Physics II Questions (Test #1
8. (11-MC) A grandfather clock is "losing" time because its pendulum moves too slowly. Assume that the pendulum is a massive bob at the end of a string. The motion of this pendulum can be sped up by (list all that work): (a) shortening the string (b) lengthening the string (c) increasing the mass of the bob (d) decreasing the mass of the bob
(a) Period must be decreased. Period is proportional to the square root of the length, so shortening the string will decrease the period. The period does not depend upon the mass of the bob, so changing the mass will not affect the period.
1. (12-MC) Do you expect an echo to return to you more quickly on a hot day or a cold day? (a) hot day (b) cold day (c) same on both days
(a) Speed of sound is not constant, but depends upon the temp. of the air. Hotter equals speed of sound greater, so less time for echo to return.
5. (11-MC) When you use the approximation sin θ ≈ θ for a pendulum, you must specify the angle θ in (a) radians only (b) degrees only (c) revolutions or radians (d) degrees or radians
(a) The small angle approximation is valid only in units of radians because the angle in radians is equal to the ratio of the arc length to the radius.
10. An organ pipe with a fundamental frequency f is open at both ends. If one end is closed off, the fundamental frequency will (a) drop by half. (b) not change. (c) double.
(a) Use equation.
10. (11-MC) Two waves are traveling toward each other along a rope. When they meet, the waves (a) pass through each other (b) bounce off of each other (c) disappear
(a) Waves are not objects that can collide. They obey the superposition principle such that at any point on the rope the total displacement is the sum of the displacements from each wave. The waves pass through each other unaffected.
15. (11-MC) A wave transports (a) energy but not matter (b) matter but not energy (c) both energy and matter
(a) Waves do not transport matter as well as energy. As shown by a transverse wave on a horizontal string, the wave transports the disturbance down the string, but each part of the string stays at its initial horizontal position.
13. (11-Q) What kind of waves do you think will travel along a horizontal metal rod if you strike its end (a) vertically from above and (b) horizontally parallel to its length?
(a) perpendicular displacement and will set up transverse waves (b) initial displacement parallel to rod and will set up primarily longitudinal waves
1. (22-MC) In a vacuum, what is the difference between a radio wave and an X-ray? (a) wavelength (b) frequency (c) speed
(a,b) All electromagnetic waves have the same velocity in a vacuum. The velocity is the product of the wavelength and frequency. Since X-rays and radio waves have different wavelengths but the same speed, they will also have different frequencies.
3. (22-MC) Which of the following travel at the same speed as light? (Choose all that apply.) (a) radio waves (b) microwaves (c) radar (d) ultrasonic waves (e) infrared radiation (f) cell phone signals (g) gamma rays (h) X-rays
(a,b,c,e,f,g,h) All electromagnetic waves travel at the speed of light. The only listed wave not an EM wave is (d) ultrasonic wave, which is a sound wave and would travel at the speed of sound.
2. (11-MC) An object oscillates back and forth on the end of a spring. Which of the following statements are true at some time during the course of the motion? (a) The object can have zero velocity and, simultaneously, nonzero acceleration. (b) The object can have zero velocity and, simultaneously, zero acceleration. (c) The object can have zero acceleration and, simultaneously, nonzero velocity. (d) The object can have nonzero velocity and nonzero acceleration simultaneously.
(a,c,d) At the turning points int the oscillation (x = ± A), the velocity is zero and the acceleration is a maximum, so (a) is true. At the center of the oscillation (x = 0), the acceleration is zero and the velocity is a maximum, so (c) is true. Since the velocity is only zero at the turning points where the acceleration is a max., there is no point where both the velocity and acceleration are zero, so (b) is not true. At all other points besides x = ± A and x = 0, both the acceleration and velocity are nonzero values, so (d) is also true.
4. (11-MC) An object of mass m rests on frictionless surface and is attached to a horizontal ideal spring with spring constant k. The system oscillates with amplitude A. The oscillation frequency of this system can be increased by (a) decreasing k (b) decreasing m (c) increasing A (d) More than one of the above (e) None of the above
(b) Amplitude does not affect the frequency. Frequency can be increased by increasing k or decreasing m.
