The kneejerk reaction/ Most expensive metals/ Shapes - what's in a name?/ Our synchronously rotating moon
Tuesday 06 January 1998
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Q Why do doctors on TV tap the knee of a patient to see if their leg jerks?
They do this to test whether the knee jerk reflex - which helps the body to control the movement of the legs during running - is working properly. If it is, this gives the "doctor" information about how well the person's nervous system is working, and they test to see if it is working on both sides of the body. A reflex is a rapid, automatic response to a stimulus: pulling your hand away from a hot pan-handle is a reflex action. A simple reflex involves communication between neurones in the peripheral nervous system and the spinal cord. The brain may be informed, but does not take part in the response.
Reflexes can also help to co-ordinate complex muscular events, such as swallowing. The knee-jerk is a "stretch" reflex - one of many which work together to help us maintain an upright posture. You can test it by holding your knee suspended, leg down, and tapping the leg just below the kneecap. Your lower leg should lift automatically.
Q What are the most expensive elements?
It depends to some extent on what state they are sold in - foil, wire, powder, rod, etc. Generally speaking, though, the most expensive are osmium, platinum, iridium, gold and europium.
Q How are mathematical shapes named?
Shapes are named from the standard Greek roots for numbers, according to the number of sides: pentagon (5), hexagon (6), heptagon/ septagon (7), octagon (8), enneagon/ nonagon (9), decagon (10), hendecagon/ undecagon (11), dodecagon (12).
Three- and four-sided figures are so familiar that we usually talk of the triangle and quadrilateral rather than the formal "trigon" and "tetragon". But most names for large numbers of sides are not used: a 27-sided shape is just called "a 27-sided polygon" - apart from the 15-sided pendecagon/ quindecagon and 100-sided chiliagon.
Q Is it just a coincidence that the Moon takes the same amount of time to rotate as it does to go round the Earth?
The two rotational rates are not a coincidence. The Moon is locked in what's known as a "synchronous rotation". The Earth raises body tides on the Moon (basically, it stretches it) which are about 20 times greater than body tides on the Earth. The enormous energy dissipation that results has slowed the rotation of the Moon to result in an equilibrium, whereby the Moon's rotation (27 days) is the same as the time it takes to revolve around the Earth. Thus it always presents the same face to us.
As a consequence of tidal forces, the Moon is actually moving away from the Earth. The angular momentum of the Earth-Moon system must remain constant; the Earth's momentum is decreasing, owing to tides raised on its surface; and, in order to conserve momentum, the Moon increases its angular momentum by moving away.
Some of the satellites of the outer planets (Jupiter Saturn Uranus and Neptune) have also become trapped in synchronous orbits.
Q Is it true that cold weather stimulates urination?
Cold weather of itself would not directly stimulate urine production or excretion. But it does lead to reduced sweat production, so proportionally more water has to be excreted via the kidneys - hence, more visits to the toilet.
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