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THE PASTA PUZZLE

If a strand of dried spaghetti is held at both ends and bent, why does it always break into three or more pieces? This is a strange phenomenon. Surely holding a strand of dried spaghetti at both ends and bending it until it breaks should produce just two pieces, but it hardly ever does – usually three or even more pieces are the result.

What do I need? Strands of dried spaghetti. Something to catch them in.

What do I do? Hold a strand of spaghetti at both ends and bend it until it breaks. Repeat with the other strands.

What will I see? In nearly all cases the spaghetti will break into three or more pieces. Even on the rare occasions when it seems to break into only two you’ll often find a stray shard or splinter has flown off into the nether regions of your kitchen.

What’s going on? First, when you bend a piece of 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 occurs near the apex where the combination of stress level and defect size reaches a critical value. This breaks the original piece into a long and a short piece. After the break, as the longer piece snaps back, the whipping action sends the tip beyond the neutral point (the original straight state of the piece of spaghetti) and activates the next defect on what was the long side. This defect has already been opened up on the outside of the curved spaghetti by the first bending, so it doesn’t take much to finish off the crack by bending it in the other direction.

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

It took Basile Audoly and Sébastien Neukirch to verify what was going on in their paper Fragmentation of Rods by Cascading Cracks: Why Spaghetti Does not Break in Half, which won them the 2006 Ig Nobel Prize for Physics. Audoly and Neukirch broke strands of spaghetti of varying thickness and length by clamping one end and bending them from the other. They found that the unexpected three-part breakage occurs because of what are known as flexural waves. When the curvature of the spaghetti reaches a critical point, the first break appears. The shock of this causes a flexural wave to ripple down each of the two resulting lengths of pasta at high speed and amplitude. The two halves formed by the initial break do not have time to relax and straighten before being hit by the flexural wave, which causes them to curve even further and suffer more breaks, leading to a cascade of cracks in the pasta. Often more than three pieces are created when this happens. Audoly and Neukirch’s work also provides important information about failures in other elongated, brittle structures, including human bones and bridge spans.

HOT CHOCOLATE

Is it true that you can measure the speed of light using nothing more than a chocolate bar and a microwave oven? The answer is yes. This is an astounding experiment that actually allows you to measure one of the fundamentals of science in your own home.

What do I need? A bar of chocolate (the longer the better). A metric rule. A microwave oven.

What do I do? Remove the turntable from your microwave oven – the bar of chocolate needs to be stationary. Put the chocolate in the oven and cook at high power until it starts to melt in two or three spots. This usually takes about 40 seconds. You should stop after 60 seconds maximum for safety.

What will I see? Because the chocolate is not rotating, the microwaves are not evenly distributed throughout the bar and spots of chocolate will begin to melt in the high-intensity areas, or “hotspots”. Remove the bar from the oven and measure the distance between adjacent globs of melted chocolate.

What’s going on? The frequency of the microwaves is the key. A standard oven will have a frequency of 2.45 gigahertz (the figure should be on the back of the oven or in the user manual). If your oven is 2.45GHz, the microwaves oscillate 2,450,000,000 times a second (you can adjust this figure depending on your oven). Microwaves are a form of electromagnetic radiation and therefore travel at the speed of light. If you know the frequency of the microwaves, finding out their wavelength will help you to calculate how fast they are travelling.

This is where the chocolate comes in. The distance between the globs of molten chocolate is half the wavelength of the microwaves in your oven, so double the measurement you have taken of the gap between the molten globs to find the microwave wavelength. In the New Scientist microwave oven the distance between the globs of molten chocolate was 6cm, so the wavelength in our 2.45 GHz oven is 12cm. To calculate the speed of light in centimetres a second you need to multiply this wavelength by the frequency of the microwaves: 12x2,450,000,000 = 29,400, 000,000, which is near to the true speed of light of 29,979,245,800cm a second (or 299,792,458 m per second).

Try it yourself, measuring as accurately as possible to get a figure even nearer to the true speed. If your chocolate bar is chilled beforehand, the molten areas tend to be more distinct when they first appear. You may find different chocolate bars, all of which taste delicious slightly melted, will aid your research. True scientists know that it is always important to double-check results.

PS: The hotspots – and consequent cold spots – that occur in ovens thanks to the wavelength of microwaves are the reason why ants can survive unscathed and uncooked inside a switched-on oven. They immediately scurry to the cooler areas and ride out the microwave storm.

CAN WE TRICK OUR SENSE OF TOUCH?

This experiment shows us that much of what we perceive through our senses is relative. We adapt to changing circumstances.

What do I need? Three bowls large enough to put your hands in. A decent depth of hand-hot water, tepid water, and cold water chilled in the fridge. Your hands.

What do I do?Place one hand in the cold water and the other in the hot water. Hold them there for 90 seconds. Then place both simultaneously into the bowl of tepid water.

What will I feel? When your hands are in the cold water bowl and the hot water bowl, one feels cold, the other hot. Yet when you place them in the tepid water they still feel different, even though they are in water of the same temperature. The hand that was originally in the hot water now feels cold, but the hand that was originally in the cold water feels warm. After both have been held in the tepid water eventually they feel the same.

What’s going on? Human senses are relative, measuring only the differences between things that we perceive. So your cold hand registers that the tepid water is warmer, and conversely your hot hand perceives the tepid water as colder. Your senses do not make absolute judgments, only the relative differences. While this may lead to a few disconcerting moments, it helps us to focus on what is important and what is changing around us, and allows us to ignore things that aren’t. It’s a process called adaptation and is not just a product of our sense of touch. It affects all our senses.

Adaptation is the reason we get used to smells and stop noticing them, or why our eyes become used to brightness or dark after a while. People who work in foul-smelling environments are aware of the odour when they first arrive at work, but after a few minutes the impact is reduced. Similarly, people who work in noisy environments are able to filter out the constant background noise, allowing them to hear what their colleagues are saying once they have adapted to the other, permanent sounds.

PS: Adaptation may be the reason why, as we age, we believe that in our youth the sky was bluer, summer days warmer and food tasted better. This was studied in the 1950s when researchers in the US sat down volunteers in front of a screen and shone coloured lights on to it for five seconds. After a lapse of five more seconds, the volunteers were asked to adjust the controls on a coloured light generator to try to reproduce the colour they had seen. The colours they chose were always brighter or deeper than those they had originally been shown.

This is because adaptation was acting on their visual sense. They were given longer than the five seconds that they had viewed the original colour to set up their matching colour. The longer they looked at the screen as they attempted to set up their match, the more the colour appeared to fade as their sensitivity to it declined. This meant that they continually turned up the brightness or depth of the colour as they worked at matching it to the original. The researchers believed that, because adaptation probably acts in the long term as well as the short term, the summer skies of our childhood seem brighter, the days warmer and the chicken more succulent. Other researchers dismiss this idea, saying ageing faculties are to blame.

WHY DON'T ANTS DIE IN A MICROWAVE OVEN?

The hot and cold spots in a microwave occur because of the wavelength of microwaves inside a switched-on oven. Ants find the cooler areas and remain unscathed.

© Mick O’Hare 2007

Extracted from How To Fossilise Your Hamster published by Profile Books on October 4 at £7.99.

For more experiments go to www.newscientist.com/hamster

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