Question

can anyone tell me from where m/2 came?​

Answer 1

It is simple brother:

$m \frac{dv}{dt}v = 0$

multiplying and dividing by 2,

$m \times ( \frac{2}{2} ) \times (\frac{dv}{dt} v) = 0 \\ \\ = > \frac{m}{2} (2 \times \frac{dv}{dt} v ) = 0 \\ \\ = > \fbox{\frac{m}{2} ( \frac{dv}{dt}v \: + \: v \frac{dv}{dt} ) = 0}$

This is how it happened!

Hope it helped you!

☺☺