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If the world鈥檚 population all met in one place and all ran in the same direction, would this affect Earth鈥檚 rotation?

Martin Gregorie, Harlow, Essex, UK

There would be no detectable effect on Earth鈥檚 rotation, no matter how fast people accelerate when they start to run. That鈥檚 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鈥檚 population is to Earth.

Herman D鈥橦ondt,聽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 脳 10²⁴ kilograms, and that of its 7.5 billion people totals about 4 脳 10¹¹kg, making the planet about 10 trillion times more massive than us. You have about 30 trillion cells this says 37 trillion (ignoring bacteria). Hence in terms of mass, the proportion of one cell to the body is comparable to that of all people to Earth 鈥� 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鈥檛 collectively as heavy as you might think. Even if 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鈥檚 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鈥檚 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鈥檚 rotation slows by a few milliseconds per day. Earth鈥檚 atmosphere is estimated to have a mass of more than 5 脳 10¹⁸ kg, which is about 1 millionth of Earth鈥檚 total mass, but far more than the mass of its people.

Simon Rockett, Ipswich, Suffolk, UK

Can I refer you to Randall Munroe鈥檚 analysis of 鈥溾�� 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?

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