Astronomy Practice Exam

What does Newton's laws suggest about total orbital energy in a system?

1. It can change with external forces

2. It remains constant without external forces

3. It is always decreased by gravity

4. It can spontaneously transform

The correct answer is: It remains constant without external forces

Newton's laws, particularly when applied to celestial mechanics, suggest that the total orbital energy of a system remains constant without the influence of external forces. This principle is grounded in the conservation of energy, which states that in an isolated system where no external work is done, the total energy—comprising both kinetic and potential energy—remains unchanged. In the context of orbital mechanics, when two bodies are in orbit around each other, the gravitational attraction between them acts as a conservative force. As one body moves closer to another, its gravitational potential energy decreases while its kinetic energy increases, and vice versa. However, the overall energy—kinetic plus potential—remains constant, so long as no external forces, such as those from a third mass or outside influence, act on the system. This explains why the correct answer highlights the constancy of total orbital energy in the absence of external forces. In scenarios where external forces are present, such as tidal forces or drag from a planetary atmosphere, the total orbital energy can be altered, but those cases are outside the scope of idealized Newtonian mechanics focusing solely on an isolated system.