• # question_answer The mass of a hydrogen molecule is $3.32\times {{10}^{-27}}kg$.   If ${{10}^{23}}$ hydrogen molecules strike, per second, a fixed wall of area $\text{2 c}{{\text{m}}^{\text{2}}}$ at an angle of $\text{45 }\!\!{}^\circ\!\!\text{ }$ to the normal, and rebound elastically with a speed of $\text{1}{{\text{0}}^{\text{3}}}\text{ m/s}$, then the pressure on the wall is nearly: [JEE Main Online 08-04-2018] A) $\text{2}\text{.35}\times \text{1}{{\text{0}}^{2}}\,\,N/{{m}^{2}}$ B) $4.70\times {{10}^{2}}\,\,N/{{m}^{2}}$ C) $2.35\times {{10}^{3}}\,\,N/{{m}^{2}}$ D) $4.70\times {{10}^{3}}\,\,N/{{m}^{2}}$

 [c] Force = rate of change of momentum (Perpendicular to area) $=n(2mu\,\,\cos \theta )$ Pressure$=\frac{Force}{Area}=\frac{n(2mu\,\,\cos \,\,\theta )}{A}$ $=\frac{3.32}{\sqrt{2}}\times \frac{{{10}^{-1}}}{{{10}^{-4}}}=\frac{3.32}{1.41}\times {{10}^{3}}=2.35\times {{10}^{3}}N/{{m}^{2}}$