3. (12-MC) The sound level near a noise air conditioner is 70 dB. If two such units operate side by side, the sound levels near them would be (a) 70 dB (b) 73 dB (c) 105 dB (d) 140 dB
(b) Sound intensity level is not a linear scale but instead a logarithmic scale. If the sound intensity doubles, the intensity level increases by about 3 dB, so the correct answer in 73 dB.
5. (12-MC) A musical note that is two octaves higher than a second note (a) has twice the frequency of the second note. (b) has four times the frequency of the second note. (c) has twice the amplitude of the second note. (d) is 3dB louder than the second note. (e) none of the above.
(b) The octave is a measure of musical frequency, not loudness. Raising a note by one octave requires doubling the frequency. Therefore, raising a note by two octaves is doubling the frequency twice, which is the same as quadrupling the frequency.
12. (12-MC) You are driving at 75 km/h. Your sister follows in the car behind at 75 km/h. When you honk your horn, your sister hears a frequency (a) higher than the frequency you hear. (b) lower than the frequency you hear. (c) the same as the frequency you hear. (d) You cannot tell without knowing the horn's frequency.
(c) No relative motion. Distance does not change.
7. (11-MC) At a playground, two young children are on identical swings. One appears to be about twice as heavy as the other. If you pull them back together the same distance and release them to start them swinging, what will you notice about the oscillations of the two children? (a) The heavier child swings with a period twice tat of the lighter one. (b) The lighter child swings with a period twice that of the heavier one. (c) Both children swing with the same period.
(c) Period of a simple pendulum is determined by the length of the swing cords and the acceleration of gravity. It does not depend upon the weight of the chilf. Since the swings are identical, they should oscillate with the same period.
3. (11-MC) An object of mass M oscillates on the end of a spring. To double the period, replace the object with one of mass: (a) 2M (b) M/2 (c) 4M (d) M/4 (e) none of the above
(c) The period is proportional to the square root of the mass. Therefore, the mass must be quadrupled (to 4M) for the period to double.
14. (11-MC) A student attaches one end of a Slinky to the top of a table. She holds the other end in her hand, stretches it to a length L, and then moves it back and forth to send a wave down the Slinky. If she next moves her hand faster while keeping the length of the Slinky the same, how does the wavelength down the Slinky change? (a) It increases. (b) It stays the same. (c) It decreases.
(c) The speed of the wave along the Slinky depends upon the mass of the Slinky and the tension caused by stretching it. Since this has not changed, the wave speed remains constant. The wave speed can also be written as the product of the wavelength and frequency. Therefore, as the frequency is increased, the wavelength must decrease.
2. (22-MC) The radius of an atom is on the order of 10⁻¹⁰ m. In comparison, the wavelength of visible light is (a) much smaller. (b) about the same size. (c) much larger.
(c) Visible light has a wavelength on the order of 10⁻⁷ to 10⁻⁸ m, which is over a thousand times larger than the size of an atom.
9. (12-MC) A guitar player shortens the length of a guitar's vibrating string by pressing the string straight down onto a fret. The guitar then emits a higher-pitched note because (a) the string's tension has been dramatically increased. (b) the string can vibrate with a much larger amplitude. (c) the string vibrates at a higher frequency.
(c) When the string is pushed down, its effective length is shortened, which shortens the wavelength and thus increases the oscillation frequency. (The wave speed doesn't change due to fretting the string.)
