The correct answer is 1) F=Rmr(uout-uin).
The force is equal to the time rate of change of momentum. The impulse is provided by the difference between a mass flow rate of rocks (Rmr) which are taken in with a velocity with respect to the boat of uin, and a mass flow rate of rocks (Rmr) which are ejected with a velocity with respect to the boat of uout.
Class response (2004):
Class response (2003):
Class response (2001):
