Short-haul flight
I鈥檝e just been watching the athletics World Cup in Madrid on TV. Why do long jumpers adopt different styles when travelling through the air after take-off? Some just hang with their arms and legs pointing forward, some cycle madly through the air, while others seem to keep running while they are airborne. Intriguingly, triple jumpers adopt nothing other than a simple floating technique during their final phase. Do any of these styles affect the distance gained or is the length of jump determined simply by take-off speed and trajectory?
鈥� To achieve the greatest distance, the jumper needs to convert horizontal sprint speed into a 45掳 launch. However, in practice the trajectory is normally lower. To take off in this manner, the jumper plants a foot on the centre of the board, rocks forward onto his toes and drives off over a straight leg to reach the highest point possible. This action is created by a rotational motion about the foot on the ground and, as the jumper leaves the ground the centre of rotation moves to their centre of gravity. The moment of inertia reduces drastically, and forward rotation increases which tends to leave the feet behind.
The simplest way of counteracting this tendency is known as 鈥渢he hang鈥�, in which the jumper stretches out to increase the moment of inertia and slow rotation. As the feet are brought forward for landing, the arms also circle forward and down, to some extent counteracting the body鈥檚 forward rotation. Additional circling of the arms throughout the jump will have a better effect, resulting in the feet landing further forward. However, this requires more skill and time in the air.
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The best results come from executing a 鈥渉itch kick鈥�, which does look superficially like running in the air. It is much harder to pull off, and is generally of little benefit unless the jumper is already extremely good. Once in the air the fully extended front leg drives backwards while the other leg recovers through a high tuck position before extending to drive back in turn. The arms circle forwards once, as in the hang. Both actions, done powerfully, counteract the body鈥檚 forward rotation and can even reverse it. A good jumper can manage one and a half cycles; a very good jumper can do two and a half, and they land with feet as far forward as possible. Unfortunately sometimes they overextend themselves and their feet are so far forward that they fall backwards, undoing all the good work.
Pat Doncaster
Schull, Ireland
The sandman cometh
My colleagues and I have been wondering, during those rare moments of reflection, what the scientific term is for the yellowish crystalline substance sometimes found encrusted on eyelids when you wake up. Some people call it 鈥渟and鈥� or 鈥渟leep鈥�, but does it have a medical name? What is its composition and why does it form?
鈥� The substance collects around the eyes because of irritation. During the day, the dried mucus consists of salts and proteins secreted by glands in response to dryness or exposure to pollution. The mucus continues to collect and dries out in the corners of your eyes while you鈥檙e asleep even though tears keep the eyes moist.
The tears have three separate components. The innermost tear layer coats the surface of the cornea and is called the mucous layer or mucin. The middle tear layer is an aqueous layer produced by the lachrymal glands and supplies salt, proteins and other compounds to the cornea. The outer tear layer is composed of oil from the meibomian sebaceous glands in the eyelids. This helps to prevent evaporation of the watery tears from the surface of the eyes.
Readers should be aware, however, that thick, frequent yellow or green mucus in the eyes is a sign of viral or bacterial conjunctivitis.
Johan Uys
Bellville, South Africa
鈥� There seems to be no widely used specific term, perhaps because the effect is seen as trivial and erratic. None the less, it is important. During the day grit, dead cells and other debris accumulate in the tears which are more than mere saltwater.
Mucoproteins cover the eyeball, curdling protectively around sharp grit to encase it in mucus, a middle salty layer is the main liquid part, and an outer, oily layer reduces evaporation. At night, movements of the eye and closed eyelids stir this orbital midden, massaging solids towards the inner corner of the eyelids. There the exposed liquid evaporates until the residual sludge forms pellets that you remove harmlessly by washing, or with your finger, the next morning.
Gritty environments such as deserts, may damage eye tissues enough to convert your tears into dilute pus. This dries on the edges of your eyelids, gluing them shut in spite of the waxy coating that normally reduces spillage and keeps their epidermis water-repellent. It can be very disconcerting to awaken from an exhausted sleep to find your eyelids sealed shut so that you think it is still dark. If this ever happens to you, soak them open gently, or you may lose some eyelashes in the sand.
Jon Richfield
Somerset West, South Africa
鈥� Neither of the above letters suggests a technical term for the substance that collects in the corner of your eye. Many were put forward but the nearest we found was 鈥渕ucopurulent discharge鈥� offered by John Devers of Bronia, Victoria, Australia 鈥� Ed
This week鈥檚 questions
Underneath the arches
On my way to work I pass over a humpback railway bridge built of brick. Some years ago this bridge was declared weak and had a traffic restriction imposed on it. How did the authorities know it was weak? How is the strength of a brick arch assessed?
Paul Hughes
No address supplied
Who needs nine lives?
A friend of mine reckons that you can drop a cat from any height and it will survive unhurt because its terminal velocity is lower than the speed at which it can land unhurt. Can someone confirm or refute this because the kittens in my house now look strangely at my friend. I鈥檓 sure this can鈥檛 be true, can it?
Anna Goodman
Oxford, UK