11. (12-MC) Two loudspeakers are about 10 m apart in the front of a large classroom. If either speaker plays a pure tone at a single frequency of 400 Hz, the loudness seems pretty even as you wander around the room and gradually decreases in volume as you move farther from the speaker. If both speakers then play the same tone together, what do you hear as you wander around the room? (a) The pitch of the sound increases to 800 Hz, and the sound is louder but not twice as loud. It is louder closer to the speakers and gradually decreases as you move to away from the speakers - except near the back wall, where a slight echo makes the sound louder. (b) The sound is louder but maintains the same relative spatial pattern of gradually decreasing volume as you move away from the speakers. (c) As you move around the room, some areas seem to be dead spots with very little sound, whereas other spots seem to be louder than with only one speaker. (d) The sound is twice as loud - so loud that you cannot hear any difference as you move around the room. (e) At points equidistant from both speakers, the sound is twice as loud. In the rest of the room, the sound is the same as if a single speaker were playing.
(c) because of destructive interference and constructive interference
2. (12-MQ) Sound waves are (a) transverse waves characterized by the displacement of air molecules. (b) longitudinal waves characterized by the displacement of air molecules. (c) longitudinal waves characterized by pressure differences. (d) both (b) and (c). (e) (a), (b), and (c).
(d) Sound waves are longitudinal waves. They can be characterized either by longitudinal displacement of the air molecules or by the pressure differences that cause the displacement.
12. (11-MC) Consider a wave on a string moving to the right. what is the direction of the velocity of a particle of string at point B (v=0)? (a) right (b) down and right (c) down (d) up (e) v = 0, so no direction
(d) The point on the string does not move horizontally so answers (a) and (b) cannot be correct. The string has zero velocity only at the turning points (top and bottom) so (e) cannot be correct. Examining the graph shows that as the wave moves to the right the crest is approaching point B, so the string at B is traveling upward at this instant.
7. (22-MC) If the Earth-Sun distance were doubled, the intensity of radiation from the Sun that reaches the Earth's surface would (a) quadruple (b) double (c) drop to 1/2 (d) drop to 1/4
(d) The radiation intensity decreases as the square of the distance. Doubling the distance will decrease the radiation intensity to 1/4 the initial intensity.
11. (11-MC) Which of the following increases the speed of the waves in a stretched elastic cord? (More than one answer may apply.) (a) Increasing the wave amplitude (b) Increasing the wave frequency (c) Increasing the wavelength (d) Stretching the elastic cord further
(d) The wave speed on a cord is related to the tension in the cord and the mass per unit length of the cord. The wave speed does not depend upon the amplitude, frequency, or wavelength. Stretching the cord increases the tension and decreases the mass per unit length, both of which increase the speed of the wave on the cord.
13. (11-MC) What happens when two waves, such as waves on a lake, come from different directions are run into each other? (a) They cancel each other out and disappear. (b) If they are the same size, they cancel each other out and disappear. If one wave is larger than the other, the smaller one disappears and the larger one shrinks but continues. (c) They get larger where they run into each other; then continue in a direction between the direction between the direction of the two original waves and larger than either original wave. (d) They may have various patterns where they overlap, but each wave continues with its original pattern away from the region of overlap. (e) Waves cannot run into each other; they always come from the same direction and so are parallel.
(d) The waves obey the superposition principle such that at any point in the lake the amplitude of the wave is the sum of the individual amplitudes of each wave. This produces the various patterns when they overlap. The waves, however, will pass through each other and continue in the same pattern after they pass.
4. (22-MC) Which of the following electromagnetic radiation travels the fastest? (a) radio waves (b) visible light waves (c) X-rays (d) gamma rays (e) All of the above travel at the same speed.
(e) All EM waves travel at the speed of light.
6. (11-MC) Suppose you pull a simple pendulum to one side by an angle of 5º, let go, and measure the period of oscillation that ensues. Then you stop the oscillation, pull the pendulum to an angle of 10º, and let go. The resulting oscillation will have a period about _____ the period of the first oscillation. (a) four times (b) twice (c) half (d) one-fourth (e) the same as
(e) Amplitude does not affect the period of oscillations. Period of a small-amplitude pendulum is determined by the length of the string and the acceleration of gravity, not the amplitude. Both oscillations will then have the same period.
