Science Bell Ringer for Grades 9–12: How Chuck Yeager broke the sound barrier below sea-level sound speed
A free science bell ringer for grades 9–12. Put the prompt on the board as students walk in, give them three to five minutes, then go over it together. The answer and teacher notes are in the box below the prompt.
The prompt
On October 14, 1947, Chuck Yeager flew the Bell X-1 faster than sound at about 662 mph, roughly 40,000 feet up. Yet at sea level on a standard day, sound travels about 761 mph. How could he break the sound barrier while flying slower than 761 mph? Use the speed-of-sound formula a = √(γRT), with γ = 1.4 and R = 287 J/(kg·K), to compare sea-level air at 288 K (15 °C) with high-altitude air at about 217 K (−56.5 °C).
Answer and teacher notes
Sea level: √(1.4 × 287 × 288) ≈ 340 m/s ≈ 761 mph (1 m/s ≈ 2.237 mph).
High altitude: √(1.4 × 287 × 216.65) ≈ 295 m/s ≈ 660 mph.
The speed of sound depends on air temperature, not on altitude directly. In the cold air near 40,000 feet, sound is slower, so 662 mph was just over the local speed of sound (662 ÷ 660 ≈ Mach 1.00). Note that speed scales with the square root of temperature: the absolute temperature drops about 25 percent, but the speed of sound drops only about 13 percent. The actual temperature that day may have differed from these standard-atmosphere values.
Sources: Britannica: Chuck Yeager; NASA Glenn Research Center: Speed of Sound
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