Frequently I see the expression F=dpdt=ddt(mv)=dmdtv+ma,
But I'm wondering if this derivation is correct, because the second law of Newton is formulated for point masses.
Furthermore if i change the inertial frame of reference, only v on the right side of the formula F=dmdtv+ma will change, meaning that F would be dependent of the frame of reference, which (according to me) can't be true.
I realize there exists a formula for varying mass systems, that looks quite familiar to this one, but isn't exactly the same, because the v on the right side is there the relative velocity of mass expulsed/accreted. The derivation of that formula is also rather different from this one.
So my question is: is this formula, that I frequently encounter in syllabi and books, correct? Where lies my mistake.
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