\(F =\frac{ d }{ d t }( M v )\)
\(F=\frac{ dv }{ dt } M + v \frac{ dM }{ dt }\)
\(F= M \frac{ dv }{ dt }+ v (\alpha v )\)
We know that the net force is zero. \(F =0\),
\(M \frac{ d v }{ dt }=- v (\alpha v )\)
\(\frac{d v}{d t}=a=-\frac{\alpha v^{2}}{M}\)