To solve this problem it is necessary to apply the concepts related to the Flow Rate. It is understood as the volumetric amount transported during a certain time and can be calculated as

Where
A = Area
V = Velocity
Our values are given as


Replacing and rearranging to find the velocity



Therefore the speed of the water at the end of the Nozzle is 4.59m/s
Formula:
F = ma
m = F÷A
m: mass F:force A: accelerarion
m = 450N ÷ 15m/s^2
= 30kg
Hope this helps :)
In energy point of view, the larger stone had more potential energy before dropping. impacting the water, the larger one, having more kinetic energy which changed from potential energy, tranfered energy to the water and formed wave. the amplitude of the wave indicate the energy of the wave. more energy more amplitude.