Two satellites $$A$$ and $$B$$ revolve round the same planet in coplanar circular orbits lying in the same plane. Their periods of revolutions are $$1 \mathrm{~h}$$ and $$8 \mathrm{~h}$$, respectively. The radius of the orbit of $$A$$ is $$10^4 \mathrm{~km}$$. The speed of $$B$$ is relative to $$A$$. When they are closed in $$\mathrm{km} / \mathrm{h}$$ is

A planet is revolving around the sun in a circular orbit with a radius $$r$$. The time period is $$T$$. If the force between the planet and star is proportional to $$r^{-3 / 2}$$, then the square of time period is proportional to

The weight of a body on the surface of the earth is 63 N. What is the gravitational force on it due to the earth at a height equal to half the radius of the earth?