Questions where additional mass is added to a moving body (or dropped onto it) and the final velocity is found using momentum conservation for variable mass scenarios.
Sand is being dropped on a conveyer belt at the rate of 2 kg per second. The force necessary to keep the belt moving with a constant speed of $3 \mathrm {~ms} ^ { - 1 }$ will be (1) 12 N (2) 6 N (3) zero (4) 18 N