1 Why do flying bees buzz? 1. They have special wings that make sounds. 2. The buzz comes from their heads. They make a buzzing noise to communicate with each other. 3. They move their wings at audible frequencies. 2 When a sound wave moves past a point in air, what happens to the density of air at this point? 1. The air is compressed after the sound wave passes. 2. There is no change in the density of air. 3. There is no air after the sound wave passes. 4. The density of air increases and then decreases as the sound wave passes. 3 Middle C has a speed of 1500 m/s in water and 340 m/s in air. Does it have a longer or shorter wavelength in water than in air and why? 1. Shorter; water is denser than air. 2. Longer; water is denser than air. 3. Longer; the speed of sound is greater in water than in air. 4. Shorter; the speed of sound is greater in water than in air. 4 If the frequency of sound is doubled, how will its speed change? How will its wavelength change? 1. Both speed and wavelength will double. 2. Its speed will not change, and its wavelength will halve. 3. Its speed will halve, and its wavelength will double. 4. Its speed will double, and its wavelength will not change. 5. There will be no change in its speed and wavelength. 5 Why does sound travel faster in warm air? 1. In warm air the wavelength of sound is shorter. 2. In warm air the air molecules travel faster. 3. In warm air the frequency of sound is higher. Quest Chapter 26 Flying bees buzz. What could they be doing that generates sound? What type of wave is sound? What is the displacement that occurs in this type of wave? Middle C is a certain frequency. If we fix the frequency and change the speed of the wave, how does λ change? From Chapter 25, what do we know about the speed of a specific type of wave in a specific medium? Double the frequency and what is the change, if any, in the wavelength? What has to happen for sound to propagate? (check notes) What does warming the air do?
6 Why can the tremor of the ground from a distant explosion be felt before the sound of the explosion can be heard? 1. The wavelength of the sound is smaller in the ground than in air. 2. The amplitude of the sound is bigger in the solid ground than in air. 3. The frequency of the sound is higher in the solid ground than in air. 4. Sound travels faster in solid ground than in air. 7 What is the wavelength of a 23500 Hz tone in air? The speed of sound in air is 340 m/s. 8 Anna was on vacation and came across an echo lake. Wanting to know how far she had to swim to get across the lake to the other side, she yelled across Hello! How wide is the lake if 10 seconds later she heard her own echo? Assume that the temperature is 20 C and the speed of sound is 344 m/s. 9 (part 1 of 2) The range of human hearing extends from approximately 22 Hz to 20200 Hz. Find the wavelength for 22 Hz when the speed of sound in air is 340 m/s. 10 (part 2 of 2) 10.0 points Find the wavelength for 20200 Hz when the speed of sound in air is equal to 340 m/s. 11 How much more intense is sound at 60 db than at 0 db? 12 Why does a sounding board on a musical instrument produce louder sound? 1. A sounding board with large surface has greater intertia, so the pitch of the produced sound will decrease. 2. A sounding board with large surface is able to set more air vibrating, so the amplitude of the produced sound will increase. 3. A sounding board with large surface vibrates with higher frequency, so the amplitude of the produced sound will increase. 4. A sounding board with large surface has greater intertia, so the pitch of the produced sound will increase. Feel the wave in the ground BEFORE hearing the wave in the air hmm I wonder what could be the reason What equations relates v, λ, and f? Hmmm You have speed and time and are asked for distance hmmm How would you solve this? Remember: The time is for the round trip. Here comes that wave equation again and again How is the energy in sound related to db? Check section 26.6 in the text.
