Answers to last issue's Do You Know?


1. Can skyscraper buildings be built from wood?
Ans: Yes indeed, as the Canadians have shown. The province of British Columbia in Canada has a strange problem: it could lose over half its pine trees to the depredations of the fearsome mountain pine beetle. The beetle, no bigger than a grain of rice, kills trees by releasing a blue stain fungus that prevents the flow of water and nutrients. As a result, the province is peppered with billions of dead, grey trees. If they are simply left standing, they will eventually either decay or burn in forest fires. In either case, they will release the carbon dioxide.
To deal with the problem, in 2009 British Columbia's parliament passed a Wood First Act that requires wood to be considered as the primary construction material in all new buildings erected with public money. The 2010 Vancouver Winter Olympics was staged in a stadium built with beetle-affected wood. 
Canadian researchers have discovered that rain and snow conveniently wash out sugars and other organic compounds from dead pine trees. By grinding it up and adding it to normal cement, they created a hybrid material that is waterproof, fire-resistant and pourable like concrete but that can be worked, cut and nailed or drilled like wood. 
British Columbia recently revamped its building codes to allow taller buildings to be made from wood, but still capped their height at a modest six storeys. (In comparison, Britain, Norway and New Zealand place no height restrictions on safely-made wooden skyscrapers.) 
If dead trees are not harvested within 10 to 15 years of being killed, they will have rotted or burned to the point of uselessness. So we might see plenty of wooden skyscrapers.

2. Could an asteroid impact knock the moon into Earth?
Ans: If an asteroid hits the moon, it will just get another crater. It would take a moon-size object to move the moon, and most likely the moon wouldn't survive. Hitting it with a much larger, denser object would be like hitting an egg with a golf club. But let us suppose that the moon and whatever is hitting it will react like solid billiard balls. None of the known asteroids larger than 150 km in diameter orbit anywhere near the moon. Well, how about if the largest known asteroid, Ceres -- which at 1500 km across is roughly the size of Tamil Nadu and Andhra Pradesh combined -- did manage to slip out of its place in the asteroid belt and set out on a collision course for the moon? 
Hardly a budge, even then. It is the equivalent of a four-year-old child trying to knock over a heavyweight boxing champion. The moon orbits the Earth at some 1.6 km per second. This orbital momentum is so great that it would overwhelm the impact force of a collision and the moon would just continue its journey around the planet.
By now it should be clear that the moon is staying put, but what could send it toward Earth? At minimum, you would need an object of the same size and density as the moon to hit it at the same speed, and in the opposite direction of its orbit. This could stop the moon in its tracks, and it would fall onto the Earth. Even if the collision only pushed the moon into a lower or less-circular orbit, that does not mean we would escape unscathed, though: if its new orbit halved its current distance from the Earth, ocean tides would get about eight times as big. 
Chennai would get very wet, for sure.

3. Whatever could be the evolutionary purpose of tickling?
Ans: You probably know that you can't tickle yourself. And although you might be able to tickle a total stranger, your brain also strongly discourages you from doing something so socially awkward. These facts offer insight into tickling's evolutionary purpose, say neuroscientists. Tickling, they say, is partly a mechanism for social bonding between close companions and helps forge relationships between family members and friends.
Laughter in response to tickling kicks in during the first few months of life. It is one of the first forms of communication between babies and their mothers. Parents learn to tickle a baby only as long as she laughs in response. When the baby starts fussing instead, they stop. This face-to-face activity also opens the door for other interactions.
Children enthusiastically tickle one another, which some scientists say not only inspires peer bonding but might help hone reflexes and self - defense skills. In 1984 a psychiatrist noted that many ticklish parts of the body, such as the neck and the ribs, are also the most vulnerable in combat. He inferred that children learn to protect those parts during tickle fights, a relatively safe activity.
Tickling while horsing around may have also given rise to laughter itself. The 'ha ha' of human laughter almost certainly evolved from the 'pant pant' of rough-and-tumble human play, say neuroscientists, who base that conclusion on observations of panting among tickle-battling apes such as chimpanzees and orangutans.
In adulthood, tickling trails off around the age of 40. At that point, the fun stops; for reasons unknown, tickling seems to be mainly for the young.

4. Why is the Moon sometimes out in the Day?
Ans: You can see the Moon in the daytime because it is big and brightly lit by the Sun. The surface of the Moon is about as reflective as a tar road -- rather dark but not totally black. When you look at the Moon, you are seeing the light which reflects off it. This is not nearly as bright as the Sun, but it is up to 100,000 times as bright as the brightest nighttime star.
During the day, the brightness of the sky washes out the light from the stars: a region of the sky including a bright star is only very slightly brighter than a region of the sky without a bright star, so your eye cannot notice the difference. However, the region of the sky containing the Moon is much brighter, so you can see it. You can also sometimes see Venus during the day if the conditions are right and you know exactly where to look, but anything dimmer is lost.
It might be useful to think of the Sun as a large light bulb, and the moon as a large mirror. There are situations where we can't see the light bulb, but we can see the light from the bulb reflected in the mirror. This is the situation when the moon is out at night. We can't see the Sun directly because the earth is blocking our view of it, but we can see its light reflected from the moon. However, there are also situations where we can see both the light bulb and the mirror, and this is what is happening when we see the moon during the day. 
You can explore this for yourself with a light and a hand mirror. Depending on which way you face (away from the light or sideways to the light) you can see either just the mirror, or both the light and the mirror.

5. I went swimming with my friend. She dived and then screamed and though I was right next to her but with my head above the water, I could hardly hear her. But once I too went underwater, I could hear her clearly. Why did this happen?
Ans: This is an example of *impedance mismatch*. Acoustic impedance - a measure of the way a sound wave interacts with the medium it is passing through - varies depending on the medium concerned. When a wave encounters a medium with a different impedance from the one it is in, most of its energy will be reflected at the boundary.
With your friend underwater and you outside, most of her sound energy is bouncing off the under-surface of the water and is not coupling into the air. In contrast to air, water is a nearly incompressible fluid and so sound transmits through it very efficiently, but with only a small amplitude that, once it reaches the surface, does not jiggle the air very efficiently. So you only hear a faint sound. 
Most people will be familiar with ultrasound scans of a fetus inside a mother's womb. For these scans, a gel is applied to the expectant mother's skin to reduce the impedance mismatch between her body and the transducer and so maximise the transfer of acoustic energy.
Despite the acoustic mismatch between air and water, our everyday experience is that sound passing through the air manages to reach receptors immersed in the fluid of the cochlea within the inner ear. For this, we can thank the design of the middle ear for impedance-matching the sound.
Vibrations are passed from the eardrum via the auditory ossicles - the three smallest bones in the human body - to a membrane in front of the cochlea called the oval window. The eardrum is attached to the first of the ossicles, called the malleus (or hammer). This is pivoted about the second bone, the incus (or anvil), which in turn is fused to the third bone, the stapes (or stirrup). The stapes drums on the oval window.
The eardrum has an area about 15 times that of the oval window. As the sound energy striking the eardrum is concentrated at the oval window, the amplitude of the sound vibrations is increased. Lever action further boosts this amplification. Without the middle ear ossicles, barely 0.1 per cent of the energy arriving at the eardrum would reach the inner ear.
The human ear is not built to hear sound transmitted in a liquid. Notice that sounds under water sound much higher pitch than those above the surface. This is again related to the density of the medium, which is a large factor in the speed of sound propagation. 

Sources: The Economist, The New Scientist, PopSci, Nasa.