Given that the energy in the universe is constant, how is it that spacecrafts traveling far distances sometimes get a boost by falling into the gravitational field of a planet? Acceleration caused by this trick needs force. Where is that force coming from? If the answer is gravity, then what is taken out of gravity so that energy is still constant?
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1Does this answer your question? Where does the extra kinetic energy come from in a gravitational slingshot? – John Rennie Feb 20 '23 at 18:04
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The force acting on the spacecraft is gravity. The energy gained by the spacecraft comes from the kinetic energy of the planet that is providing the gravity assist. Of course, the planet's kinetic energy is so large that the small amount lost to the spacecraft is negligible in comparison.
gandalf61
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The planet transfers some momentum to the spacecraft via the gravitational interaction. The planet's motion is changed imperceptibly because of its large mass, but the small spacecraft has a very noticeable change in its motion.
Edit: from a force point of view, the planet and spacecraft exert equal forces on each other, but the spacecraft accelerates much more because its mass is much less.
Rich006
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