A satellite moves eastwards in a circular orbit of radius $2R$ in the plane of the equator, where $R = 6400\ \text{km}$ is the Earth's radius. Take $g = 10\ \text{m/s}^2$ at the Earth's surface, and the Earth to turn once (eastwards) in $24\ \text{h}$. How often does the satellite pass straight over a given point on the equator?