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Dune

Question: When the tide recedes on a sandy beach, ripples are left in the sand with a fairly constant length and direction. What influences their direction, length and height?

Answer: Ripples on beaches are generated by waves and/or currents. Those formed by waves should have a symmetrical profile, with pointed crests and rounded troughs.

These crests will, typically, divide along their length. The distance between successive ripples (wavelength) is determined by the grain size of the sediment, wave height and water depth. Waves are only able to move sediment in water depths of less than half their wavelength.

Ripples formed by currents have an asymmetric profile with a steeper lee face and a gentler upstream face. These ripples migrate in the direction of the current. Their wavelength deepens with the grain size of the sediment, the current flow velocity and the water depth.

The alignment of wave ripples is parallel to that of the waves forming them while current ripples are perpendicular to the direction of the current flow. Where a combination of the two processes is seen, the alignment results from the interaction of the two vectors.

Stephen Lokier

by e-mail, no address supplied

Snowed under

Question: I have recently undertaken a study about the effects of avalanches and was intrigued to find that many of the victims are found close to the surface. Why is this? Shouldn’t humans sink due to the fluidity of moving snow?

Answer: This is probably due to the cornflake-settling effect. When a packet of cornflakes is shaken, the smaller flakes slip through the gaps left between the larger ones. So the large flakes end up on top and the small dusty bits are found at the bottom of the packet.

The density of humans is not very different from that of ice. So in the maelstrom of an avalanche, the smaller lumps of ice and compacted snow would be more likely to end up below the larger humans than above them.

Michael Brimicombe

Aylesbury, Buckinghamshire

If you have the misfortune to be hit by an avalanche (as I was) you’ll find it’s rather like being in a big surf. You are swept along at speed and tumbled head over heels. Most mountaineers attempt to follow the well-known practice of swimming in the flowing snow (it’s actually more flailing than swimming) to stay on the surface. When the snow stops it can set like concrete, and if you are even a short distance below the surface, or the wrong way up, you can suffocate—Ed

Rock around the Earth

Question: Why is the Moon round? If it is a bit of rock with no atmosphere, what process has caused it to become round? Also, is it a coincidence that the Moon appears to exactly cover the Sun during a solar eclipse or is there a scientific reason?

Answer: The Moon is round because it is large. This has nothing to do with the erosional effects of atmosphere. Gravity tries to shape any object into a sphere, because every point on a sphere’s surface is as close to the centre of gravity as it can get.

For smaller objects, like bricks or asteroids, the internal strength of the rock is stronger than the gravitational force acting on the surface, so their irregular shape persists. On the Earth, for instance, it is impossible for mountains to be higher than about 15 000 metres (only a small fraction of the Earth’s diameter). Yet on Mars, a planet smaller than the Earth, the highest mountain is 25 000 metres high, because the gravity of Mars is weaker. What’s more, most large planetary objects were formed hot and partially molten, easing gravity’s sphere-shaping job.

There is no scientific reason for the fact that the Sun and Moon are apparently equal in size. It is sheer coincidence. The surface of the Earth is the only planetary surface in the Solar System from which a perfect natural solar eclipse can be observed, with the Moon exactly covering the surface of the Sun.

Govert Schilling

Utrecht, The Netherlands

Answer: Watch the Sun and Moon long enough (over millions of years) and you will see that the coincidence you refer to is temporary. If you had been contemplating an eclipse from the back of a triceratops, you would have seen the Moon eclipsing the Sun with a considerable overlap. Loiter for another few hundred million years, and you will find the complete solar eclipse a thing of the past.

Tidal effects are moving the Moon further from the Earth into slower orbits and lengthening our day in the process. This will not stop until the Moon is in geostationary orbit and the Earth’s day length equals the month length.

Jon Richfield

Dennesig, South Africa

Why wood it?

Question: I used a magnet to find nails in the ash from timber burnt in our wood-fired heater. The magnet attracted copious amounts of black ash as well as nails, the ash adhering to the magnet in the same way that iron filings cling to a magnet in primary school experiments. But how can wood ash be magnetic?

Answer: Any iron nails in the wood will inevitably have rusted to a greater or lesser extent, and rust is, in effect, hydrated iron (III) oxide. When the wood is burnt the charcoal (carbon) which is produced acts as a reducing agent and re-forms metallic iron from the rust, although by now it is in a finely dispersed state.

This reduction process is an exact parallel of the way iron is extracted from its ore in the first place. Prior to the Industrial Revolution charcoal was used rather than coke. It is this finely dispersed iron powder, along with the charcoal adhering to it, which is attracted to the magnet.

John Bibby

Rochdale, Lancashire

Answer: Even ash from untreated wood will contain traces of iron oxides and possibly even smaller traces of reduced iron, which may be weakly attracted to a magnet. In your case, rust from your heater and from the nails would have supplied a lot more magnetic material.

Some pesticides used on commercial wood could leave traces of metal salts which could remain as oxides in the ash and account for still more of the attraction.

If you feel curious, you can collect a lot of the ash by magnet, powder it finely and refine it by repeatedly passing the magnet over it. The least messy way to do this is to apply the magnet from behind a sheet of polythene, then soak the most strongly magnetic 10 per cent or so of the ash in hydrochloric acid and see what dissolves. The resulting fluid should give you a rough idea of what metals were responsible for making the ash cling to the magnet.

Jon Richfield

Dennesig, South Africa

Answer: Take your magnet outside and run it through the dirt a few times. You will get the same result as when you ran it through the ashes.

Sidney Biesterfeld

by e-mail, no address supplied

Answer: There is a large influx of meteorite material to be found in the average garden. About 40 000 tons per year strikes the atmosphere of which between 400 and 3000 tons reaches the Earth’s surface. Around 10 per cent of this is iron. However, the best place to find this iron is not in your garden but by running your magnet through your gutter where the tiny particles have been carried by the rain.

Duncan Steel

Adelaide, South Australia