POWER WORDS (adapted from the Yahoo! Kids Dictionary)

As a disc representing a stone plunges into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in the video, the air is filled with smoke for visibility) shoots through the shrinking opening at supersonic speed.

But there was another difficulty: It’s hard to see and measure air. To solve that problem, the scientists filled the air above the water with smoke and illuminated the smoke with a laser, which made the moving air easier to see. (To make the smoke, Gekle said, they used a smoke machine like the ones that provide the dramatic effects seen onstage at theaters.)

Finally, because everything happens so fast when the rock moves through water, the scientists had to find a way to slow down time. As the disc moved through the water, the scientists took pictures with a camera that captured 15,000 frames every second. (That’s faster than most movie cameras.) After the experiment, the researchers could slow down the movie and, aided by computer simulations, calculate the speed of air as it blew out of the hourglass-shaped tube.

But there’s one aspect of supersonic air that Gekle and his team didn’t observe. When a jet exceeds the speed of sound, the air around it produces a noise like thunder, called a sonic boom. So far, however, Gekle says the tiny air jets aren’t making even a teeny, tiny boom — but the researchers will keep listening.

speed of sound About 760 miles per hour, through air at sea level.

supersonic Faster than the speed of sound.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics and electromagnetism, as well as in modern fields including atomic and nuclear physics, solid-state physics, particle physics and plasma physics.

force The capacity to do work or cause physical change.

pressure Force applied uniformly over a surface, measured as force per unit of area.