Why does a flat ball skip better?

Question

I posted this in physics this morning. This question is just looking for a qualitative answer, so maybe I'll get one here.

Some waterpolo players I have known like to skip their shots off the water. (This usually makes the shot easier for the goalie to block, but that's beside the point of my question). These players figured out that if they sneak in before a game and let some air out of the game balls, their shots bounce much livelier off the surface.

So why does a flatter ball bounce better?

I suspect it may have to do with surface tension--the harder ball penetrates the surface more, the softer ball flattens out and skips off.

Does anyone here know a physics-ish answer or have a link to any clue?

Answer 1

Alright Bekki I'll weigh in with my two-bits.

I think the surface tension direction is correct.  A flat ball, on a rebound will have to overcome the surface tension of the water, but that force is limited to gamma(L) where L is the circumference of the flat ball, the inner part being concave we might assume does not support a surface.    So the force should go something like:

dF = 2 PI R sin(T) cos(p) dT

p is the contact angle and Rsin(T) is the radius made by the surface for a ball whose depth is given by R-Rcos(T).   This should be integrated from Ti to Tf, Ti>Tf >0

This gives F ~ const (cos(Tf) - cos(Ti))

For a fully inflated ball, the total force will be the integral of all the circumferences from L(determined by how deep the ball goes) to 0.  As above we have

dF = 2 PI R sin(T) cos(p) dT

but we integrate from Ti to Tf=0, so we get

F~const (1 - cos(Ti))

Lets take Ti = 10 deg and Tf = 5 deg (flattened) and Tf = 0 for full, then we get

Fflat ~ const (0.011)

Ffull ~ const (0.015)

so this at least gives a rough estimate of 40% larger downward force on the full ball.

This could be tested (next time I'm at the pool), but lifting a fully inflated ball, which seems to me does have a sort of "suction" type force as you pull it out.  

Of course if you take a completely flattened ball, this  will fail since now we have to worry about the fluid mechanics of the sharp edges contacting the water.