A glass plante of negligible mass and thickness is held against the end of a tube and pushed 10 cm under the surface of water. When released, the plate does not fall off. What depth of kerosene oil of relative density 0.8 must be poured into the tube so that the plate just falls off?

Explanation

Let `A cm^(2)` be the area of the base of the plate and h cm be height of kerosene column at which the plate just falls off. Then downward thrust due to h cm of kerosene = upward thrust due to 10 cm of water column.
i.e., ` h xx 0.8 xx g xx A = 10 xx 1 xx g xx A`
or ` h=10/0.8 = 12.5 cm`.