Why are railway tracks laid on large-sized wooden sleepers?

In a $U$-tube as shown in the figure,water and oil are in the left and right sides of the tube,respectively. The heights from the bottom for the water and oil columns are $15 \; cm$ and $20 \; cm$,respectively. The density of the oil is ...... $kg/m^3$. [Take $\rho_{\text{water}} = 1000 \; kg/m^3$]

$A$ square gate of size $1\,m \times 1\,m$ is hinged at its mid-point. $A$ fluid of density $\rho$ fills the space to the left of the gate. The force $F$ required to hold the gate stationary is

If two liquids of same masses but densities $\rho_1$ and $\rho_2$ respectively are mixed,then the density of the mixture is given by:

In making an alloy,a substance of specific gravity $s_1$ and mass $m_1$ is mixed with another substance of specific gravity $s_2$ and mass $m_2$. Then,the specific gravity of the alloy is:

The pressure at half the depth of a lake is equal to two-third pressure at the bottom of the lake. So the depth '$h$' of the lake is ($\rho$ = density of water in the lake, $g$ = acceleration due to gravity, $P_0$ = atmospheric pressure).