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1. 1.4

2. If the density of air at sea level is 1.225 kg/m$^3$, what is the average spacing of the molecules? Molecular mass of air is 28. For what size of bodies would you expect major problems in trying to use a continuum approximation? If the free path length is 6.6 $10^{-8}$ m, at what size would you expect that normal equations of motion (like the Euler and Navier-Stokes equations) become unusuable? When the body size becomes comparable to the mean molecular spacing or to the mean free path length?

3. Repeat for 200 km height, where the number of molecules is 8 $10^{15}$/m$^3$ and the free path length 200 m.

4. Derive the net pressure force per unit volume in the $y$-direction on a little cube of fluid.