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Why We Invented Travel Cups

Can you walk and chew gum at the same time? What about walking while holding a cup of coffee? Spill any?

This photo was provided courtesy of the American Physical Society.
This photo was provided courtesy of the American Physical Society.

H. C. Mayer and R. Krechetnikov of UC-Santa Barbara’s Department of Mechanical Engineering figured out why.  They analyzed the way we walk, the fluid dynamics in the mug, and what happens when we focus on the fact that we’re hold a mug of coffee versus when we walk along with our mug and don’t think about it.

So, they developed “a simple mechanical model based on the fully nonlinear spherical pendulum equations with forced oscillations.” In other words, they modeled the coffee moving in the cup, and how walking, including starting, speeding up, and moving along, moves the coffee in the mug.

They came up with some surprising conclusions: when we walk more quickly, the mug moves forward more smoothly, but jiggles more from side to side. If we speed up quickly, we can expect to spill quickly, but just walking quickly doesn’t mean we’ll spill.

On the whole, we usually spill within 7-10 steps. That’s because most of the coffee spilling happens when you start to walk, during the “acceleration” period of your walk. It’s not the swirling of coffee in your mug, but the sloshing back and forth. Mayer and Krechetnikov also realized that we usually have to walk on uneven surfaces, up and down steps, over the dog, etc. So they also figured out that if you are careful, you may not spill as much.

scarlett_burned_travel_mug_1They suggest either a flexible container, or one with interior baffles. Or, I’d say, a travel cup. Most of all, though, if you notice how much sloshing around is going on in your mug, and change your gait, you might get to drink the whole mugful, instead of wearing it.

P.S. This research was done on coffee, but we have reason to believe that the same would be true for tea, or any other drink in a mug.

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Rachel Gilker
Rachel Gilker
Rachel's interest in sustainable agriculture and grazing has deep roots in the soil. She's been following that passion around the world, working on an ancient Nabatean farm in the Negev, and with farmers in West Africa's Niger. After returning to the US, Rachel received her M.S. and Ph.D. in agronomy and soil science from the University of Maryland. For her doctoral research, Rachel spent 3 years working with Maryland dairy farmers using management intensive grazing. She then began her work with grass farmers, a source of joy and a journey of discovery.

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