How Come?

Archive for the ‘physics’ Category

How come yelling on a mountain can start an avalanche?  asks reader Paul Grinde.

Scream and bring down a roaring cascade of snow?  It seems plausible, especially since a single person skiing in the wrong place can trigger an avalanche above.  But researchers say that a human voice doesn’t have the power to move mass quantities of snow.

A powder snow avalanche

According to snow researchers, the idea that avalanches can be triggered by ordinary noise is a (popular) myth.  At an International Snow Science conference in Switzerland this year, researchers presented a paper comparing the human voice to aircraft noise, sonic booms, and deliberate explosions.

All sounds produce pressure waves in the air.  But even the loudest scream produces a pressure wave just one-tenth the amplitude, or magnitude, of a passing plane.  The pressure wave produced by a sonic boom — created by a plane flying faster than the speed of sound — has an amplitude 100 times as large as a scream.  A detonation of explosives produces pressure waves more than seven times as strong as the boom’s.

Scientists say that whether a snow mass shifts depends on the pressure wave amplitude.  They note that a short-lived pressure wave must be at least as strong as that produced by a sonic boom to trigger an avalanche.  So even a low-flying jet  — let along a human scream — won’t start an avalanche.

The avalanche myth may have its origins in the idea that a human voice, singing the right note, can cause a glass to shatter.  That idea, it turns out, is true.

Resonance is the phenomenon behind glass-breaking.  Have you ever swung on a swing, and noticed that if you pump your legs in the right rhythm, the swing will go higher and higher?  Your rhythm has exactly matched the natural rhythm, or frequency, of the swing, and the swing swings higher and faster.

Something similar happens when a musical note shatters an empty wine glass.  A thin, lead-crystal wineglass has its own natural frequency, which we can hear by lightly tapping the glass with a spoon.  A sound of the same frequency will make the glass vibrate.  And if the right sound is also loud and long, the glass may shatter.

But while an amplified voice singing the right note was known to break glass, no one had absolute proof that an un-amplified voice would also work.  Enter the show “Mythbusters,” which used a crate of wine glasses to demonstrate that the human voice can shatter glass.  Watch the “Mythbusters” wine glass episode here.  (But don’t try this at home!)

While a wine glass poses the risk of flying shards, resonating bridges can be even more dangerous.  In England in 1831, marching cavalry soldiers were crossing a suspension bridge in Manchester.  The bridge began to sway, and the troops automatically matched their marching to the motion.  Like a swing powered by pumping legs, the bridge’s swinging steadily increased.  As the bridge oscillated, a bolt broke, and the soldiers fell into the water below.  These days, soldiers are instructed to march out of time with each other when they cross a bridge — just in case.

How come it’s so hard to break a piece of dry spaghetti in two?  Why does it break into more pieces?  Also, when you slurp up a strand of cooked spaghetti, why does the pasta sauce fly off?  asks a reader.

Spaghetti cooking

Before you is a pile of dry spaghetti.  Your job, should you decide to accept it, is to break each piece in half, so that the pasta fits easily into a small saucepan.  Ready, set, snap…Oops.  What should be a snap is actually frustratingly difficult, as tiny, broken bits of pasta litter the table.

Scientists tried to solve the broken pasta problem for years.  Most famously, the late physicist (and Nobel Prize winner) Richard Feynman spent an evening with friend (and supercomputer expert) W. Daniel Hillis, snapping spaghetti.  At the end of the night, there was a pile of broken spaghetti, but no satisfying theory.

But in 2005, two physicists in Paris may have solved the spaghetti puzzle.  The scientists took high-speed images of breaking spaghetti, and applied a mathematical equation describing how waves travel through a stressed object.  What they found:  As a piece of spaghetti is bent until it can curve no longer, it breaks.  The sudden release causes a burst of “flexural waves” to travel through the remaining pieces, causing them to curve sharply, too — leading to more breaking.

So in a split second, your pasta breaks into three or four pieces, instead of neatly in two.  (Watch dry spaghetti bend and fragment at www.youtube.com/watch?v=8GutricnMNc.)

Once you’ve cooked your broken (or intact) spaghetti and added sauce, you may be in the mood for slurping.  But while it’s fun to hoover up strands of spaghetti,  you could find the tablecloth–and everyone around you–covered in a fine spray of crushed tomatoes.

How come?  According to physicist Jearl Walker, of Cleveland State University, the culprit is…wait for it…the Spaghetti Effect.  It turns out that the Spaghetti Effect doesn’t just apply to pasta drawn into your mouth, but also to paper, metal, and other materials pulled into machinery.

Walker says that when a spaghetti strand is lifted from your plate, it already has some sideways swinging motion.  As you suck up the spaghetti, you leave less and less of the strand hanging free.  So the energy of motion–the kinetic energy of the strand — is concentrated in a smaller and smaller piece of strand.  Just before the strand disappears into your mouth, its sideways motion becomes violent enough to fling sauce across the table.

You can also see the Spaghetti Effect in action between meals, whenever you use the vacuum cleaner.  After you’re finished sweeping the living room, unplug the vacuum and press the cord rewind button.  As the spaghetti-like cord is sucked quickly into the base, it may begin to whip around, turning the metal-tipped plug into a moving hazard.  The solution:  Retract the cord slowly, with several gentle pushes of the button.  (And if you must slurp pasta, try to do it in slow motion.)