The science behind my spilled coffee.
Hydrodynamics and resonant frquencies combine to cause me to spill my coffee partway back to my seat in the far corner of the cafe. My steps are less certain than of yore, thanks to age and my wounded body. I hold the handle of the cup in a good grasp, in the way I was encouraged to hold a camera or a handgun. "As though you are holding a live bird", as I recall. Not too tightly as that causes strain in muscles and tendons, yet firmly enough to resist gravity and momentum and provide stability. I watch my foot placement, plan the next footstep, avoid the scattered bags and laptop charger cables. I move slowly, deliberately. Still my coffee spills, inevitably.
It turns out that the natural sway of my body (and everyone elses'!) means that the liquid in the cup moves, and sooner or later, hits a resonant frequency, as happens when pushing a child in a swing. These subtle low-frequency vibrations build up until the liquid is sloshing back and forth with enough momentum that it clears the rim of the cup and, devoid of a saucer, spills all over the oche. It's now down my jacket sleeve, my freshly-pressed trousers and my newly-polished shoes. Not to mention laving on the floor a slippery hazard for the next soul to come the same way.
I recall reading a study in which a team of people researched this phenomenon, and discovered that on average, the spill takes between seven and nine footsteps. ¹ Apparently it's inevitable , this sloshing. Subtle vibrations build up over time. Rocket makers are familiar with this, and design their fuel tanks with baffles to reduce sloshing that can build up with disastrous results, even more so than my coffee spill. This can lead to what rocket designers euphemistically call RUD, "Rapid Unscheduled Disassembly",i.e. a flight-ending explosion. Spectacular to watch, but that's an end to an expensive rocket and its extremely expensive payload. Many is the satellite lost to fuel slosh. So whilst my coffee spill is inconvenient, at least there's not a billion-dollar loss.
Now it's not 100% the case that my coffee spills; sometimes whatever dice the universe rolls means I make my saving throw. Barring that, seven steps is all it might take, no matter how careful I am, how I hold the cup , no matter how many hands I use to cradle it. I ask for a rag to mop up the slippery mess, gripe gently that the cafe should provide a saucer for each cup. Seriously, when did that stop being a thing? "One less thing to break", is the predictable answer. And whilst i understand that, I need you, the business, to understand that I want to sit down and enjoy my coffee without spilling even a mouthful, and not to leave coffee rings on the table that someone meticulously cleaned earlier in the day. Also, i am a stickler for tradition and expect a saucer.
Seven steps is all it takes to create sufficient hydrodynamic instability, no matter what care we take. Somewhere in this there's a life lesson for society.
¹ I looked for the paper, cannae find it.
$ xclip -o | wc -w
523