Two fat astronauts each of mass \(120 kg\) are traveling in a closed spaceship moving at a speed of \(15 km s^{-1}\) in the outer space far away from any other object. The total mass of the spaceship including the astronauts is \(660 kg\).
If the astronauts try to do slimming exercise & succeed in bringing their weights down to \(90 kg\) each (this is sure to make the women jealous), with what velocity will the spaceship finally move in \(km s^{-1}\)?
You can also try (Un) Realistic Situation #2.