13 If the handle of a vibrating tuning fork is held solidly against a table, the sound from the tuning fork becomes louder. Why, and how will this affect the length of time the fork keeps vibrating? 1. Because a more massive surface is set into more lower frequency vibration, the sound becomes louder. By conservation of energy, this reduces the length of time the fork keeps vibrating. 2. Because a more massive surface is set into vibration, the sound becomes louder and the length of time the fork keeps vibrating becomes longer. 3. Because a greater surface is set into vibration, the sound becomes louder. By conservation of energy, this reduces the length of time the fork keeps vibrating. 4. Because a greater surface is set into vibration, the sound becomes louder. By conservation of momentum, this reduces the length of time the fork keeps vibrating. 14 Why do soldiers break step in marching over a bridge? 1. They do that to avoid making a lot of noise. 2. Usually a bridge is too narrow for the soldiers to march. 3. If they do not break step in marching over a bridge, they are likely to expose themselves to their enemy. 4. There is no special reason. 5. The regular step could set the bridge into a resonance which could destroy the bridge. 15 The sitar, an Indian musical instrument, has a set of strings that vibrate and produce music, even though they are never plucked by the player. These sympathetic strings are identical to the plucked strings and are mounted below them. How does this work? 1. Scientists are still studying to find the reason. 2. The lower strings are resonating with the upper. 3. The lower strings are plucked by a ghost. 4. The upper strings are connected to the lower strings with invisible strings. Check section 26.6 in the text. Check section 26.8 in the text. Check section 26.8 in the text.
16 A special device can transmit sound out of phase from a noisy jackhammer to its operator using earphones. Over the noise of the jackhammer, the operator can easily hear your voice while you are unable to hear his. Why? 1. These devices reduce jackhammer noise by using destructive interference to cancel the noisy sound, so he can hear your voice clearly. 2. The operator s earphonex amplifies your voice, so he can hear your voice clearly. 3. The operator s earphones are connected to your microphone, so he can hear your voice clearly. 4. The operator s earphones change the frequencies of your voice, so he can hear your voice clearly. 17 Two sound waves of the same frequency can interfere, but to create beats, two sound waves have to have different frequencies. Why? 1. To alternate between constructive and destructive interference requires different frequencies. 2. Two waves of different frequencies interfere constructively, independent of their phase difference. 3. Waves of the same frequency interfere constructively, independent of their relative phase. 4. Two waves of different frequencies interfere destructively, independent of their phase difference. 5. Waves of the same frequency interfere destructively, independent of their relative phase. 18 When two tuning forks of 126 Hz and 153 Hz, respectively, are sounded simultaneously, how many beats per second are heard? What must happen to reduce or eliminate the effect of the jackhammer sound on the operator? How can a sound wave s amplitude be reduced? Check section 26.10 in the text. Remember: f beat = Δf
19 What beat frequencies are possible with tuning forks of frequencies 246, 249, and 251 Hz? 1. 3 possible: 246 Hz, 249 Hz, and 251 Hz. 2. 3 possible: 492 Hz, 498 Hz, and 502 Hz. 3. 4 possible: 2 Hz, 3 Hz, 5 Hz, and 7 Hz. 4. 3 possible: 2 Hz, 3 Hz, and 5 Hz. 5. 2 possible: 2 Hz and 3 Hz. 20 When a tuning fork of frequency 263 Hz vibrates beside a piano string, beats are heard. The string is tightened slightly and the beats go away. What was the original frequency of the string? 1. less than 263 Hz 2. 263 Hz 3. greater than 263 Hz 21 Potential energy and kinetic energy are forms of what kind of energy? 1. electromagnetic 2. nuclear 3. heat 4. mechanical 5. chemical 22 (part 1 of 2) A football is thrown hard horizontally and it hits and sticks in a cart that is on a track at position A in the diagram. The cart with the football in it then moves along the hilly frictionless track to position B, where the cart stops. What is conserved in the collision of the football with the cart at position A? 1. mechanical energy and momentum 2. more information is needed to answer 3. neither mechanical energy nor momentum 4. momentum only 5. mechanical energy only 23 (part 2 of 2) What is conserved as the cart with the football in it moves from position A to position B? 1. mechanical energy only 2. more information is needed to answer 3. momentum only 4. neither mechanical energy nor momentum 5. mechanical energy and momentum Remember: f beat = Δf So, calculate the three differences and find the matching beat frequencies in the answers. Tightening the string does what to the frequency (pitch)? So, the original pitch of the piano string was higher than, the same as, or lower than the fork? Review question. Look it up in the book or in your notes. Review problem. Note the energy mentioned in the answers is limited to mechanical energy. What has changed in the motion of cart at B compared to A? What is different: Momentum? Mechanical Energy? What has not changed?