Standard rocket propulsion problem. We are supposed to derive an expression for the final velocity of a rocket being launched into space based upon the exhuast velocity (Ve), final mass (Mf), intial mass (Mo), g, and time. The only forces acting on the rocket throughout the motion is the force of thrust and gravity, with the force of gravity being constant.(adsbygoogle = window.adsbygoogle || []).push({});

Now this is what my teacher has taught me, and this is the explanation that I have recieved on many websites. The derivation for the constant force of thrust is:

Fthrust = Ve(dm/dt)

This I understand. But then apparently when doing a summation of forces on the rocket it should be as follows:

[tex]F_{thrust}[/tex] - [tex]F_{g}[/tex] = ma

What I do not understand is why the summation of the forces can be set equal to ma. Obviously the velocity is increasing, so the rocket is experiencing an acceleration, but isn't the mass of the rocket itself changing? In this case the summation of forces should be:

[tex]F_{thrust}[/tex] - [tex]F_{g}[/tex] = d(mv)/dt

My physics teacher understands the point I am trying to make, and says he does not know for sure why the changing mass is disregarded. I have the feeling that my challenge is wrong, but I am not sure why. Can anyone help me with this problem?

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# Questioning logic used for highschool rocket propulsion problem

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