PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
In studying planetary motion, a student encounters the statement: “Objects bound to a planet in a circular orbit possess negative total mechanical energy.” The student finds this puzzling because kinetic energy is always positive.
(a) Explain in words why the total mechanical energy of a planet–satellite system is negative for a bound circular orbit, even though the kinetic energy of the satellite is positive.
(b) Describe qualitatively how the kinetic energy and gravitational potential energy of a satellite change as the satellite moves from the point of closest approach to the point of farthest separation in an elliptical orbit about a planet.
(c) Suppose an external agent does just enough work on a satellite in a circular orbit to make the total mechanical energy exactly zero. Explain what happens to the satellite’s subsequent motion and why the system’s energy sign change is significant.