The radius of the Moon's approximately circular orbit is 384,000 km and its period is 27.3 days. Estimate the mass of the Earth. [G=6.67×10−11 N m2 kg−2.] The actual mass is 5.97×1024 kg. What is the main reason for the error in your estimate?
An artificial satellite is to be placed in a circular orbit around the Earth so as to be `geostationary'. What must the radius of its orbit be? [The period of the Earth's rotation is 23 h 56 m, not 24 h. Why?]