Happy returns
Question: Why do boomerangs come back?
Answer: A boomerang is like two spinning aeroplane wings joined in the middle. It is held almost vertically before it is thrown end over end. Because it spins in this way, the top wing actually goes away from you faster than the bottom wing. This makes the sideways push on the top wing (similar to lift on an aeroplane wing) stronger than that on the bottom wing, so the boomerang gets tilted over, just as you would be if someone pushed on your shoulder, and its flight pattern begins to curve.
Similarly, if you ride a bicycle and lean over, the bicycle will turn, eventually going in a circle. The boomerang does too.
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Alan Chester
Sheffield, South Yorkshire
Answer: Returning boomerangs work by a combination of aerodynamic and gyroscopic effects. A boomerang is essentially a rotating wing with two or more aerofoil-shaped blades. It is thrown with its plane of rotation at about 20 degrees to the vertical and so that it spins rapidly (typically about 10 revolutions per second) with the uppermost blades travelling in the direction of overall motion. Therefore, the blade at the top moves through the air faster than the lower one. The faster-moving blades generate more lift than the slower-moving ones. This produces an overall force in the direction of turn, plus an overturning torque.
The rotation of the boomerang makes it behave like a gyroscope. When the overturning torque occurs, the gyroscopic effect makes the boomerang turn (or precess) about a different, near-vertical, axis. This continuously changes the boomerang鈥檚 plane of rotation, causing it to travel around an arc back to the thrower.
Other effects are also evident in the boomerang鈥檚 motion, such as its tendency to lie flat as it returns to the thrower 鈥� its plane changes from 20 degrees from the vertical initially, to horizontal at return. This is caused by a number of aerodynamic effects combined again with gyroscopic precession. The most significant effect is that the blades on the leading side of the rotating boomerang generate more lift force than the blades on the trailing side, because of the disturbed airflow on the trailing side. This again causes rotation which leads the boomerang to spin towards the horizontal plane. An article by Felix Hess in the November 1968 edition of Scientific American explains this process in detail.
Richard Kelso and Philip Cutler
University of Adelaide
South Australia
Answer: The simple answer to this question is that most boomerangs don鈥檛 come back and were never intended to do so. The Australian Aboriginal people made the boomerang for hunting and fighting rather than for sport or play, so they did not make the so-called returning boomerang over most of the Australian continent. For them, the real returns of boomerang throwing came in the form of fresh food or the besting of an enemy.
I have seen the Warlpiri people throw a karli boomerang and hit a target at well over 100 metres. Particularly skilled users of the karli throw this deadly weapon with surprising ease. The Warlpiri also manufacture the wirlki (also known as the 鈥渉ooked鈥� or 鈥淣umber 7鈥� boomerang) which is used for fighting.
Across Australia, even in those areas where the boomerang is not made, there is near universal use of paired boomerangs as rhythm instruments in ceremonial contexts. Such boomerangs are still traded for ritual use across thousands of kilometres.
There are and have been an astonishing variety of boomerangs from Australia. For an accessible account see Boomerang: Behind an Australian Icon by Philip Jones, published by the South Australian Museum.
Chips Mackinolty
Nightcliff, Northern Territory
Red hot
Question: What causes the colours that form on a clean iron or steel surface after it has been heated and cooled for tempering? The colours range from yellow when the metal was heated to about 200 掳C, through gold, brown, purple, blue and finally black when heated to about 600 掳C. And because the oxidised blue or purple finishes on steel mechanisms have often survived unmarked in clocks from the last century, what is the physical nature of this transparent and very durable coloured layer?
Answer: The hot furnace gases that are used for heat-treating steel oxidise the alloying elements, such as chromium, to form a thin surface film. These surface films interfere with visible light waves to produce the colours that your correspondent mentions.
The thickness of the films determines the apparent colour of the steel as it interacts with light of different wavelengths. Thinner films, which are formed at lower temperatures, seem yellow or gold. Thicker films make the steel appear light blue. The thickest films seem midnight blue and finally black.
Temper colours on clean, bare steel are actually quite fragile, and are quickly lost if rusting thickens the surface film by depositing layers of hydrated iron oxides. Many parts of the hundred-year-old clocks mentioned in the question owe the durability of their temper colours to the practice of dipping the tempered steel in sperm whale oil. The sperm oil gives a transparent, waxy protective covering to the oxide films, preserving their colours for posterity. Widespread use of this technique has had the obvious disadvantage of producing a serious shortage of sperm whales.
Dale McIntyre
Dhahran, Saudi Arabia
This week鈥檚 question
Round and about: On a sunny afternoon a few weeks ago, I saw a circular rainbow. It had a relatively small diameter, and the sky surrounding it had a lighter colour than the sky enclosed by it. I have never seen this before. How was it formed?