9. (11-MC) Consider a wave traveling down a cord and the transverse motion of a small piece of the cord. Which of the following is true? (a) The speed of the wave must be the same as the speed of a small piece of the cord. (b) The frequency of the wave must be the same as the frequency of a small piece of the cord. (c) The amplitude of the wave must be the same as the amplitude of a small piece of the cord. (d) All of the above are true. (e) Both (b) and (c) are true.
(e) As a wave travels down the cord, a point on the cord will move vertically between the lowest point of the wave and highest point of the wave. The wave and the point have the same amplitude. The point on the cord completes one up and down oscillation as each wavelength passes that point. Therefore, the motion of the point on the cord has the same frequency as the wave. The speed of the wave on the string is determined by the wavelength and frequency. It is constant in time. The point on the cord moves perpendicular to the wave with a speed that varies with time. The max speed of the point is proportional to the wave amplitude and the wave frequency. Changing the amplitude will change the max speed of the point on the cord, but it does not change the wave speed. The wave speed and string speed therefore are not equal.
8. (12-MC) A guitar string vibrates at a frequency of 330 Hz with wavelength 1.40 m. The frequency and wavelength of this sound in air (20ºC) as it reaches our ears is (a) same frequency, same wavelength. (b) higher frequency, same wavelength. (c) lower frequency, same wavelength. (d) same frequency, longer wavelength. (e) same frequency, shorter wavelength.
(e) As string oscillates, it causes the air to vibrate at the same frequency. The speed of sound in the string is the product of wavelength and frequency, 462 m/s. So the wavelength will be shorter in air than on the string.
13. (12-MC) A guitar string is vibrating at is fundamental frequency f. Which of the following is not true? (a) Each small section of the guitar string oscillates up and down at a frequency f (b) The wavelength of the standing wave on the guitar string is λ = v/f, where v is the velocity of the wave on the string. (c) A sound wave created by this vibrating spring propagates through the air with frequency f. (d) A sound wave created by this vibrating string propagates through the air with wavelength λ = v/f, where v is the velocity of sound in air. (e) The wavelength of the standing wave on the guitar string is λ = L, where L is the length of the string.
(e) As the string vibrates, each part of the string (other than the nodes) oscillates at the same frequency, so answer (a) is true. This oscillation excites the air to vibrate at that frequency, so answer (c) is true. The wave relationships in answers (b) and (d) are true for any wave, so both are true in this case as well. However the speed of the wave on the string is determined by the tension and mass of the string, and the speed of sound in the air is determined by the temp., pressure, and density of the air. The two speeds are not necessarily the same. Since the sound wave and wave on the string have the same frequencies, but not necessarily the same wave speeds, they do not necessarily have the same wavelengths. Thus, (e) is not true.
7. (12-MC) When a sound wave passes from air into water, what properties of the wave will change? (a) frequency (b) wavelength (c) wave speed (d) both frequency and wavelength (e) both wave speed and wavelength
(e) Frequency does not change as it passes from air to water. Speed becomes greater. Since frequency cannot change, the wavelength increases.
6. (12-MC) In which of the following is the wavelength of the lowest vibration mode the same as the length of the string or tube? (a) a string (b) an open tube (c) a closed tube at one end (d) all of the above (e) none of the above
(e) In a string or open tube, the lowest vibration mode is equal to half of a wavelength. In a tube closed at one end the lowest vibration mode is equal to a quarter of a wavelength.
4. (12-MC) To make a given sound seem twice as loud, how should a musician change the intensity of the sound? (a) Double the intensity. (b) Halve the intensity. (c) Quadruple the intensity. (d) Quarter the intensity. (e) Increase the intensity by a factor of 10.
(e) The ear interprets loudness on a logarithmic scale. For something to sound twice as loud, it must have an intensity that is 10 times as great.
1. (11-MC) A mass on a spring in SHM has amplitude A and period T. At what point in the motion is the velocity zero and acc. zero at the same time? (a) x = A (b) x > 0, but x < A (c) x = 0 (d) x < 0 (e) none of the above.
