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QUESTION: we know the rockets that send from earth can move using the 3rd law of NEWTON within the air, but after that when it moves in space, how it can move? please give me a brief discussion.

ANSWER: According to Newton's Third Law any time you apply a force to an object, that object applies the exact opposite force to you. The resulting effect of that force application is irrelevant! So, when a rocket burns fuel and exhausts that fuel out the rear of the rocket at high velocity, the force exerted on the fuel to throw it out the back will cause an identical force, but in the opposite direction, to be applied to the rocket. It is this reaction to the force being applied to the fuel that causes the rocket to be propelled in the opposite direction. For every action there is an equal, but opposite, reaction.

---------- FOLLOW-UP ----------

QUESTION: do the rocket then use the theory of relativity E=M*C*C by Einstein? i.e the mass of the fuel then converted to the kinetic energy?

Not at all! This is strictly Newtonian. It has nothing to do with Special Relativity. It is the chemical energy of the fuel (that is the "electrostatic energy") that is being converted into kinetic energy. (Technically, the loss of mass is true but it is irrelevant in this case.)


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James J. Kovalcin


I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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