I'm a class 12th student and this thought just struck me from nowhere.
Assume a situation where every human on this planet turns towards west. All of them start walking simultaneously in west direction (exactly west i.e. antiparallel to Earth's rotation and none of their paths cross each other).
As Earth spins from west to east would this activity cause the earth's rotation speed to increase?
(There is frictional force acting on the earth due to motion by nearly a billion people which should produce enough torque to increase angular velocity of the earth.)
p.s. If the magnitude of friction force not enough to rotate the earth then assume each person to weigh more than 100 kg.
Also if it does happen would earth remain in the same orbit around the sun with just increased angular velocity about axis? (or would path around the sun also change due to this? )
I would really appreciate an easy solution cause I am still in class 12th.
Answer
The friction due to a single person would be on the order of ~100N. There are currently 7.2 billion persons on Earth. I don't think this will affect revolution around Sun but surely it will affect the rotation of Earth about its own axis.
I did calculated this effect on the back of envelope. Assuming a very not-so-realistic assumption that all people will march along the equator simultaneously, we can calculate an upper limit on the total external torque opposing the rotation of the earth. The angular acceleration due to such a march will be so small that it will take something on the order of 100,000,000 years for all the people to march with no stop to finally being able to bring the rotation to a halt. And, obviously no one can do this for such a long time. So, we are safe.
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