So I was at the pool today with my daughter. She had brought along a rubber inflatable ball, about 4" across, that we were tossing back and forth. At one point I said " Watch this!" And pushed the ball as far under the water as I could reach. ( maybe a couple feet) I expected that it would rise quickly to the surface and pop out, which I thought she might enjoy ( she’s 5).
But to my surprise, the ball did rise to the surface, but did not “breech” as I expected. It just sort of scooted sideways at the surface (I assumed that was probably because of some small amount of spin I had imparted when releasing it).
But the really strange part was while experimenting with the ball I found that if I held the ball just below the surface ( instead of 2 ft down) it WOULD pop up and breech the surface by a couple inches.
I would have expected the exact opposite to be true. What is happening that is causing this ( at least to me) non-intutive result?
Possible relative facts.
ball is about 4 inches across, inflatable vinyl. Would the experiment be the same if it were a solid, yet buoyant material ( styrofoam, solid rubber)?
salt water pool
this particular ball has tiny, uniform spikes over its whole surface. Is that affecting something?
@turtlescanfly - I went through the same thing with my daughter like a decade back. Your post reminded me of this problem and I googled it and found some excellent recent research on it.
God I love the Dope! Post an obscure question, and WITHIN THE HOUR get a response that not only validates the question, but with a link to a scientific article that explains the phenomenon.
Thank you, am77494. My only regret is that the answer is so definitive that it is effectively a threadkiller.
I thought the phrase “water exit dynamics” is worth the thread alone. And it’s interesting that the experiment was done with rigid steel spheres and they still saw the behavior - I would have bet it had to do with the fact that pool toy balls are inflatable or otherwise flexible, so they compressed at depth, and had something to do with the expansion during ascent causing rippling and turbulence at the surface of the ball.
Fascinating - it definitely is not intuitive that the velocity of balls released lower in the pool would be lower than balls released near the surface, given that it seems like they have more time to accelerate. I guess the balls reach a terminal upward velocity relatively quickly, and that most of the energy imparted to the rising ball is lost to the oscillating motion instead of the rising motion of the ball? I wonder if there’s any way to keep a deep ball in a mostly vertical acceleration scheme instead of oscillating (eg. by spinning it?).
What I cant wrap my head around… at some point, a ball released 2 ft underwater becomes a ball just below the surface. Why then does it not act like a ball that started just below the surface?. How does it “know” its previous state, or rather, what forces are acting on it that cause it to act differently?
