Many analyses of orbital motion set the gravitational potential energy of a system of two masses equal to zero when the separation between the masses approaches infinity. Under this choice, the potential energy at any finite separation is negative.
(a) Explain qualitatively why the potential energy takes on negative values at finite separations when the zero is defined at infinite separation for an attractive interaction.
(b) An astronaut asserts, “Because the energy is negative, the system really has no energy.” Evaluate this statement, making clear reference to the role of the reference level for potential energy.
(c) Describe one practical advantage of choosing the reference level at infinite separation when analyzing the motion of satellites or planets.