Given, area of the square base is 1.0 m 2 , area of the small hinged door is 20 cm 2 , relative density of the acid is 1.7 , and the tank is filled with water and acid up to a height of 4.0 m .
When the liquids is filled into the tank then the pressure difference across the door is,
P d =( ρ 1 gh− ρ 2 gh )
Here, ρ 1 is density of the acid ( ρ 1 =1.7× 10 3 kgm -3 ) and ρ 2 is the density of the water.
Substitute the values in the above equation.
P d =( 1700×4×9.8−1000×4×9.8 ) =2.744× 10 4 Pa
Now the force exerted by the acid and liquid is,
Force exerted by the acid and the liquid=pressure difference×area of the door F d = P d × A d =2.744× 10 4 ×20× 10 −4 ≈55 N
Hence, the force necessary to keep the door close is 55 N .