In which case does the potential energy decrease?

$A$ sphere of solid material of relative density $9$ has a concentric spherical cavity and floats just submerged in water. If the radius of the sphere is $R$,then the radius of the cavity $(r)$ is related to $R$ as:

$A$ wooden block floating in a bucket of water has $(4/5)$th of its volume submerged. When a certain amount of oil is poured into the bucket,it is found that the block is just under the oil surface,with half of its volume under water and the other half in oil. The density of oil relative to that of water is

Why is it easier to swim in sea water than in river water?

$A$ box when hung from a spring balance shows a reading of $50 \,kg$. If the same box is hung from the same spring balance inside an evacuated chamber,the reading on the scale will be

$A$ small solid ball is dropped from a height above the free surface of a liquid. It strikes the surface of the liquid at $t = 0$. The density of the material of the ball is $500 \ kg/m^3$ and that of the liquid is $1000 \ kg/m^3$. If the ball comes momentarily to rest at $t = 2 \ s$,then the initial height of the ball from the surface of the liquid was ..... $m$ (neglect viscosity).