**Question:**

Two fat astronauts each of mass $120 \mathrm{~kg}$ are travelling in a closed spaceship moving at a speed of $15 \mathrm{~km} / \mathrm{s}$ in the outer space far removed from all other material objects. The total mass of the spaceship and its contents including the astronauts is $660 \mathrm{~kg}$. If the astronauts do slimming exercise and thereby reduce their masses to $90 \mathrm{~kg}$ each, with what velocity will the spaceship move?

**Solution:**

In a closed spaceship, there is no external force (not even gravity). So the spaceship will move with a constant speed of $15 \mathrm{~m} / \mathrm{s}$.