Quick questions on Gravitation and orbital motion: WACE Year 12 Physics Unit 3

A projectile moves in a uniform downward field, so it has constant horizontal velocity and constant vertical acceleration $g$. The two directions are independent. Horizontally,

A geostationary satellite stays above the same point on the equator, which requires its period to equal one sidereal day, $T=8.64\times10^4\ \text{s}$. Rearranging Kepler's third law $T^2=\dfrac{4\pi^2}{GM}r^3$ for $r$ gives a single fixed radius of about $4.2\times10^7\ \text{m}$ (roughly $3.6\times10^4\ \text{km}$ above the surface). The orbit must also lie in the equatorial plane and travel west to east; otherwise the satellite would drift relative to the ground. This is why communications and weather satellites that must point at a fixed dish all sit at the same altitude.