5.5 billion people in the world. All land used for production of wheat. 45 times greater than current population. Projected to exceed allowed limit before 2100. The first starships are on their way.
Rockets are self-contained.
A spring or explosive pushes the two blocks apart. The blocks go in opposite directions. The smaller block moves faster. In a rocket, the burning of fuel pushes material away from the rocket at high speed. The faster the exhaust material, the greater the acceleration.