∴ The mass of water M water = ρ water V water = 1.× 13kg/m3× 5×-6m3 = .5kg,
The pressure of the bottle on the sponge:
F=G water +G bottle.
(2) To make the pressure on the sponge equal when the bottle is inverted,
a piece of cardboard with an area of .2m2 can be placed under the bottle when it is inverted;
a: (1) the pressure on the sponge is 3 pa when the pure water is placed upright.
(2) pieces of cardboard with an area of .2m2 are placed under the bottle when it is inverted.