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Physicists Dive into Oscillation Frequency of Coffee
Scientists puzzle out when and why coffee spills
At a recent math conference, Rouslan Krechetnikov watched his colleagues gingerly carry cups of coffee. Why, he wondered, did the coffee sometimes spill and sometimes not? A research project was born.
Although the problem of why coffee spills might seem trivial, it actually brings together a variety of fundamental scientific issues. These include fluid mechanics, the stability of fluid surfaces, interactions between fluids and structures, and the complex biology of walking, explains Krechetnikov, a fluid dynamicist at the University of California, Santa Barbara.
In experiments, he and a graduate student monitored high-speed video of the complex motions of coffee-filled cups people carried, investigating the effects of walking speed and variability among those individuals. Using a frame-by-frame analysis, the researchers found that after people reached their desired walking speed, motions of the cup consisted of large, regular oscillations caused by walking, as well as smaller, irregular and more frequent motions caused by fluctuations from stride to stride, and environmental factors such as uneven floors and distractions.
Coffee spilling depends in large part on the natural oscillation frequency of the beveragethat is, the rate at which it prefers to oscillate, much as every pendulum swings at a precise frequency given its length and the gravitational pull it experiences. When the frequency of the large, regular motions that a cuppa joe experiences is comparable to this natural oscillation frequency, a state of resonance develops: the oscillations reinforce one another, much as pushing on a playground swing at the right point makes it go higher and higher, and the chances of coffee sloshing its way over the edge rise. The small, irregular movements a cup sees can also amplify liquid motion and thus spilling. These findings were to be detailed at a November meeting of the American Physical Society in Baltimore.
Once the key relations between coffee motion and human behavior are understood, it might be possible to develop strategies to control spilling, such as using a flexible container to act as a sloshing absorber, Krechetnikov says. A series of rings arranged up and down the inner wall of a container might also impede the liquid oscillations.
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Fiber optics
The Romans must have been particularly pleased with themselves the day they invented lead water pipes around 2000 years ago. At last, they had an easy way to carry their water from one place to another. Imagine what they'd make of modern fiber-optic cables"pipes" that can carry telephone calls and emails around the world in a seventh of a second!
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Please note that in some countries, including the UK, fiber optics is spelled "fibre optics." If you're looking for information online, it's always worth searching both spellings.
What is fiber optics?
We're used to the idea of information travelling in different ways. When we speak into a landline telephone, a wire cable carries the sounds from our voice into a socket in the wall, where another cable takes it to the local telephone exchange. Cellphones work a different way: they send and receive information using invisible radio wavesa technology called wireless because it uses no cables. Fiber optics works a third way. It sends information coded in a beam of light down a glass or plastic pipe. It was originally developed for endoscopes in the 1950s to help doctors see inside the human body without having to cut it open first. In the 1960s, engineers found a way of using the same technology to transmit telephone calls at the speed of light (186,000 miles or 300,000 km per second).
Try this fiber-optic experiment!
This nice little experiment is a modern-day recreation of a famous scientific demonstration carried out by Irish physicist John Tyndall in 1870. It's best to do it in a darkened bathroom or kitchen at the sink or washbasin. You'll need an old clear, plastic drinks bottle, a flashlight (torch), some aluminum foil, and some sticky tape.
1. Take the plastic bottle and wrap aluminum foil around the sides, leaving the top and bottom of the bottle uncovered. If you need to, hold the foil in place with sticky tape.
2. Fill the bottle with water.
3. Switch on the flashlight and press it against the base of the bottle so the light shines up inside the water. It works best if you press the flashlight tightly against the bottle. You need as much light to enter the bottle as possible, so use the brightest flashlight you can find.
4. Standing by the sink, tilt the bottle so the water starts to pour out. Keep the flashlight pressed tight against the bottle. If the room is darkened, you should see the spout of water lighting up ever so slightly. Notice how the water carries the light, with the light beam bending as it goes!
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