The Earth Moved.......

[Stav]

If everyone in the world ran in the same direction, in theory could we speed up the rotation of the Earth, or slow it down/stop it??

[incitatus]

Ill have to think about it but my gu feeling is no. The reason being we would have nothing to push agianst.....being on earth we are all part of the same system. us running around it doesnt apply friction or any other retarding force to the global system....I think one could make it wobble, but to slow it down you would need something to act against...

[j2buttonsw]

Is this not a take on the relativity type questions? Like the the velocity of a hovering fly in a speeding aeroplane? I think the movement of the people is not relative to the forces making the earth spin.

[darth vader]

I would disagree with incitatus (sorry old bean!) and say yes in theory. One of Newton's physical laws says that "for every action there is an equal an opposite reaction". By taking a step forward you push the hind leg back therefore your body is propelled forward, but there is an equal force from your leg pushing back, so you would exert some force on the planet. I remember reading a famous question a few years ago which went "if everyone in China jumped up at the same time would it cause an earthquake?" and the scientists' reply was that the combined force would result in almost no effect in relation to the earth's size due to the immense difference in ratio's - hence me saying yes in theory, but i believe the combined energy from people running would be extremely small and certainly not noticeable

[Stav]

Thank you all. I was thinking the same thing as Darth - the force exerted by the people would not be enough to make any difference, but it could be done in theory - like if you stand on top of a big ball and start running, you can make the ball move whilst staying on top of it. Cheers people!

[incitatus]

darth, i think you are wrong. Use newton: conservation of angular momentum. case1: I run in the direction of rotation of the earth. my angular moentum increases, the earths decreases. the total angular momentum remains the same. I stop running. the earths angular momentum increases, mine decreases, angular momentum conserved. The whole system remains unchanged by this. If you were to keep running forever, then maybe. or if you could act against something......

[wildwood]

No, because the propulsion force of the bodies is compensated for by the collection of kinetic energy by the moving bodies. We could slow it down temporarely if we all started at the same time, but once the maximum acceleration is reached by the bodies, the Earth would compensated by over-rotating slightly.

[sddsddean]

As Incitatus says , we are talking about angular momentum. As we only want to affect the speed of the earth, the actual velocity is irrelevant, so we can estimate any effect by inspection of the masses involved. There are approx 6,000,000,000 people on earth. Say each has an average weight of 80kg, this gives a combined mass of 480,000,000 tonnes. The Earth's mass is about 6,000,000,000,000,000,000,000 tonnes. So the Earth has a mass approximately 10,000,000,000,000 times more than its population. By inspection, it is so much greater that the mass of humanity would have no visible affect. For comparison the same ratio would apply to a 1gram grain of rice compared to 40 x 250,000 super tankers.

[MrPooHead]

Told!

[darth vader]

i did say "in theory" sddsddean - the change would be so Infinitesimally small as to not be noticed IMHO. I'm not really convinced yet so I will wait till my friend the Physics degree graduate comes back off holiday on monday, before I give in :-)

[ukkid76]

Darth Vader said, "For every action there is an equal an opposite reaction" ok by taking or running a person propels his/her self forward they would be pushing against the earth and that person would be moving forward so that's the equal reaction, they are not standing still with the earth spinning underneath them are they!!!. Anyway if everybody were to congregate in one small part of this planet say the Isle of Man to try out this experiment how would the bus's cope?

[orionone]

Yes, it would affect the rotation but by a very very very small amount. I remember in physics class we did a similar calculation, if everyone on earth was in one area (which is the size of jacksonville, florida area wise) and jumped all at once, then how much would the earth move in its orbit. i dont remember the exact amount, but it was something in micrometers that the earth would move, which is very insignificant anyways