We demonstrate the rebound characteristics of three balls impacting the water surface at shallow angles: a Superball® a racquetball, and a water bouncing ball (Waboba®). On a solid surface the three balls have coefficients of restitution of 0.9, 0.8, and 0.2, respectively, yet a lower rebound coefficient results in better water skipping behavior. In Figure 1, a Waboba is thrown at the surface of a lake, skipping an astonishing 23 times before landing on the opposite shore approximately 46 m away. In fact, we have observed skipping distances of 100 m which is by no means an upper limit!

FIG. 1.

Images of a Waboba thrown across a lake, skipping 23 times before landing on the shore 46 m away. Arrows indicate the position of the ball.

FIG. 1.

Images of a Waboba thrown across a lake, skipping 23 times before landing on the shore 46 m away. Arrows indicate the position of the ball.

Close modal

Looking at the first impact from the side of the cavity, the Superball plunges underwater without skipping due to the negligible deformation and large mass ratio (Figure 2(a)) (see video). Conversely, the racquetball has a lower mass ratio and greater elastic deformation resulting in 1-3 successful skips downstream (Figure 2(b)). Vibration induced by impact results in a small deformation in the right hand side of the cavity before exit (frames 3-5) and two large spray patterns are flicked off of the ball surface (frame 5). The Waboba flattens inside the cavity due to its large elastic deformation, causing the portion in contact with the water to resemble the shape of a skipping stone (Figure 2(c)). This allows it to skip more easily off of the surface resulting in skipping behavior similar to that observed in Figure 1. The Waboba is more compliant than the other sports balls and thus less likely to interact with its own cavity. However, if thrown too slowly or at too high of an angle the ball will deform within the cavity, creating the dimpled cavity appearance similar to the racquetball, and skip fewer times (Figure 2(d)).

FIG. 2.

The first skip cavity formation viewed from the side for three different sports balls: Superball (a), racquetball (b), and Waboba (c and d) (enhanced online). [URL: http://dx.doi.org/10.1063/1.4746071.1]

FIG. 2.

The first skip cavity formation viewed from the side for three different sports balls: Superball (a), racquetball (b), and Waboba (c and d) (enhanced online). [URL: http://dx.doi.org/10.1063/1.4746071.1]

Close modal

The spherical Waboba elastically deforms at each impact, which eliminates the need for the precise attack angle required for proper stone skipping,1 thus removing the skill necessary for skipping objects on the water surface.

1.
C.
Clanet
,
F.
Hersen
, and
L.
Bocquet
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Secrets of successful stone skipping
,”
Nature (London)
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29
(
2004
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