21. Bob stands at a distance 15 (m) away from a point sound source with an average power output of 250 (W). The point sound source moves toward the observer with constant speed u = 120 (km/hr). Suppose that the speed of sound is v = 343 (m/s). Calculate the wave intensity observed by Bob. (A) 0.057 W/m² (B) 0.092 W/m² (C) 0.080 W/m² (D) 0.032 W/m²

10. The number of collisions a molecule makes per second is called collision frequency. At one atmosphere and 0 °C, estimate the collision frequency of air molecules. [The diameter of an air molecule under one atmosphere and 0 °C is approximately 3×10⁻¹⁰ (m), the average molecular speed is approximately 460 (m/s), and the molecular number density is approximately 2.7×10²⁵ molecules/m³]. (A) 3.5×10⁵ (collisions/s) (B) 3.5×10⁷ (collisions/s) (C) 3.5×10⁹ (collisions/s) (D) 3.5×10¹¹ (collisions/s)

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