I think we will use the law of conservation of linear momentum; M1V1 = M2V2 M1 = 4 kg (mass of the water balloon launcher) V1=? M2= 0.5 kg ( mass of the balloon) V2 = 3 m/s
if the elevator is moving upward with the constant speed the spring scale will read 18 N which is the mass of each of the two blocks attached by separate springs to the scale at opposite ends.
