Once a satellite has been launched into orbit, the only force governing its motion is the force of ______.

The correct answer is A, gravity. A satellite is given the required height and speed by its launch vehicle, and after that no engine is needed: the gravitational pull of the earth alone acts on it and provides the centripetal force that keeps it in a curved path, so the satellite keeps falling towards the earth while moving forward fast enough to miss it. This is why the satellite and everything inside it are in free fall and objects appear weightless. Option B, elasticity, is a property of materials and has no role in orbital motion. Option C, friction, is almost absent at the height of a satellite because there is practically no air there, although a trace of atmospheric drag does slowly lower the orbit of satellites in very low orbits. Option D is wrong because fuel is spent only during launch and for occasional small corrections of the orbit, not to keep the satellite moving.