fish! It should be posting here :-)

#### clem

if you put a fish in a plastic ball filled with water, can the fish make the ball roll by swimming.

just like my hamster can make the ball roll by walking.

thanx.
The ball will roll backwards as the fish swims forward to keep the center of mass of the fish-water-ball system from moving forward.

"Can the fish move the ball?"

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