SACE Stage 2 Physics — Motion 1.2

Propulsion

In deep space there is nothing to push against. The only way to change momentum is to throw mass the other way — and that is exactly what a rocket does. Total momentum of the system stays zero.

Mode

Sail type

Ion species

Momentum vectors

Key idea

Before firing, the rocket is at rest — total momentum is zero. After ejecting exhaust gas backward, the exhaust carries momentum in one direction, so the rocket must carry equal momentum in the opposite direction. p_rocket + p_exhaust = 0, always.