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Q & A: astronomical energy conservation

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Most recent answer: 01/15/2010
Q:
I have always wondered, say the hypothetical scenario existed where an asteroid is at rest in space. A planet or other large body passes nearby, and the asteroid is pulled torwards the large body by gravity. where does this kinetic energy come from? Does this defy the conservation of energy?
- Raiden (age 13)
Del Rio, TN, USA
A:
Nice question. Initially the planet and the asteroid are far apart, so there's some gravitational potential energy. That's just like the gravitational potential energy of lifting some rock up off the ground. As the two get closer, some of that is lost and repalced with kinetic energy. Both the planet and the asteroid speed up. That's just like the kinetic energy when a rock falls.

If the planet and asteroid don't collide but end up getting far away again, the potential energy goes back to it's starting value. However, although the kinetic energy of the planet + the asteroid also goes back to the starting value. In the simplest case, each goes back to its own starting value, and the asteroid returns to rest (in our chosen coordinate frame). Add a little complication (some other nearby planets), and you can end up with the asteroid still moving, with the energy coming from the kinetic energy of the planets.

Mike W.

(published on 01/15/2010)

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