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Flying blind
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THE LAST WORD Light as air While on the scales this morning I wondered, would passing gas affect the weight of the human body at sea level and, if so, in which direction? (Continued)
n Whether the presence of gas in the body (and its subsequent release) causes an increase or decrease in body weight depends on Archimedes’ principle, which states that the buoyant force exerted on a body immersed in fluid (in this case air) is equal to the weight of the fluid the body displaces. Thus the effect depends on the mass difference between the particular gas in the body and the same volume of air. Take a typical vent volume of 100 millilitres. This amount of air has a mass of about 0.12 grams. The same volume of methane is 0.07 grams, so it would give slight buoyancy to the body, and its
“Flatus is a mixture of air, carbon dioxide and hydrogen, varying with each individual” expulsion an apparent weight gain of 0.05 grams. Conversely, 100 millilitres of carbon dioxide has a mass of about 0.18 grams, so expulsion of this gas would give an apparent weight loss of 0.06 grams. The extreme case is hydrogen: 100 millilitres has a mass of only 0.008 grams, thus giving a weight gain of about 0.11 grams on expulsion. Flatus is a mixture of air, carbon
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. Reed Business Information Ltd reserves all rights to reuse all question and answer material that has been
dioxide, methane and hydrogen in variable amounts depending on the individual and their recent dietary history. Therefore it can be lighter or heavier than air. There are three minor complications to any calculation of this nature. Gas inside the body, at 37 °C, is hotter and therefore about 6 per cent less dense than the outside air, assuming outside temperatures of 20 °C. Conversely, the intestinal gas will be under slightly higher pressure, increasing its density. Finally, internal gases are saturated with water vapour, so the density difference will depend on the humidity of the surrounding air, because humid air is less dense. Martin Pitt Department of Chemical and Biological Engineering University of Sheffield, UK
Bathed in heat I had a hot bath one evening and decided not to let all the heat go to waste. So I left the plug in until the water had given up all of its heat to the house. But when is the best time to pull the plug out? Should I wait until the water is the same temperature as the ambient indoors air, or should I keep the extra thermal mass in the bathtub, and then pull the plug when the temperature outside the house is at its lowest?
n The water serves two functions. It provides a source of heat until it cools to the ambient temperature, and it is also a thermal mass.
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to lastword@newscientist. com or visit www.newscientist.com/topic/ lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
Assuming the water equilibrates with the surroundings of the dwelling and the heat only leaves the house at a set rate, the additional mass of the full tub will cause the house to cool at a slower rate. But the interior of the house will also take longer to heat up. The most efficient thing to do is to drain all of the water out of the bath at the point when the lowest interior house temperature has been reached – you may need to have a thermometer to hand. Draining the bath at this point will lower the thermal mass and will then allow the interior of the house to heat up faster when the outside temperature increases once more, because heat will not be wasted in increasing the water temperature. Neil Ayre Kalgoorlie, Western Australia
This week’s questions Flying blind
Do large modern jet airliners need windows for the pilots? In theory, could a Boeing 747 be flown from London to Sydney without the pilots needing to see outside? Peter Marston London, UK Web of intrigue
This cobweb appeared in a toilet cubicle (see photo). The segmented object to the left is, I assume, a captured insect. There is an opening at the top and a dark shape within that is presumably the spider. Stuck to the web are peculiar whitish lumps that may be eggs. Does anyone know what spider makes this kind of nest? Charlie Wartnaby Cambridge, UK
Will we ever speak dolphin? The latest book: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphins
THE LAST WORD Light as air While on the scales this morning I wondered, would passing gas affect the weight of the human body at sea level and, if so, in which direction? (Continued)
n Whether the presence of gas in the body (and its subsequent release) causes an increase or decrease in body weight depends on Archimedes’ principle, which states that the buoyant force exerted on a body immersed in fluid (in this case air) is equal to the weight of the fluid the body displaces. Thus the effect depends on the mass difference between the particular gas in the body and the same volume of air. Take a typical vent volume of 100 millilitres. This amount of air has a mass of about 0.12 grams. The same volume of methane is 0.07 grams, so it would give slight buoyancy to the body, and its
“Flatus is a mixture of air, carbon dioxide and hydrogen, varying with each individual” expulsion an apparent weight gain of 0.05 grams. Conversely, 100 millilitres of carbon dioxide has a mass of about 0.18 grams, so expulsion of this gas would give an apparent weight loss of 0.06 grams. The extreme case is hydrogen: 100 millilitres has a mass of only 0.008 grams, thus giving a weight gain of about 0.11 grams on expulsion. Flatus is a mixture of air, carbon
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. Reed Business Information Ltd reserves all rights to reuse all question and answer material that has been
dioxide, methane and hydrogen in variable amounts depending on the individual and their recent dietary history. Therefore it can be lighter or heavier than air. There are three minor complications to any calculation of this nature. Gas inside the body, at 37 °C, is hotter and therefore about 6 per cent less dense than the outside air, assuming outside temperatures of 20 °C. Conversely, the intestinal gas will be under slightly higher pressure, increasing its density. Finally, internal gases are saturated with water vapour, so the density difference will depend on the humidity of the surrounding air, because humid air is less dense. Martin Pitt Department of Chemical and Biological Engineering University of Sheffield, UK
Bathed in heat I had a hot bath one evening and decided not to let all the heat go to waste. So I left the plug in until the water had given up all of its heat to the house. But when is the best time to pull the plug out? Should I wait until the water is the same temperature as the ambient indoors air, or should I keep the extra thermal mass in the bathtub, and then pull the plug when the temperature outside the house is at its lowest?
n The water serves two functions. It provides a source of heat until it cools to the ambient temperature, and it is also a thermal mass.
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to lastword@newscientist. com or visit www.newscientist.com/topic/ lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
Assuming the water equilibrates with the surroundings of the dwelling and the heat only leaves the house at a set rate, the additional mass of the full tub will cause the house to cool at a slower rate. But the interior of the house will also take longer to heat up. The most efficient thing to do is to drain all of the water out of the bath at the point when the lowest interior house temperature has been reached – you may need to have a thermometer to hand. Draining the bath at this point will lower the thermal mass and will then allow the interior of the house to heat up faster when the outside temperature increases once more, because heat will not be wasted in increasing the water temperature. Neil Ayre Kalgoorlie, Western Australia
This week’s questions Flying blind
Do large modern jet airliners need windows for the pilots? In theory, could a Boeing 747 be flown from London to Sydney without the pilots needing to see outside? Peter Marston London, UK Web of intrigue
This cobweb appeared in a toilet cubicle (see photo). The segmented object to the left is, I assume, a captured insect. There is an opening at the top and a dark shape within that is presumably the spider. Stuck to the web are peculiar whitish lumps that may be eggs. Does anyone know what spider makes this kind of nest? Charlie Wartnaby Cambridge, UK
Will we ever speak dolphin? The latest book: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphins