Moonflight
If I were an Olympic high jumper would I be able to jump high enough to escape the moon鈥檚 gravity? I suspect not, but some astronauts on the moon鈥檚 surface seemed to float in the air for a long time. Exactly how high and how fast would I have to jump to escape the moon鈥檚 pull, or simply to fly over its surface? As there is no atmosphere, are my aerodynamics irrelevant? (continued)
鈥� The maximum height a high jumper can attain on the moon and the one quoted by all your correspondents so far (30 August) is the 鈥渦rban myth鈥� height of 6 times the current high-jump record on Earth.
Actually, the correct figure is about 9 metres,not 15 metres. This is because the energy that a high jumper must expend is that required to lift their centre of mass over the bar, and not that required to lift their feet. The centre of mass of the average human is about 1.1 metres from the ground. Most Olympic high jumpers, who are tall and slim, will have a centre of mass further from the ground than this.
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Therefore, the world record holder for the high jump expended the energy necessary to raise their centre of mass by about 1.35 metres, and not 2.45 metres (the current record). The position of their centre of mass will be the same both on the Earth or the moon and consequently the height jumped on the moon will be 6 times 1.35 metres, plus the height of their centre of mass above the lunar surface, giving a grand total of 9.2 metres.
So I鈥檓 afraid that we are even more puny than originally thought.
Paul Robinson
West Bridgford, Nottinghamshire, UK
Leading astray
I have noticed that there are strange cylinders around the leads of most computer peripherals. On further investigation these seem to contain two crescent-shaped magnets that form the outside of the cylinder, while the lead runs through the middle. What do these do and how do they do it?
鈥� The strange cylinders are made of ferrite and are there to reduce electromagnetic pollution from the computer and also to prevent external signals from interfering with the computer鈥檚 operation.
Ferrite consists of particles with magnetic properties mixed with a binding agent. The two half-cylinders are held together with a plastic clamp and can therefore be fitted to the cable after the connectors have been fitted.
Placing a cylinder of magnetic material around the cable concentrates the magnetic field produced by the current flowing in the cable. This is the same effect that can be observed when a steel nail inserted into a coil of wire carrying a direct current is near a compass needle. The deflection of the compass needle is greater when the nail is inserted. In the case of the computer lead, the positions of the wire and the nail are reversed but the effect is the same.
The computer lead carries high-frequency alternating current which produces an alternating electromagnetic field around the cable. This causes the cable to radiate like a radio aerial, adding to the background radio noise level. Because the ferrite concentrates the magnetic field in one place, the radio-frequency energy is absorbed in the ferrite, reducing the amount radiated from the cable. The granular structure of the ferrite makes it many times more effective at absorbing high-frequency energy than a solid cylinder of iron or steel. This effect also works in reverse: the computer lead picks up signals from devices such as mobile phones, and these are absorbed by the ferrite and prevented from entering the computer and causing problems.
Recent legislation has enforced stricter limits on the amount of radio-frequency radiation that devices such as computers are permitted to emit and the levels of signals that they have to withstand. Hence the fitting of these ferrite chokes.
Ian Shardlow
Solihull, West Midlands, UK
鈥� Ferrite is semi-magnetic and made from iron oxide mixed with other metals. The ferrite cylinder is coated with plastic, and slips over or snaps around the cable so that the lead runs through the middle.
Computer components tend to be fairly noisy. There are many different oscillators to drive the processor chip, video card, keyboard and so on, and each of these has the potential to broadcast radio signals at the frequency of the particular oscillator. The various cables connected to the system also act as long antennas, and can broadcast the signals they carry. These can interfere with other devices such as TVs and radios. As well as broadcasting, they can also receive and channel signals back into the computer, and this interference can cause problems.
The ferrite eliminates this unwanted interference and turns it into heat, stopping unwanted interference both in and out of the computer. Different types of ferrite are available, each removing signals with a particular frequency range.
Gareth Downes-Powell
Banbury, Oxfordshire, UK
This week鈥檚 questions
It鈥檚 not fair!
My husband is in his fifties, weighs 105 kilograms, smokes, drinks and never, ever, takes any exercise. Yet his body is perfectly firm, no sagging whatsoever, very much like a freshly caught mackerel. The rest of us need incessant trips to the gym to avoid looking like jellyfish. Can his incredible luck be explained in biological terms? And given that a nice firm body is an obvious advantage in the breeding game, why isn鈥檛 it more widespread?
Ingebj酶rg Storeide
Stavanger, Norway
Bow how?
I was reading a travel brochure that told me that my cruise ship had a bow bulb on the prow of the ship. The guide says: 鈥淭he bow bulb is just below the waterline and displaces the same amount of water that would be pushed out of the way by the ship鈥檚 bow. This virtually eliminates a bow wave, resulting in some fuel saving as less energy is needed to push the ship forward.鈥� Intuitively, one would think that a sharp prow would be best. How does a rounded bow bulb eliminate the bow wave, and how does it improve efficiency?
Berdell Funke
North Dakota, US