Which of the following best explains why the gravitational binding energy of an isolated satellite–planet system is negative?
(a) The kinetic energy of an orbiting satellite is negative while its potential energy is positive, so the total energy is negative.
(b) Gravitational binding energy equals −rGMm, which is negative because G,M,m, and r are positive.
(c) Zero potential energy is defined at infinite separation, so any finite separation corresponds to a lower (negative) energy state.
(d) Because the gravitational force is attractive, by convention both potential and total energies must be negative.