(e) x = plus or minus A, velocity is zero, but acceleration is at max. At x = 0, the acceleration is zero, but the velocity is at max. At all other points, the acceleration and velocity are nonzero.
7. (22-Q) In the electromagnetic spectrum, what type of EM wave would have a wavelength of 10³ km? 1 km? 1 m? 1 cm? 1 mm? 1 µm?
-Wavelength of 10³ m: sub-radio waves (very long radiowaves) -Wavelength of 1 km: radio waves. -Wavelength of 1 m: TV signals and microwaves. -Wavelength of 1 cm: microwaves and satellite TV signals. -Wavelength of 1 mm: microwaves and infrared waves. -Wavelength of 1 µm: infrared waves.
8. (11-Q) What is the approximate period of your walking step?
1 m equals length of leg. Use period equation for simple harmonic motion.
8. (12-Q) Explain how a tube might be used as a filter to reduce the amplitude of sounds in various frequency ranges. (An example is a car muffler.)
A tube of a given length will resonate (permit standing waves) at certain frequencies. Only frequencies close to resonate frequencies will produce a sound that persists, because of standing waves. Frequencies far from resonant frequencies will "die out" quickly. The length of a tube can therefore be chosen to "filter" certain frequencies if those filtered frequencies are not close to resonant frequencies.
27. (11-Q) AM radio signals can usually be heard from behind a hill, but FM often cannot. That is, AM signals bend more than FM. Explain. (Radio signals are carried by elctromagnetic waves whose wavelength for AM is typically 200 to 600 m and for FM about 3 m.)
AM radio waves have a much longer wavelength than FM radio waves. How much waves bend, or diffract, around obstacles depends on the wavelength of the wave compared with the size of the obstacle There will be a "shadow" region with FM waves because a hill is much larger than the wavelength of FM waves. AM waves will just bend around the hill.
13. (12-Q) Traditional methods of protecting the hearing of people who work in ares with very high noise levels have consisted mainly of efforts to block or reduce noise levels. With a relatively new technology, headphones are worn that do not block the ambient noise. Instead, a device is used which detects the noise, inverts it electronically, then feeds it to the headphones in addition to the ambient noise. How could adding more noise reduce the sound levels reaching the ears.
Active noise reduction devices work on the principle of destructive interference. If the electronics are fast enough to detect the noise, invert it, and create the opposite wave (180º out of phase with the original) in significantly less time than one period of the components of the noise, then the original noise and the created noise will be approximately in a destructive interference relationship. The person wearing the headphones will then hear a net sound signal that is very low in intensity.
7. (11-Q) Two equal masses are attached to separate identical springs next to one another. One mass is pulled so its spring stretches 40 cm and the other is pulled so its spring stretches only 20 cm. The masses are released simultaneously. Which mass reaches the equilibrium point first?
Both reach the equilibrium point at the same time. The period is independent of the amplitude.
23. (11-Q) Why do the strings used for the lowest-frequency notes on a piano normally have a wire wrapped around them?
By wrapping the string with wire, the mass per unit length of the string can be greatly increased without changing the length or the tension of the string. This gives the string a low fundamental frequency.
3. (22-Q) Ca EM waves travel through a perfect vacuum? Can sound waves?
EM waves can travel through a perfect vacuum. The energy is carried in the oscillating electric and magnetic fields, and no medium is required to travel. Sound waves cannot travel through a perfect vacuum. A medium is needed to carry the energy of a mechanical wave such as sound, and there is no medium in a perfect vacuum.
17. (12-Q) Fig. 12-32 shows various positions of a child on a swing moving toward a person on the ground who is blowing a whistle. At which position, A through E, will the child hear the highest frequency for the sound of the whistle? Explain.
Highest frequency at position C, while child is swinging forward. Assuming child is moving with SHM, the highest speed is at the equilibrium point, point C. To have an increased pitch, the relative motion of the source and detector must be toward each other. The child would also hear the lowest frequency at point C, while swinging backwards.
