Do you also have trouble walking with a cup of coffee without it sloshing over the edge? Two American physicists investigated why this is the case. Their conclusion: it’s our own fault, the way we walk makes spilling coffee inevitable.
When physicist Rouslan Krechetnikov and his doctoral candidate Hans Mayer saw at a scientific conference how their colleagues struggled to walk without a spill with a full mug of coffee, their idea for a new study was born. They wondered what happens in such a mug, which makes it seem inevitable that the coffee runs over the edge while walking.
The natural swing
A question that is perfectly suited to the two who are active in fluid dynamics at the University of California at Santa Barbara – the study of fluid movement. By the way, realize how much science is behind something “simple” like walking with a cup of coffee. In addition to the walking movement that differs per person, you are dealing with a complicated interplay of forces between the coffee molecules in the mug.
Moving coffee back and forth in a mug is similar to a crank. Just like a pendulum, coffee has a certain natural frequency. This is the frequency with which the coffee naturally fluctuates. Factors such as the shape and size of the mug or the amount of coffee determine how great the natural frequency is. Krechetnikov and Mayer determined this frequency for a standard type of coffee mug.
Pay attention from step seven
The duo discovered – by running test subjects with a coffee mug at different speeds – that the natural frequency of the coffee almost corresponds to the rhythm of a person’s walking movement. In other words: every step you take gives an extra boost to the swing of the coffee, as if you were pushing a swing. Due to our gait, we automatically rock our coffee!
Whether it actually spills depends mainly on irregularities in your walking pace. For example, if you start to walk faster and faster – which people often tend to do – or drift a little. Using cameras that recorded the subjects’ walking motion and a sensor that monitored the “coffee level” in the mug, the researchers found that between the seventh and tenth steps, the coffee is most likely to slosh over the edge. Just so you know.
Keep an eye on your mug
Can spillage be prevented? Yes, although the tips the researchers offer feel like an open door, with the difference that they are now scientifically substantiated. One: walk slowly. The slower you walk, the more likely that the fluctuations of the coffee remain within bounds (read: edges of the mug). Two: don’t overfill your mug. A distance of one-eighth of the diameter of the mug from the top edge should suffice. That’s about an inch.
And third, look at your mug as you walk. The experiments showed that subjects who watched their mug were significantly less likely to spill. Probably because people automatically slow down or because they correct excessive fluctuations in the mug with their hand. In short, even though the research seems obvious, there is more to it than you think. So a contender for an Ig Nobel Prize.