Sand is dropped on a conveyor belt at rate of M kg/s Force necessary to keep the belt moving with a constant velocity v m/s will be

$(a)\;zero \quad (b)\;Mv\;N \quad (c)\;2\;Mv\;N \quad (d)\;\frac{Mv}{2}\;N$

Force $=\large\frac{d}{dt}$$(mv)$
$\qquad=v \large\frac{dm}{dt}$ as v is constant
Therefore $F=Mv$ N
Hence b is the correct answer.
edited May 28, 2014