When a jet hits a stationary flat plate perpendicularly, the water's momentum changes. The force on the plate equals the rate of change of momentum of the water jet. Since the water splashes out parallel to the plate, its final velocity perpendicular to the plate is zero. The initial velocity perpendicular to the plate is the jet velocity, $u$ = 10 m/s. The mass flow rate is $\frac{dm}{dt}$ = density × area × velocity = 1000 kg/m³ × (2 × 10^-4 m²) × 10 m/s = 2 kg/s. The force is $F=\frac{dm}{dt}\times u$ = 2 kg/s × 10 m/s = 20 N.

Final Answer: 20 N