Practice question · Multiple choice
A rocket accelerates in vacuum with nothing to push against. Which conservation law makes that unmysterious?
Hints
- Ask what the system consists of: just the rocket, or the rocket and its propellant?
- Total momentum before launch is zero. What must be true after?
Show the answer
C. Conservation of momentum, which needs no external medium
Why
The system is rocket plus propellant, and its total momentum is conserved, throw mass backwards and the remainder moves forward. Nothing external is needed, which is why exhaust velocity matters so much and why the 1920 objection to Goddard was simply a misreading of the third law.
Practise Newton's Laws and Forces
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