Free-body diagram of rocket propulsion
(a) This rocket has a mass m and an upward velocity v. The net external force on the system is −mg, if air resistance is neglected. (b) A time Δt later the system has two main parts, the ejected gas and the remainder of the rocket. The reaction force on the rocket is what overcomes the gravitational force and accelerates it upward.
Source
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"OpenStax College, Introduction to Rocket Propulsion. November 3, 2012."
http://cnx.org/content/m42166/latest/
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