10000 small balls, each weighing 1 g, strikes 1 cm² of area per second with a velocity $100 {\mathrm{ms}}^{-1}$ in a normal direction and rebound with the same velocity. The value of pressure on the surface will be

In 1 cm² area, 10⁴ balls are striking per second. Therefore, in 1 m² area, 10⁸ balls will strike per second.
Change in momentum of each ball per second will be 2 mu.

$\therefore$  Pressure $=\frac{F}{A}=N\frac{(\Delta p/\Delta t)}{A}=n\times 2m\times u\left[\because n=\frac{N}{A\Delta t}={10}^8\right]$

                      $= {10}^8\times 2\times {10}^{-3}\times 100$

                       $= 2\times {10}^7 {\mathrm{Nm}}^{-2}$