A 30 cm bottle is filled with water. A cap is put on the bottle and it is turned upside down. There is no air inside the bottle. The area of the cap in contact with the water is 4.0×10⁻⁴ m². The density of water is 1000 kg/m³ and the acceleration due to gravity is 9.8 m/s². Find the force exerted by the water on the cap.

Given,  
Density (ρ)=1000kg/m³
Height (h) =30 cm = 0.3m
g=10 m/s²
Area  (A)=4×10⁻⁴ m²

We know that,  Force (F)=Pressure(P) x Area(A) and;   P=ρgh. 
Substituting the given values in the above formula,
P=ρgh=1000 × 10 × 0.3=3000 Pa. 

We know the pressure is given as:
Pressure = $\frac{\mathrm{Force}}{\mathrm{Area}}$

$\therefore$F=3000 ×4×10⁻⁴=1.2N

Therefore, the force exerted by the water on the cap will be 1.2 N.