杏吧原创

Burning rope trick

Question: When I was working at a fire research centre some years ago, a
co-worker and I noticed that burning droplets of polyethylene emitted a buzzing
sound as they fell. We never worked out why.

Recently, I duplicated the phenomenon and photographed the test. I took a
clear polyethylene bag of the kind used by dry cleaners and laundries to protect
clean clothes, and formed it into a rope with knots along its length. I then
hung the rope outside on a windless night and set it alight at the bottom.

I had set up my camera about 30 centimetres from the falling droplets. Each
time a droplet looked ready to fall, I opened the shutter, and kept it open
until the droplet had passed.

I suspect that the frequency with which the burning droplets brighten and dim
as they fall matches that of the buzzing sound. But can anyone explain why the
droplets dim this way?

Answer: Polyethylene, when lit, forms both liquid and gaseous states due to
the high temperatures of combustion. As the solid polyethylene burns, it quickly
melts to a liquid that forms burning droplets. As these droplets fall through
the air, more oxygen is made available, further raising their temperature.
Evolving gas forms a bubble within the droplet and ignites explosively when it
bursts through the surface of the droplet, creating an audible pop. These gas
bubbles form and ignite with rapid and consistent frequency generating a buzzing
sound. The ignitions can be clearly seen in photographs as bright flashes.

Bernie Sunderhauf

Ashgrove, Queensland

Answer: The phenomenon described will be familiar to readers of the
Fabulous Furry Freak Brothers cartoon books by Gilbert Shelton. The Freak
brothers called it a 鈥渮ilch鈥�, presumably because of the noise it makes.
Unfortunately when Fat Freddy tried it, he set fire to the ceiling and the
waterbed in the bedroom above鈥攖he resulting deluge extinguishing the fire
and provoking the reaction: 鈥淕ee, I didn鈥檛 know these old buildings had a
sprinkler system!鈥�

Chris Tyler

Isle of Skye

Pasta puzzle

Question: If you bend a piece of dried spaghetti it breaks into three pieces
with the middle piece flying out. Why does this happen?

In answer to this question we first reproduce a letter that appeared
in New 杏吧原创 on 18 February 1995 from Oliver and Richard Nickalls and J.
E Reeve鈥擡d

Your recent letter on Feynman鈥檚 joke (New 杏吧原创, 14 January, 1995)
reminded us of the passage in the book No Ordinary Genius (edited by
Christopher Sykes, 1994) in which Danny Hills describes his and Feynman鈥檚
experiments with spaghetti: 鈥淚f you get a spaghetti stick and break it, it turns
out that instead of breaking in half, it will almost always break into three
pieces. Why is this true鈥攚hy does it break into three pieces? . . . Well
we ended up at the end of a couple of hours with broken spaghetti all over the
kitchen and no real good theory about why spaghetti breaks in three.鈥�

The remaining replies below attempt to solve the problem鈥擡d

Answer: When you bend a piece of uncooked spaghetti, it does not usually
break at the apex of the bend where the stresses are highest, because failure in
the spaghetti is controlled by defects in the pasta.

The first break will occur at a point near the apex where the combination of
stress level and defect size reaches a critical value. This initially breaks the
piece into a long and a short piece. After the break, as the longer side snaps
back, the whipping action sends the tip beyond the neutral point and activates
the next defect on the long side. This defect has already been opened up on the
outside of the curved spaghetti by the first bending, so it doesn鈥檛 take much to
finish off the crack by bending it in the other direction.

Because of the whipping motion that section of the spaghetti is travelling
forward at that point of the second break and so the liberated small piece
continues in that direction.

If the critical defect does happen to occur right at the apex, which it does
only rarely, the spaghetti breaks into only two pieces.

Stephen Claeys

Niagara Falls, New York

Answer: The sequence of events can be determined be looking at the broken
ends. When a break occurs the fracture starts cleanly on the stretched convex
side, and ends slightly raggedly on the compressed concave side where a small
splinter is usually torn away from one side of the break. Careful inspection of
the ejected middle piece will reveal evidence of spicule formation at both ends
and that these are on opposite sides. This shows that the two breaks which
generate the middle piece each occur while the spaghetti is bending in opposite
directions, which is consistent with the dynamics of linear spaghetti
structures.

Oliver Nickalls

London University

Dick Nickalls

City Hospital, Nottingham

Double trouble

Question: I recently purchased a box of eggs, each of which was guaranteed to
have two yolks. And the claim was correct. How does the supplier ensure that
each egg has two yolks?

Answer: These special eggs are a natural phenomenon over which we have no
control. Double yolk eggs are larger than those laid by the majority of the
flock and are set aside to be tested individually.

Demand for double yolkers far outstrips supply and we need to be very sure
that they do in fact contain two yolks. Each egg is therefore checked by holding
it against a bright light. During this process (still known as candling from the
days when a candle provided the source of light) the number of yolks will be
clearly visible as shadows.

Graham Muir

Stonegate Farmers Limited

Hailsham, Sussex

Try it at home鈥攜ou鈥檒l be surprised how much of the inside of an egg
you can see鈥擡d

This week鈥檚 question

Acid test: How does putting a few drops of vinegar into water that is being
used for boiling eggs prevent the eggs cracking?

Sandy Fung

Cambridge