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Kettles Stop Whistling in the Dark

British physicist Lord Rayleigh is best known for his discovery of argon and for explaining, in 1871, why the sky is blue.

But he also puzzled over this.

Rayleigh knew that a kettle makes that sound when steam jets through the hole in a thick lid that has a gap in the middle.

He speculated that the jet becomes unstable inside that gap, setting up an acoustic feedback loop within the gap.

But he couldn't prove it.

Now two engineers at Cambridge University claim to have solved the puzzle and proved Rayleigh wrong.

The work is in the journal Physics of Fluids.

The engineers found that a kettle actually whistles in two distinct ways.

It starts off with air vibrating in the gap between the layers of the lid, like when you do THIS and THIS.

But as the pressure builds, vortices of steam peel off from the jet exiting the lid.

Each vortex creates sound waves at a frequency that depends on the length of the spout and the pressure inside it.

Rising temperature means rising pressure, which produces a rising whistle which means it's time for tea.

 

Mine Injuries May Rise Right after Daylight Saving Time

Don't forget to move your clocks forward this weekend.

And then don't forget to be more careful in the days after you adjust your clocks,

because a recent study found that the hour of lost sleep was related to increased job-related injuries.

Probably because sleepy workers were less alert.

The work appeared in the Journal of Applied Psychology.

Organizational behavioral psychologists Christopher Barnes and David Wagner analyzed reports filed with the National Institute for Occupational Safety and Health about injuries that took place in mines.

The study looked at all reports between 1983 and 2006.

And they found that compared with other days, more injuries happened on the Monday after daylight saving time went into effect, and the injuries were more severe.

Survey data found that people sleep on average 40 minutes less on the Sunday night right after the time change.

The data also revealed that on Mondays after the switch to standard time, when we gain an hour of sleep, there's no significant differences in sleep or injuries.

So enjoy that extra hour of sunlight in the evening. Carefully.

转载于:https://www.cnblogs.com/huangbaobaoi/p/8481808.html