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Rise and fall
THE LAST WORD Repulsive coffee What causes coffee grounds to dry into the pattern shown at the bottom of this coffee cup (see photo)?
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a clear space around them, then a very concentrated ring of fine material. A New Scientist article many years ago called “The thrill of the spill” (25 October 1997) explains this effect in detail. Simon Iveson School of Engineering, Faculty of Engineering and Built Environment University of Newcastle Callaghan, New South Wales, Australia
Rise and fall Why are the largest tide ranges in the world – of up to 17 metres – found in the Bay of Fundy, on Canada’s Atlantic coast?
n The coffee dregs essentially n To understand what happens consist of two components – very in the Bay of Fundy, start with a large coffee grains which remain hand basin half-full of water. Push settled in one place and very fine down on the surface on one side colloidal material that moves with the palm of your hand and with the liquid. Because of the water will rise on the other, surface-tension effects, the large after which it will slosh back grains retain a pool of liquid and forth like a liquid see-saw. around them. As this liquid By pushing down even more on droplet dries, evaporation happens preferentially at the edge “An incoming ocean tide effectively appears as of the droplet because it is more a huge wave advancing exposed. This creates a flow of towards the bay” liquid towards the edge which carries with it the fine colloidal particles. As a result, the majority each side at the same time as the level on that side is falling, of them end up being deposited the rise and fall of the surface at the droplet periphery, which will increase – and can be made to remains anchored in place overflow the rim of the basin. The some distance from the particle sloshing of the water has a natural until nearly all of the moisture frequency and your additional has evaporated. input resonates with it, increasing This explains the resultant the amplitude of the see-saw pattern of large particles, with
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
wave. At the central axis of the basin the level remains unchanged, although water moves to and fro horizontally. Now imagine the basin cut vertically down that central axis and consider just half of it. The half that is left corresponds to the Bay of Fundy, the axis-edge marks
“If the bay opened into the Atlantic it would not have such tides, but it opens into the Gulf of Maine” the opening of the bay, and the missing half of the basin is replaced by the open Atlantic Ocean. An incoming tide effectively appears in the ocean as a huge wave advancing towards the bay. As it reaches the continental shelf at the bay’s opening it plays the role of the high half of the see-saw and happens to coincide with the low water level at the far end of the bay. By the time this wave has moved to the innermost section of the bay – raising its water level to a peak – the dip in the ocean surface corresponding to the low tide has reached the continental shelf. The exceptionally high tides occur because the successive incoming tides appear at nearly the same frequency as water sloshing into and out of the bay, just as happened in the wash basin. It is a resonance effect. If the Bay of Fundy opened directly into the Atlantic it would not have such high tides because the natural period of water moving to and fro in it would
Do Polar Bears Get Lonely? Our latest collection serious enquiry, brilliant insight and the hilariously unexpected Available from booksellers and at www.newscientist.com/ polarbears
be only about 9 hours, which is not close enough to the 12.5-hour period of the tides to lead to significant amplification of the wave motion. However, the Bay opens into the Gulf of Maine and together they have a natural frequency of just over 13 hours. Richard Holroyd Cambridge, UK
This week’s questions Piles of reasons
Why do rugs placed on carpets appear to move of their own volition? How can a bunch of inert fibres exert the force needed to move my rug 15 centimetres in just two weeks when there is a substantial armchair on it? Ann Duncombe Falkirk, Stirlingshire, UK Shooting star
Yesterday I tripped over a fence post I had just pulled from the ground. When my face hit the concrete around the bottom of the post, I literally saw stars. Today I have a shiny new black eye and a few questions. What are those stars and what causes them? Victor Stanwick Staten Island, New York, US Revision tactics
Is it better to stay up late on the night before an exam, learning those last-minute facts, or to get up early and revise in the morning before the exam? Andrew Maeer Exmouth, Devon, UK
Last words past and present, plus questions, at www.last-word.com
a clear space around them, then a very concentrated ring of fine material. A New Scientist article many years ago called “The thrill of the spill” (25 October 1997) explains this effect in detail. Simon Iveson School of Engineering, Faculty of Engineering and Built Environment University of Newcastle Callaghan, New South Wales, Australia
Rise and fall Why are the largest tide ranges in the world – of up to 17 metres – found in the Bay of Fundy, on Canada’s Atlantic coast?
n The coffee dregs essentially n To understand what happens consist of two components – very in the Bay of Fundy, start with a large coffee grains which remain hand basin half-full of water. Push settled in one place and very fine down on the surface on one side colloidal material that moves with the palm of your hand and with the liquid. Because of the water will rise on the other, surface-tension effects, the large after which it will slosh back grains retain a pool of liquid and forth like a liquid see-saw. around them. As this liquid By pushing down even more on droplet dries, evaporation happens preferentially at the edge “An incoming ocean tide effectively appears as of the droplet because it is more a huge wave advancing exposed. This creates a flow of towards the bay” liquid towards the edge which carries with it the fine colloidal particles. As a result, the majority each side at the same time as the level on that side is falling, of them end up being deposited the rise and fall of the surface at the droplet periphery, which will increase – and can be made to remains anchored in place overflow the rim of the basin. The some distance from the particle sloshing of the water has a natural until nearly all of the moisture frequency and your additional has evaporated. input resonates with it, increasing This explains the resultant the amplitude of the see-saw pattern of large particles, with
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
wave. At the central axis of the basin the level remains unchanged, although water moves to and fro horizontally. Now imagine the basin cut vertically down that central axis and consider just half of it. The half that is left corresponds to the Bay of Fundy, the axis-edge marks
“If the bay opened into the Atlantic it would not have such tides, but it opens into the Gulf of Maine” the opening of the bay, and the missing half of the basin is replaced by the open Atlantic Ocean. An incoming tide effectively appears in the ocean as a huge wave advancing towards the bay. As it reaches the continental shelf at the bay’s opening it plays the role of the high half of the see-saw and happens to coincide with the low water level at the far end of the bay. By the time this wave has moved to the innermost section of the bay – raising its water level to a peak – the dip in the ocean surface corresponding to the low tide has reached the continental shelf. The exceptionally high tides occur because the successive incoming tides appear at nearly the same frequency as water sloshing into and out of the bay, just as happened in the wash basin. It is a resonance effect. If the Bay of Fundy opened directly into the Atlantic it would not have such high tides because the natural period of water moving to and fro in it would
Do Polar Bears Get Lonely? Our latest collection serious enquiry, brilliant insight and the hilariously unexpected Available from booksellers and at www.newscientist.com/ polarbears
be only about 9 hours, which is not close enough to the 12.5-hour period of the tides to lead to significant amplification of the wave motion. However, the Bay opens into the Gulf of Maine and together they have a natural frequency of just over 13 hours. Richard Holroyd Cambridge, UK
This week’s questions Piles of reasons
Why do rugs placed on carpets appear to move of their own volition? How can a bunch of inert fibres exert the force needed to move my rug 15 centimetres in just two weeks when there is a substantial armchair on it? Ann Duncombe Falkirk, Stirlingshire, UK Shooting star
Yesterday I tripped over a fence post I had just pulled from the ground. When my face hit the concrete around the bottom of the post, I literally saw stars. Today I have a shiny new black eye and a few questions. What are those stars and what causes them? Victor Stanwick Staten Island, New York, US Revision tactics
Is it better to stay up late on the night before an exam, learning those last-minute facts, or to get up early and revise in the morning before the exam? Andrew Maeer Exmouth, Devon, UK