What is buoyant force?

$A$ spherical ball of radius $r$ and relative density $0.5$ is floating in equilibrium in water with half of it immersed in water. The work done in pushing the ball down so that the whole of it is just immersed in water is: (where $\rho$ is the density of water)

$A$ cylindrical block of area of cross-section $A$ and of material of density $\rho$ is placed in a liquid of density one-third of the density of the block. The block compresses a spring,and the compression in the spring is one-third of the length of the block. If the acceleration due to gravity is $g$,the spring constant of the spring is:

$A$ hot air balloon is carrying some passengers and a few sandbags of mass $1 kg$ each,so that its total mass is $480 kg$. Its effective volume providing buoyancy is $V$. The balloon is floating at an equilibrium height of $100 m$. When $N$ number of sandbags are thrown out,the balloon rises to a new equilibrium height of $150 m$ with its volume $V$ remaining unchanged. If the variation of the density of air with height $h$ from the ground is $\rho(h) = \rho_0 e^{-\frac{h}{h_0}}$,where $\rho_0 = 1.25 kg m^{-3}$ and $h_0 = 6000 m$,the value of $N$ is:

The tension in a massless cable connected to an iron ball of $100 \,kg$ when it is submerged in sea water is (Given: $\rho_{\text{iron}} = 8 \times 10^3 \,kg/m^3$, $\rho_{\text{sea water}} = 1000 \,kg/m^3$, $g = 10 \,m/s^2$). (in $\,N$)

$A$ $0.5 \,kg$ block of brass (density $=8 \times 10^3 \,kg \,m^{-3}$) is suspended from a string. What is the tension in the string if the block is completely immersed in water? $(g=10 \,ms^{-2})$