12. (12-Q) In figure 12-16, if the frequency of the speakers is lowered, would the points D and C (where destructive and constructive interference occur) move farther apart or closer together? Explain.
If the frequency of the speakers is lowered, then the wavelength will be increased. Each circle will be larger, so the points C and D will move farther apart.
5. (12-Q) What evidence can you give that the speed of sound in air does not depend on frequency?
If the speed of sound in air depended significantly on frequency, then the sounds that we hear would be separated in time according to frequency. For example, we would hear high, middle, and low notes at a different time. This effect is not heard for a large range of distances, indicating that the speed of sound in air does not depend significantly on frequency.
5. (22-MC) In empty space, which quantity is always larger for X-ray radiation than for a radio wave? (a) amplitude (b) wavelength (c) frequency (d) speed
In empty space, X-ray radiation and a radio wave both travel at the speed of light. X-ray radiation has a much smaller wavelength than a radio wave. Since wave speed is the product of the wavelength and the frequency, and both waves travel at the same speed, the X-ray radiation has a higher frequency than a radio wave. The amplitude depends upon the intensity of the wave, so either the X-ray radiation or the radio wave could have the greater amplitude.
7. (12-Q) How will the air temp. in a room affect the pitch of the organ pipes?
In the basic equations for pitch of pipes, the frequency is proportionall to the speed of sound in air. Since the speed is a function of temp., and the length of any particular pipe is very nearly constant over the relatively small range of temps. in a room, the frequency is also a function of temp. Thus, when the temp. changes, the resonant frequencies of the organ pipes change. Since the speed of sound increases with temp, as the temp, increases, the pitch of the pipes increases.
1. (12-Q) What is the evidence that sound travels as a wave?
Interference is a wave phenomenon, and sound produces interference (such as beats). Diffraction is a wave phenomenon, and sound can be diffracted (such as sound being heard around corners). Refraction is a wave phenomenon, and sound exhibits refraction when passing obliquely from one medium to another. Sound also requires a medium, a characteristic of mechanical waves.
1. (22-Q) The electric field in an EM wave traveling north oscillates in an east-west plane. Describe the direction of the magnetic field vector in this wave. Explain.
Magnetic field must oscillate up and down. For and EM wave, the direction of travel, the electric field, and the magnetic field must all be perpendicular to each other.
20. (11-Q) When a sinusoidal wave crosses the boundary between two sections of cord, the frequency does not change (although the wavelength and velocity do change). Explain why.
Media is continuous. Whatever happens to one happens to the other.
10. (11-Q) Why can you make water slosh back and forth in a pan only if you shake the pan at a certain frequency?
Natural frequency due to resonance, standing wave pattern.
4. (22-Q) When you flip a light switch on, does the light go on immediately? Explain.
No. Electromagnetic waves travel at a very large but finite speed. When you flip on a light switch, it takes a very small amount of time for the electrical signal to travel along the wires from the switch to the lightbulb.
4. (12-Q) When a sound wave passes from air into water, do you expect the frequency or wavelength to change?
Particles must be in contact in order for the wave to be transmitted from one medium to the other, so the frequency does not change. Since the wave speed changes in passing from air into the water and the frequency does not change, we expect the wavelength to change. Sound waves travel about four times faster in water than in air, so we expect the wavelength in water to be about four times longer than it is in air.
9. (11-Q) What happens to the period of a playground swing if you rise up from sitting to a standing position?
Period will decrease, and therefore, frequency will increase.
2. (11-Q) Real springs have mass. Will the true period and frequency be larger or smaller than given by the equations for a mass oscillating on the end of an idealized massless spring? Explain.
Real spring has mass, and mass at end of spring is not the only mass. A larger mass equals smaller frequency by used of the frequency equation. The frequency will be smaller than the frequency with the massless spring approximation. Since true frequency is smaller, the true period is larger. One-third of the mass of the spring contributes to the total mass.
5. (11-Q) A tire swing hanging from a branch reaches nearly to the ground. How could you estimate the height of the branch using only a stopwatch?
