MOMENTUM
2 marks · EasyExplain why a rocket moves forward when burning fuel, even in the vacuum of space.
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As the rocket burns fuel, gases are expelled backwards. By the conservation of momentum, the rocket gains forward momentum equal and opposite to the momentum of the exhaust gases.
Also accept: The exhaust gases move backward, so the rocket moves forward to conserve momentum.; Momentum before and after burning fuel is conserved, so the rocket gains momentum.
Common mistakes: Saying the rocket pushes against air or ground
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Marking points
B1 · 1 mark · States that the rocket moves forward because gases are expelled backwards.
B2 · 1 mark · Links to conservation of momentum - rocket's gain is equal and opposite to gases expelled.
Worked solution
When the rocket burns fuel, fast-moving gases are pushed backwards out of the rocket. Because momentum must be conserved, the rocket gains equal momentum in the opposite direction, so it moves forwards.