A First Course in Physics · Ch. XVII, §§ 444–447 · Chapter 17: Nature and Transmission of Sound
51. Where sound comes from and how fast it travels
Goal: Your student can show that sound comes from vibrating things, that it travels through solids, liquids and gases but not a vacuum, and use the speed of sound in problems.
You'll need
- The book, open to these pages
- A ruler or rubber band to twang
- A table to put an ear against
- A stopwatch, a tape measure or a long straight sidewalk, two people and two pot lids or wooden blocks to bang
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Sound comes from vibrating bodies (a string, a tuning fork); sound travels through wood and water; a bell in an exhausted jar fades; the speed of sound measured with cannon in 1738 and 1832 (331.2 m per second at 0° C); speed in water and iron; speed rises about 60 cm per second for each degree. Six problems follow.
The lesson, step by step
Hook question
SayYou see lightning, then hear thunder several seconds later. Why the delay, and what can it tell you?
Read together
Read §§ 444–447. The book's conclusion:
From these familiar facts we conclude that sound arises from the motions of matter.
Try it: feel the vibration
Hold a ruler flat on a table with part of it hanging over the edge and twang it: you can see and hear it vibrate. Touch your throat while humming. Put your ear on one end of a table while someone scratches the other end lightly: louder through wood than through air (§ 445).
Try it: time an echo or a bang
Stand 100 m or more apart in an open place. One person bangs two pot lids together with arms high; the other starts a stopwatch when they see the bang and stops it when they hear it. Divide distance by time. (It is hard to time well; average several tries.) The speed should be roughly 340 m per second (about 1100 ft per second).
Watch only: the bell in a vacuum
The bell-in-a-jar experiment (§ 445) needs an air pump; look up a video of a teacher's "bell in a vacuum" demonstration. Describe the result: as the air is pumped out the ringing fades, so sound needs matter to travel. Light and heat from the sun cross empty space; sound cannot.
Explain the idea
Sound travels about 331 m/s at 0° C and about 60 cm/s faster for every degree (§ 447): about 343 m/s (1126 ft/s) at 20° C. It is faster in water (about 1400 m/s, today measured nearer 1480) and much faster in iron (about 5100 m/s). Count the seconds between lightning and thunder and divide by 3 to get the distance in kilometers (by 5 for miles).
Say it back
Your student explains how the 1738 French commission measured the speed of sound and why they fired cannon from both ends.
Problems
Work problems 1–6 on book pages 345–346. Use 1126 ft/s or 343.2 m/s at 20° C.
Afterwards
Next thunderstorm (from indoors), count the seconds from flash to thunder and work out how far away the strike was.
Answers to the book's problems
1. About 6000 ft. (5.5 × 1087 ft. at 0° C is 5980 ft.; at 20° C, 6190 ft.), a little over a mile. 2. Taking the earth's circumference as about 25,000 miles, about 700 miles an hour (about 1000 ft. per second). 3. About 3490 ft. 4. 2252 ft. (686 m), or any whole multiple of it. 5. About 7.0 seconds (6.39 seconds for the stone to fall, plus 0.58 second for the sound to come up). 6. About 4.4 seconds sooner (4.69 seconds through air, 0.32 second through iron).
Worked out for this site from the scan. If a number in your copy differs, trust the book.