WAEC 2021 Physics · Question 34

A rotating disc contains a set of holes in a circle. An air jet is directed onto the holes and a note of…

Question

A rotating disc contains a set of holes in a circle. An air jet is directed onto the holes and a note of frequency 480 Hz is produced. If the number of holes is 20, calculate the speed of rotation of the disc.

A.10 rev/s
B.20 rev/s
C.24 rev/s
D.8 rev/s.

Answer: C. 24 rev/s

Explanation written with AI and checked against the answer key

Analyze the question: This is a siren. As the disc rotates, each hole lets a puff of air through. The frequency of the note is the number of puffs per second. Concept: Frequency of note = (number of holes) × (rotations per second) f = n × N where f = 480 Hz, n = 20 holes, and N = rotation speed in rev/s. Solution: N = f / n N = 480 / 20 N = 24 rev/s Tip: For siren questions, remember that each hole gives one pulse per revolution, so frequency = holes × revolutions per second. If the question asks for a different quantity, just rearrange the formula. Always check that your units are rev/s and not rev/min.

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