Thank you all guys.
And this make me enlightened. Also I think over the formula I mentioned about the relationship between the force(thrust) and the velosity.
Fnet = m(vjfe - va )
The m is the mass of air in a certain time goes into the turbo.That means in a length of time
t，the total mass of the air should be
m*t, which I called it
M. So accroading the Momentum law：
F*t=Mv
－－ F*t=m*t*v
－－ F＝mv
So the formula Fnet = m(vjfe - va ) make sense.
