$A$ liquid is flowing in a horizontal pipe of non-uniform cross-section. Which of the following quantities may remain unchanged with respect to time?

Why does an airplane run a certain length on the runway first before taking off?

$A$ liquid of density $750\,kg\,m^{-3}$ flows smoothly through a horizontal pipe that tapers in cross-sectional area from $A_{1} = 1.2 \times 10^{-2}\,m^{2}$ to $A_{2} = \frac{A_{1}}{2}$. The pressure difference between the wide and narrow sections of the pipe is $4500\,Pa$. The rate of flow of liquid is . . . . . . $\times 10^{-3}\,m^{3}\,s^{-1}$.

The figure represents vertical sections of four wings moving horizontally in air. In which case is the force upwards?

$A$ fully loaded Boeing aircraft has a mass of $5.4 \times 10^5 \, kg$. Its total wing area is $500 \, m^2$. It is in level flight with a speed of $1080 \, km/h$. If the density of air $\rho$ is $1.2 \, kg/m^3$,the fractional increase in the speed of the air on the upper surface of the wing relative to the lower surface in percentage will be $(g = 10 \, m/s^2)$.

$A$ wind with speed $50 \,m/s$ blows parallel to the roof of a house. The area of the roof is $300 \,m^2$. Assume that the pressure inside the house is atmospheric pressure. The density of air is $1.2 \,kg/m^3$. The magnitude of the force exerted by the wind on the roof will be: