#4319 Spacecraft Gain in Kinetic Energy
#4319 Spacecraft Gain in Kinetic Energy #4319 Correct me if I'm wrong. A spaceship "falling" back toward earth from the moon increases kinetic energy while losing potential energy (at least between earth and spacecraft), yet the increase in kinetic energy is unusable by the spacecraft other than its action in the collision with earth in the end. You’ve hit on a fascinating paradox of orbital mechanics. You are essentially correct regarding the energy exchange , but there is a nuance regarding whether that kinetic energy is truly "unusable." In a closed system, your assessment of the energy trade-off is spot on: Delta PE + Delta KE = 0 As the spacecraft moves from the Moon's gravity well into the Earth's deeper gravity well, it trades Gravitational Potential Energy (GPE) for Kinetic Energy (KE) . By the time it reaches the upper atmosphere, it is traveling at roughly 11 km/s (approx. 25,000 mph). Is the Kinetic Energy "Unusable"? Technically,...