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To me, if an object has a mass which is increasing with time, the force to
maintain its constant vel can be found from F=vdm/dt.
So for the rotor blade, I don't quite see why dm/dt is the rate of mass flow
through the rotor blades. However, in mathematical derivation, F=mdv/dt +
v dm/dt,
v of the object is constant in the second term (vdm/dt).
Leigh, I can see your point in starting from first principle (i'm actually
teaching my students to solve from first principle, i.e considering the rate
of change of momentum (impulse-momentum form)), but I still can't reconcil
with using vdm/dt. All the texts are using this method (find F from mdv/dt)
without starting from first principle. Another classic example is to find
the extra force to maintain constant vel of the conveyer belt if luggage is
dropping on it at a constant rate (but the luggage itself increases vel from
0 to that of the belt).
Leigh, could you help me to resolve this confusion, 'cause some students are
asking me, and I think I can't say the approach of using the formulae of
F=vdm/dt is WRONG, particularly if it still gives the correct answer?