Sand is being dropped on a conveyor belt at the rate of M / kg s. The force necessary to keep the belt moving with a constant velocity of $v\: m/s$ will be :

Question

$\begin {array} {1 1} (1)\;Zero & \quad (2)\;mv\: newton \\ (3)\;2mv\: newton & \quad (4)\;\large\frac{mv}{2}\: newton \end {array}$