New Scientist - 26.10.2019

54 | New Scientist | 26 October 2019

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?

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.

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 × 10^24 kilograms, and that of its

7.5 billion people totals about

4 × 10^11 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

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

atmosphere is estimated to have

a mass of more than 5 × 10^18 kg,

which is about 1 millionth of

Earth’s total mass, but far more

than the mass of its people.

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. ❚

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

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

The back pages Almost the last word

YIPENGGE/GETTY IMAGES

How can spiders move

so rapidly on their webs

without getting entangled?