Asphere of radius $R$ and made of material of relative density $\sigma$ has a concentric cavity of radius $r$. It just floats when placed in a tank full of water. The value of the ratio $R/r$ will be
${\left( {\frac{\sigma }{{\sigma - 1}}} \right)^{1/3}}$
${\left( {\frac{{\sigma - 1}}{\sigma }} \right)^{1/3}}$
${\left( {\frac{{\sigma + 1}}{\sigma }} \right)^{1/3}}$
${\left( {\frac{{\sigma - 1}}{{\sigma + 1}}} \right)^{1/3}}$
A stream-lined body falls through air from a height $h$ on the surface of liquid. Let $d$ and $D$ denote the densities of the materials of the body and the liquid respectively. If $D > d$, then the time after which the body will be instantaneously at rest, is
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 liquid is $1000\ kg/m^3$ If the ball comes momeritariiy at rest at $t = 2\ sec$ then initial height of the ball from surface of liquid was ..... $m$ (neglect viscosity)
A metal ball of density $7800\ kg/m^3$ is suspected to have a large number of cavities . It weighs $9.8$ $kg$ when weighed directly on a balance and $1.5$ $kg$ less when immersed in water . The fraction by volume of the cavities in the metal ball is approximately ....... $\%$
Why is it easier to swim in sea water than in river water ?
A stone is projected vertically up from the bottom of a water tank. Assuming no water resistance it will go up and come down in same time but if water drag is present then the time it takes to go up, $t_{up}$ and the time it takes to come down, $t_{down}$ are related as