When the sand is dropped on the conveyor belt its velocity will become equal to that of the conveyor. Therefore, v=5ms−1.
The force required to keep the conveyor belt moving will be equal to the gain in the momentum by the sand per second.
⇒F=dtdmv⇒F=0.5×5=2.5N
Now the power required to keep the conveyor belt moving will be,
P=Fv⇒P=2.5×5=12.5W.