A student asked me this question recently, and I am not
sure about my answer.
He asked, "Imagine a rocket hovering off the ground,
applying a thrust equal to the rocket's weight. Gas is
moving downwards getting downwards momentum. What is
getting upwards momentum?"
Okay formulate your answer....
(Scroll down to see mine.)
I said that we should decide that the direction that the
gas is moving is negative, and opposite of that direction
is positive. The gas gets negative momentum and the rocket
gets positive momentum due to conservation of momentum, but
the rocket is pulling on the planet -- the planet doesn't
just pull on the rocket but the rocket also pulls on the
planet! Though the distance between the rocket and the
planet doesn't increase, the rocket and planet, together,
get some velocity in the positive direction.
This is before the gas hits the ground. After the gas
starts hitting the ground the situation becomes more
complicated. In the simplest case, the gas imparts its
momentum to the planet which stops the rocket's and the
planet's motion in the positive direction.
What say you?
Marc "Zeke" Kossover
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