Question

a boat full of steel balls is made to float on water in a tank.one by one the balls are thrown in the water tank how will it affect the water level

Answer 1

here the water level goes down.
this is something unusual to hear but its true becuse the density of the balls is more than that of water so they sink, and due to this fact, the amount of water displaced after sinking is more than that they were displacing when they were floating, so water levels falls.

this can be remembered as a rule that
whenever a solid body is droped from a boat into water;
(i) water level falls if density of solid is more than that of liquid and body sinks.
(ii) water level remains same if density of solid is equal to or less than that of water and the body floats.