For an incompressible flow field , $$\overrightarrow {V,} $$ which one of the following conditions must be satisfied?

A spherical balloon with a diameter of $$10m,$$ shown in the figure below is used for advertisements. The balloon is filled with helium $$\left( {{R_{He}} = 2.08\,\,\,\,kJ/kg.K} \right)$$ at ambient conditions of $${15^ \circ }C$$ and $$100\,\,kPa.$$ Assuming no disturbances due to wind , the maximum allowable weight (in Newton) of ballon material and rope required to avoid the fall of the balloon $$({R_{air}} = 0.289kJ/kg.K)$$ is _____________.

Water flows through a $$10$$ $$mm$$ diameter and $$250$$ $$m$$ long smooth pipe at an average velocity of $$0.1$$ $$m/s.$$ The density and the viscosity of water are $$997kg/{m^3}$$ and $$855 \times {10^{ - 6}}$$ $$N.s/{m^2},$$ respectively. Assuming fully-developed flow, the pressure drop (in $$Pa$$) in the pipe is ______________

Consider laminar flow of water over a flat plate of length $$1$$ $$m.$$ If the boundary layer thickness at a distance of $$0.25$$ $$m$$ from the leading edge of the plate is $$8$$ $$mm,$$ the boundary layer thickness (in $$mm$$), at a distance of $$0.75$$ $$m,$$ is __________________