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Physics/Equal and Opposite Reaction


Newton's third law of motion states that. , Every action has an equal and opposite reaction.    If a rocket ship is accelerating at one G, and the rocket has an elevator at the bottom of the ship, if the elevator moves from the bottom of the ship to the top under one G of artificial gravity, what is the equal and opposite reaction to the elevators motion?

Hello Richard,

Let's assume the elevator moves because a motor in the elevator car starts turning a gear that engages a sprocket in the elevator's channel. That sprocket is firmly attached to the rocket body. The turning of the gear causes a downward force on the sprocket. The reaction to that force is that the sprocket pushes the car upward. Notice that the active force is essentially downward on the rocket body. Therefore the rocket acceleration decreases while the elevator accelerates.

When the elevator gets to the top and decelerates, the action/reaction at the gear/sprocket interface is reversed. It gives an upward force to the rocket, in addition to the engine's thrust. That briefly increases the acceleration of the rocket to a value greater than it would have been without the elevator doing anything. Essentially the energy stolen by the elevator is then paid back to the rocket. So it was only a temporary decrease in acceleration when the elevator started going up.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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