Have the last word

Why does tea change colour when you add lemon? How fat do you have to be to become bulletproof? What would be the effect on the Earth, if an alien spaceship came and dragged the moon away?

In 1994, the New Scientist magazine began publishing a weekly column called ‘The Last Word’ with questions and answers on everyday science; some weird, some witty, others downright wacky. More than a decade later, the column still continues and today, The Sunday Times launches ‘YoY’ in similar vein.

Readers are requested to send in their questions, and also answers to the questions that we will publish weekly. As the New Scientist put it, The Last Word is devoted to the small questions in life.

Published here are two sample questions and answers from the New Scientist:

High brow

Q: Why do people have eyebrows?

Ben Holmes
Edmonton, Canada

My father has alopecia, so he doesn’t have eyebrows. In warm weather, sweat runs into his eyes and makes them sore; in wet weather he has to keep wiping the rain out of his eyes. So your eyebrows divert sweat droplets and raindrops from running directly into your eyes. You would be very uncomfortable without them.

Valerie Higgins
Telford, Shropshire, UK

We use our exceptionally mobile eyebrows to communicate our emotions. The position of the eyebrows emphasises expressions on the human face, thus giving others an accurate picture of the individual’s mood. This gives good indication of whether a person is friendly, or whether they might be dangerous to approach.

Smiles come in many forms, from expressions of merriment or contentment to leers, smirks and even anger. The position of the brow, emphasised by the eyebrows, is what gives us a visual cue to what an individual is really feeling.

The importance of eyebrow position as a guide to mood was brought home to me, when a friend had Botox injections in the lines on her forehead and couldn’t raise or lower her eyebrows. Talking to her became a disconcerting experience – the bottom half of her face remained mobile, but her eyebrows did not move. I couldn’t deduce her mood accurately by looking at her expression, and needed to use other cues, such as her actions and speech.

Alison Venugoban
Ngunnawal, ACT,
Australia

Eyebrows are important in expressing emotions. Perhaps most important is the ‘eyebrow flash’, a rapid up-and-down flick of the eyebrows that conveys recognition and approval. The ability to telegraph friendly intentions from a safe distance would have had obvious survival value for our ancestors.

Eyebrow signalling of various kinds is widespread among primates, although only in humans are the eyebrows highlighted by setting them against bare skin Ed.

Who needs nine lives?

Q: 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 kittens in my house now look strangely at my friend. I’m sure this can’t be true, can it?

Anna Goodman
Oxford, UK

I’m reminded of a study reported in the Journal of the American Veterinary Medicine Association in 1987 by W. O. Whitney and C. J. Mehlhaff, two New York vets, entitled ‘High-rise syndrome in cats’. The study was also summarised in Nature a year later. Briefly, the authors examined injuries and mortality rates in cats that had been brought to their hospital, following falls ranging from between two and 32 storeys. Overall mortality rates were low, with 90 percent of the cats surviving, a fact that supports the correspondent’s ailurophobic friend. However, the study unexpectedly found that the incidence of injuries and death peaked for falls of around seven storeys, and then actually decreased for falls from greater heights.

The Nature article presents three main variables that determine injury and mortality rate – the speed reached by the moggy, the distance in which said moggy is brought to a stop, and the area of moggy over which the stopping force is spread. While concrete streets work in nobody’s favour when it comes to stopping falling items, cats suffer relatively little injury (compared to their owners), because they do indeed reach lower terminal velocities and absorb the shock of stopping so much better. A falling cat has a higher surface area to mass ratio than a falling human, and so reaches a terminal velocity of about 100 kilometres per hour (about half that of humans). They are also able to twist themselves, so that the impact is spread over four feet, rather than our two. Also as they are more flexible than humans, they can land with flexed limbs and dissipate the impact forces through soft tissue.

To answer the paradoxical increase in survival rates once seven storeys has been reached, the authors suggested that an accelerating cat tends to stiffen up, reducing its ability to absorb the impact. However, once terminal velocity is reached, there is no longer any net force acting on the cat, and so it will relax, increasing both its flexibility and the cross-sectional area over which the impact is dissipated once the cat hits the ground.

I’d still keep your friend away from your kittens, if I were you. Few buildings in your home town of Oxford are seven storeys high, but there are plenty of rivers about.

John Bothwell
Marine Biological
Association, Plymouth, Devon, UK

Vets quite commonly see jaw injuries in cats, usually as a result of their taking too high a leap from a wall - Ed.

I don’t know what the terminal velocity of the average cat is, but this question did remind me of a joke. Because cats always land on their feet and toast always lands buttered side down, you can construct a perpetual motion machine by simply strapping a slice of buttered toast to a cat’s back. When the cat is dropped it will remain suspended and revolve indefinitely due to the opposing forces.

Catherine
Staffordshire University, Stoke-on Trent, UK

The risks to different animals of taking a fall were laid out in 1927 by the biologist J. B. S. Haldane, in Possible Worlds and Other Essays. He wrote, “Gravity, a mere nuisance to Christian, was a terror to Pope, Pagan and Despair. To the mouse and any smaller animal it presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft, and, on arriving at the bottom, it gets a slight shock and walks away.

“A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal’s length, breadth and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force. An insect, therefore, is not afraid of gravity; it can fall without danger, and can cling to the ceiling with remarkably little trouble.”

John Forrester
Edinburgh, UK