My teacher told me that conservation of momentum is always applicable to a rocket because the system, which consists of the rocket and the exhaust from the rocket, is isolated. But how is momentum conserved when there is drag force (air resistance), gravitational force and thrust force acting on the rocket, causing it to accelerate? Wouldn't that mean the net force acting on the rocket is not equal to zero, and that in turn would mean that momentum is not conserved?
P.S. I asked my teacher the same question and told me that drag force and gravitational force doesn't affect momentum. If that's true, why is that? But first of all, how is that true?
Best Answer
As this is found by many students to be a difficult conceptually, the derivation of the rocket equation is often done without the complication of there external forces (gravitational attraction and drag) present.
So the rate of change of momentum of the system which consists of the rocket, fuel and exhaust gasses is put equal to zero (sum of external forces acting).
If there are external forces acting then the rate of change of momentum of the rocket etc system is no longer zero and now equal to the sum of the external forces acting on the system.