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Web sense
The back pages Almost the last word How can spiders move so rapidly on their webs without getting entangled?
Run the world If the world’s population all met in one place and all ran in the same direction, would this affect Earth’s rotation?
Herman D’Hondt Newtown, New South Wales, Australia This would have about the same effect as if one of the cells in your body started running around you. In other words, the total effect is too small to be measurable. Earth has a mass of around 6 × 1024 kilograms, and that of its 7.5 billion people totals about 4 × 1011 kg, making the planet about 10 trillion times more massive than us. You have about 30 trillion cells (ignoring bacteria). Hence in terms of mass, the proportion of one cell to the body is comparable to that of all people to Earth – and the effect will be similar. There is another factor to take into account. The main effect of the runners on Earth would happen when they start. Once running, they only have to push against Earth with a much smaller force to overcome the friction of the air. When they stop, they exert a push in the opposite direction, which cancels out the starting push. Hence, the overall effect consists only of the force needed to overcome air resistance. Hillary Shaw Newport, Shropshire, UK Humanity isn’t collectively as heavy as you might think. Even if 54 | New Scientist | 26 October 2019
YIPENGGE/GETTY IMAGES
Martin Gregorie Harlow, Essex, UK There would be no detectable effect on Earth’s rotation, no matter how fast people accelerate when they start to run. That’s because their mass is so much less than that of the planet. An ant running over my foot while I float in the International Space Station would have more effect on my rotation because its weight is more comparable to mine than that of Earth’s population is to Earth.
atmosphere is estimated to have a mass of more than 5 × 1018 kg, which is about 1 millionth of Earth’s total mass, but far more than the mass of its people.
This week’s new questions Web sense How are spiders able to move so quickly along their webs without getting their legs entangled? Mantz Yorke, Manchester, UK Playing dirty In view of the need to reduce emissions because of climate change, which sport has the highest carbon footprint after considering all factors? Chris Humphries, Scunthorpe, Lincolnshire, UK
there were 8 billion people with an average mass of 100 kg each, that would be a total of 800 million tonnes. This equates to a block of land 1 square kilometre by 250 metres deep; or a square km of water 800 m deep; or a volume of air 30 km by 30 km by 1 km. Our mass is tiny even compared with the moving air masses we call weather. Think of ocean currents and tides too. Typically all this air and water will be going in different directions at any one time, resulting in the forces cancelling each other out. So all humanity running together will have no more effect than a fly buzzing round a car has on the car’s balance and momentum. Malcolm Cunningham Paraparaumu, New Zealand To quantify the effect, we use the physical law of conservation of angular momentum. This states that the angular momentum of a closed system remains constant. Once the group of people are
running, they have a certain amount of angular momentum, so the counterrotating Earth must have an equal and opposite angular momentum. This expression is dominated by the ratio of the mass of people to the mass of Earth. If everyone ran at 10 kilometres per hour, the opposite velocity of the surface of Earth would be about 10-13 metres per second. It would take people running at this velocity around 10 years to move Earth’s surface by the width of a human hair. Richard Swifte Darmstadt, Germany Seasonal climatic shifts, by comparison, do have a measurable effect. In January and February, Earth’s rotation slows by a few milliseconds per day. Earth’s
Simon Rockett Ipswich, Suffolk, UK Can I refer you to Randall Munroe’s analysis of “everyone in the world jumping together” as a reason not to try this, even to answer a science question. The key point being: what happens to the world while everyone is away, and how do they all get home?
Swim for it My preferred form of exercise is swimming. For a given distance, which stroke is the best to use in terms of calories burned (continued)?
Harry Parkes London, UK Previous correspondents stated that front crawl is the fastest stroke, but this isn’t strictly true. There are two underwater strokes that are faster: the dolphin kick and the fish kick. These involve moving the legs together up and down while flexing the body and keeping one arm straight out in front in the direction of travel. With dolphin kick, a person typically swims facing downwards. With the fish kick, the swimmer is on their side. The fish kick is the fastest of the two. They aren’t recognised as official strokes, but as an enthusiast of underwater hockey, I and my teammates regularly break into dolphin kick to get that extra bit of speed when racing towards the opponent’s goal. ❚
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms
Run the world If the world’s population all met in one place and all ran in the same direction, would this affect Earth’s rotation?
