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When a drop falls into a tub of water, we see a small drop of water rising above the surface of water. This situation does not conserve energy (because initial drop has more kinetic energy and surface energy with respect to the second raised drop). As there is no net force there, but it does not conserve momentum (because initial momentum is downward with greater magnitude but final momentum is upward with lesser magnitude).

Where is the fault in my assumption in the above situation? How does this happen failing these fundamental laws!!!

This happens not only with water drop, but anything other than gases.

Sandesh Goli
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1 Answers1

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a slow-motion video of this process reveals the following: when the droplet strikes the liquid surface, it pushes down on the liquid against the restoring force of surface tension, creating a downward-protruding concavity in the surface. Surface tension then acts to pull the concavity back up again, but because the system contains very little damping, the rising surface overshoots its equilibrium (level) position and rises further to form an upward bulge. As the bulge rises, it is accompanied by the development of a velocity field in the liquid in which the velocity vectors are all pointed towards the highest point in the bulge. Along the central axis of the bulge, those vectors are pointing almost straight up which means that the fluid parcels in the centermost and topmost portion of the bulge really want to continue rising even after the outermost portions of the bulge have reversed direction and have begun to fall back.

The net result is the ejection of a secondary droplet upwards off the tip of the bulge.

niels nielsen
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