A rocket moves forward in the vacuum of space because
- A. the exhaust gases push against the air behind it
- B. the momentum carried away by the exhaust gases is balanced by an equal forward momentum of the rocket
- C. gravity pulls it forward
- D. it becomes lighter as fuel burns
Explanation
Conservation of momentum, equivalently Newton's third law, means expelling mass backwards at speed drives the rocket forwards, and no surrounding medium is needed. This is why rockets work best in a vacuum, where there is no air resistance. Losing mass does help the acceleration but is not the source of the thrust.
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