This is a fun problem that I can't figure out how to get started. There are 6.2 billion people in the world. Assume all the people are distributed uniformly over the entire surface of the earth. Further assume that the average mass of each person in the world is 60kg and that their relative motion to the surface of teh earth is essentially zero and that the average day is 24 hours. Suppose all the people of the world agreed to gather around the equator evenly spaced out along its circumference. When we all get there suppose we all start running uniformly in the easterly direction along the equator at a uniform speed of 2 m/s. How long will a day last when this happens?