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THE LAST WORD

Hot youth All of the radioactive elements that made up the early Earth started out in the hot ash of ancient supernova explosions. This means we’ve been working through their half-lives for at least 5 billion years since our planet was born. How much hotter was Earth’s interior then? What would the heat from nuclear reactions mean for tectonic activity and the evolution of life in our world’s feverish youth? (Continued) n An earlier answer to this question said Earth’s internal heat is generated by the radioactive decay of uranium, thorium and potassium. There was, however, no mention of the gravitational heating caused by the pull of the moon in its elliptical orbit around Earth. That pull must have been considerable in the distant past, when the moon was much closer in to our planet. The heating effect is particularly evident today on Jupiter’s satellite Io.

In Earth’s case, has anyone calculated how much heating is due to radioactive decay and how much to the moon’s gravitational pull, both now and in the past? Neil MacnaughtanEdinburgh, UK

Drug cocktailTonic water contains quinine and was originally drunk with gin in tropical climates in order to counteract malaria – but how exactly does quinine treat the disease?

n Quinine is poisonous, but fortunately it is less harmful to you than to malaria parasites.

When feeding, the parasites break down haemoglobin in your erythrocytes, or red blood cells, releasing a toxic waste product called haem. Normally the parasites dispose of it by storing it in harmless insoluble clots. Although the precise way in which quinine and related drugs work is still obscure, it seems they interfere with the storage process, poisoning the parasites with their own wastes.

In appropriate doses, quinine does little harm to the body, but it can have some nasty effects if abused. It can cause heart, nerve, eye and kidney damage, and pregnant women should certainly not take it.

The drug was once also prescribed for night-time cramps. However, tests of its efficacy have given such variable and sometimes alarming results that its use for this purpose is now widely discouraged.

Quinine is one of the most bitter substances known, and has become an ingredient in some bitters used in cocktails – but it is present in such small quantities that it probably amounts to less of a health hazard than the alcohol. Jon RichfieldSomerset West, South Africa

Paindrops a’ falling This morning it was raining so hard the drops were painful. What causes this? Is it drop size and the height from which they fall? Or is there another mechanism that increases the speed at which drops fall? The rain seemed to be coming down especially hard, and drops hitting the ground bounced high into the air.

n Two things affect the energy of a raindrop and hence how painful it feels when it hits your head: its size, and the speed at which it falls. In practice, raindrops cannot grow bigger than about 5 millimetres in diameter, as air resistance will cause drops above this size to disintegrate into smaller ones.

Now, the larger the raindrop, the higher its maximum speed, or terminal velocity. When it reaches this speed, air resistance balances the downward force of gravity, so the drop no longer accelerates.

A 5-mm-diameter drop rapidly reaches its terminal velocity of around 9 metres per second, depending on the prevailing air temperature and humidity, so the height from which a raindrop falls has no bearing on its pain-inducing ability.

The school in which I teach is housed in a tall building with narrow stairwells where there is minimal horizontal air movement – ideal for investigating the behaviour of “raindrops” we make.

We find that a 4-mm-diameter drop falling two floors into a

puddle produces a splash or “bounce” of about 50 mm. The bounce for one falling four floors is about 150 mm, and for one falling six floors, just over 200 mm. This last result is no surprise as drops probably reach terminal velocity after falling about five floors.

Our experiments show that a drop 4 mm in diameter falling six floors on to skin feels like being hit by grains of rice from a few metres away. Drops 5 mm in diameter have slightly greater terminal velocity, but they are also almost twice as heavy. This means they have about twice as much energy and are mildly painful when falling on to skin from six floors up.

Thanks to bald colleagues for their assistance.David MuirScience departmentPortobello High SchoolEdinburgh, UK

This week’s questionTomaTo PligHTSometimes tomatoes that are kept in my fridge succumb to a whitish mould. The first bits to turn white are the stems, so if I spot mould, I pluck these off. But my husband says removing the stems shortens the fridge life of the tomatoes, so it is better to leave them on. Is either of us right? And what is the white mould, and is there any way I can stop it appearing?Paula CoswayPeterborough, Cambridgeshire, UK

“ Quinine is in some bitters used in cocktails, where it is probably less of a health hazard than the alcohol”

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