P = rho water GH =1.0×103kg/m3× 9.8n/kg× 0.06m = 588pa.
(2) The change of water depth after putting pebbles in the cup:
△h=5.5cm-4cm= 1.5cm，
The volume of small stones:
V stone = s △ h = 20cm2×1.5cm = 30cm 3 = 3x10-5m3.
(3)∵ The pebble floats before and after being put into the cup.
Gravity of small stones:
G stone =△F floating = ρ water g△V row =1.0×103kg/m3× 9.8n/kg× (0.09m-0.06m )× 2×10-3m2 = 0.588n,
Density of small stones:
ρ stone =m stone v stone =G stone v stone g=0.588N3× 10? 5m3×9.8N/kg=2× 103kg/m3。
Answer: (1) In Figure A, the pressure of water in the bucket at the bottom of the cup is 588Pa；;
(2) The volume of pebbles is 3×10-5m3;
(3) The density of pebbles is 2× 103kg/m3. /m3。