Back at the lab, Krechetnikov and Mayer set up an experiment: They asked a person to walk at different speeds along a straight path with a filled coffee mug in hand. The volunteer did this in one of two ways-either focusing on the coffee mug, or looking straight ahead. A camera recorded the person's motion and the mug's trajectory, while a tiny sensor on the mug recorded the instant of spillage.
A fluid's back-and-forth movement has a certain natural frequency, and this is determined by the size of its container. In their paper published last week in Physical Review E, Krechetnikov and Mayer show that everyday mug sizes produce natural frequencies that just happen to match those of a person's leg movements during walking. This means that walking alone, without any other interference, is tuned to drive coffee to oscillate in a mug. But the researchers also found that even small irregularities in a person's walking are important: These amplify the wilder oscillations, or sloshing, which bumps up the chance of a spillage.
"This is a very cool study," says Lei Ren, a specialist in the biomechanics of walking at the University of Manchester in the United Kingdom. "It reveals the sophisticated interplay between human body dynamics and the fluid mechanics of spilling coffee."
Their advice? Don't walk too fast while carrying coffee, don't fill the cup to the brim, and watch what you're doing. There's more, which you can read at Science Now. Link -via reddit
(Image credit: H.C. Mayer and R. Krechetnikov)