Herman D’Hondt Newtown, New South Wales, Australia This would have about the same effect as if one of the cells in your body started running around you. In other words, the total effect is too small to be measurable. Earth has a mass of around 6 × 1024 kilograms, and that of its 7.5 billion people totals about 4 × 1011 kg, making the planet about 10 trillion times more massive than us. You have about 30 trillion cells (ignoring bacteria). Hence in terms of mass, the proportion of one cell to the body is comparable to that of all people to Earth – and the effect will be similar. There is another factor to take into account. The main effect of the runners on Earth would happen when they start. Once running, they only have to push against Earth with a much smaller force to overcome the friction of the air. When they stop, they exert a push in the opposite direction, which cancels out the starting push. Hence, the overall effect consists only of the force needed to overcome air resistance. Hillary Shaw Newport, Shropshire, UK Humanity isn’t collectively as heavy as you might think. Even if 54 | New Scientist | 26 October 2019
YIPENGGE/GETTY IMAGES
Martin Gregorie Harlow, Essex, UK There would be no detectable effect on Earth’s rotation, no matter how fast people accelerate when they start to run. That’s because their mass is so much less than that of the planet. An ant running over my foot while I float in the International Space Station would have more effect on my rotation because its weight is more comparable to mine than that of Earth’s population is to Earth.
atmosphere is estimated to have a mass of more than 5 × 1018 kg, which is about 1 millionth of Earth’s total mass, but far more than the mass of its people.
This week’s new questions Web sense How are spiders able to move so quickly along their webs without getting their legs entangled? Mantz Yorke, Manchester, UK Playing dirty In view of the need to reduce emissions because of climate change, which sport has the highest carbon footprint after considering all factors? Chris Humphries, Scunthorpe, Lincolnshire, UK
there were 8 billion people with an average mass of 100 kg each, that would be a total of 800 million tonnes. This equates to a block of land 1 square kilometre by 250 metres deep; or a square km of water 800 m deep; or a volume of air 30 km by 30 km by 1 km. Our mass is tiny even compared with the moving air masses we call weather. Think of ocean currents and tides too. Typically all this air and water will be going in different directions at any one time, resulting in the forces cancelling each other out. So all humanity running together will have no more effect than a fly buzzing round a car has on the car’s balance and momentum. Malcolm Cunningham Paraparaumu, New Zealand To quantify the effect, we use the physical law of conservation of angular momentum. This states that the angular momentum of a closed system remains constant. Once the group of people are
running, they have a certain amount of angular momentum, so the counterrotating Earth must have an equal and opposite angular momentum. This expression is dominated by the ratio of the mass of people to the mass of Earth. If everyone ran at 10 kilometres per hour, the opposite velocity of the surface of Earth would be about 10-13 metres per second. It would take people running at this velocity around 10 years to move Earth’s surface by the width of a human hair. Richard Swifte Darmstadt, Germany Seasonal climatic shifts, by comparison, do have a measurable effect. In January and February, Earth’s rotation slows by a few milliseconds per day. Earth’s
Simon Rockett Ipswich, Suffolk, UK Can I refer you to Randall Munroe’s analysis of “everyone in the world jumping together” as a reason not to try this, even to answer a science question. The key point being: what happens to the world while everyone is away, and how do they all get home?
Swim for it My preferred form of exercise is swimming. For a given distance, which stroke is the best to use in terms of calories burned (continued)?
Harry Parkes London, UK Previous correspondents stated that front crawl is the fastest stroke, but this isn’t strictly true. There are two underwater strokes that are faster: the dolphin kick and the fish kick. These involve moving the legs together up and down while flexing the body and keeping one arm straight out in front in the direction of travel. With dolphin kick, a person typically swims facing downwards. With the fish kick, the swimmer is on their side. The fish kick is the fastest of the two. They aren’t recognised as official strokes, but as an enthusiast of underwater hockey, I and my teammates regularly break into dolphin kick to get that extra bit of speed when racing towards the opponent’s goal. ❚
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms