Row v = v = 3x10-3m3,

Buoyancy of buoy p:

F float = ρ water gv row =1.0×103kg/m3×10n/kg× 3×10-3m3 = 30n;

(2)∫f floating =gp+f pulling force

∴f pulling force =f floating-gp = 30n-0.4kg×10n/kg = 26n;

(3) The cover Q has just been pulled open,

∴g cover +f pressure =f tension =26n,

F pressure =f tension -g cover =26n-0.2kg× 10n/kg=24n,

F pressure = PS = rhoghs,

∴h=

F pressure

ρgs

=

24n

1000kg/m3× 10n/kg×8× 10？ 3 square meters

=0.3m。

Answer: (1) The buoyancy on the buoy is 30n；;

(2) The pulling force of the thin line on the buoy P is 26n；;

(3) The water depth in the water tank is 0.3m. 。