Beats (periodic amplitude variation from interference of close frequencies) are used to tune instruments: adjust until beats disappear (frequencies match). Also, in Kundt's tube or other methods, beat frequency helps measure sound speed by comparing known and unknown frequencies. Memory tip: 'Beats: tuning (zero beat = matched frequency); speed measurement (beat frequency = |f₁-f₂|)'. This application question tests wave phenomena uses, frequently appearing in competitive exams. Always link physical phenomena to practical applications; competitive exams emphasize real-world relevance of physics concepts.
Explanation:
Elastic PE: U = ½kx² ⇒ k = 2U/x². x = 3 cm = 0.03 m. k = 2×18/(0.03)² = 36/0.0009 = 40000 N/m. Unit conversion critical: cm to m. Memory aid: 'k = 2U/x²; always use SI units'. Tests spring energy formula application with unit conversion, frequently appearing in competitive exams.
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