Answer
$F=3mN$
Work Step by Step
As the rate of change of mass is given as:
$\frac{dm}{dt}=\frac{12}{2}=6\frac{g}{s}$
Now we can find the force:
$F=\frac{dp}{dt}$
$F=\frac{d}{dt}(mv)$
$\implies F=v\frac{dm}{dt}$
We plug in the known values to obtain:
$F=50\times 6 \ dyne$
$F=3mN$
