An Earth satellite in a circular orbit is at an altitude of $3185.5 \mathrm{~km}$. The acceleration due to gravity at that distance is $4.36 \mathrm{~m} / \mathrm{s}^{2}$, and the mean radius of the Earth is $6371 \mathrm{~km} .(a)$ What is the radius of the orbit? (b) Find the speed of the satellite.