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Hydrogen molecules, each of mass 3.32 * 10⁻²⁷ kg, strike a fixed wall of area 2 cm² at a rate of 10²³ molecules per second. They hit the wall at 45 deg to the normal with speed 10³ m/s and bounce back elastically with the same speed. The pressure exerted on the wall is approximately:

1. 4.70 * 10³ N/m²
2. 2.35 * 10² N/m²
3. 4.70 * 10² N/m²
4. 2.35 * 10³ N/m²

Correct answer: 2.35 * 10³ N/m²

Solution

Each elastic bounce reverses the normal velocity component, so the momentum transferred per molecule is 2*m*v*cos45. Multiplying by the strike rate and dividing by area gives the pressure.

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