Simple harmonic motion with a small amplitude. Measure period of the oscillations, and calculate the length of the pendulum from period expression. The length l is the distance from the branch to the center of the tire. The total length is l plus the length from the center of the tire to the ground.
12. (11-Q) Explain the difference between the speed of a transverse wave traveling along a cord and the speed of a tiny piece of the cord.
Speed of transverse wave is speed at which the wave disturbance moves along the cord. That speed is constant if uniform cord and depends on the tension in the cord and the mass density of the cord. The speed of a tiny piece of the cord is how fast the piece of cord moves perpendicularly to the cord as the disturbance passes by.. That speed is not constant. If a sinusoidal wave is traveling on the cord, the speed of each piece of the cord will be given by the speed relationship of a simple harmonic oscillator, which depends of the amplitude of the wave, the frequency of the wave, and the specific time of observation.
14. (11-Q) Since the density of air decreases with an increase in temperature, but the bulk modulus B is nearly independent of temperature, how would you expect the speed of sound waves in air to vary with temperature?
Speed of waves in a gas equation with B and rho. A decrease in the density due to a temp. increase therefore leads to a higher speed of sound. We expect the speed of sound to increase as temp. increases.
14. (12-Q) Consider the two waves shown in Figure 12-31. Each wave can be thought of as a superposition of two sound waves with slightly different frequencies, as in Fig. 12-18. In which of the waves, (a) or (b), are the two component frequencies farther apart? Explain.
The frequency of beating is higher in wave (a) - the beats occur more frequently. The beat frequency is the difference between the two component frequencies, so since (a) has a higher beat frequency, the component frequencies are farther apart in (a).
16. (12-Q) If a wind is blowing, will this alter the frequency of the sound heard by a person at rest with respect to the source? Is the wavelength or velocity changed?
The moving air has the same effect as if the speed of sound had been increased by an amount equal to the wind speed. The wavelength will be increased by the same percentage that the wind speed is relative to the still-air speed of sound. No Doppler effect because since wavelength and velocity change at same percentage, frequency does not change. Alternatively, the wind has the same effect as if the air were not moving but the source and the listener were moving at the same speed in the same direction. No relative motion equals no Doppler effect.
9. (22-Q) If a radio transmitter has a vertical antenna, should a receiver's antenna (rod type) be vertical or horizontal to obtain best reception?
The receiver antenna should also be vertical for obtaining the best reception. The oscillating carrier electric field is up and down, so a vertical antenna would "pick up" that signal better. That is because the electrons in the metal antenna would be forced to oscillate up and down along the entire length of the vertical antenna, creating a stronger signal.
10, (12-Q) A noisy truck approaches you from behind a building. Initially you hear it but cannot see it. When it emerges, its sound is suddenly "brighter" you hear more of the high frequency noise. Explain.
The sound waves that you are hearing when you cannot see it are arriving due to diffraction. Long wavelengths are diffracted more than short wavelengths, so you are initially only hearing sound with long wavelengths, which are low-frequency sounds. When you see the truck, you receive all the frequencies and hear a "brightened" sound due to your hearing more high-frequency components.
6. (12-Q) The voice of a person who has inhaled helium sounds very high-pitched. Why?
The speed of sound will be much higher than normal, since the speed of sound waves in helium is about 3 times that in air. Thus, the person's frequencies will go up by about a factor of 3. This is about a 1.5 octave shift upwards.
5. (22-Q) Are the wavelengths of radio and television signals longer or shorter than those detectable by the human eye?
The wavelengths of radio and TV signals are much longer than those of visible light. Radio waves on the ord of 3m-30,000 m. TV waves are on the order of 0.3m-3m. Visible waves are on the order of 10⁻⁷m.
15. (12-Q) Is there a Doppler shift if the source and the observe move in the same direction, with the same velocity? Explain.
There is no Doppler shift because Doppler shift is cause by relative motion between the source and observer. If both source and observer move in the same direction with the same velocity, there is no relative motion.
3. (11-Q) How could you double the max speed of a simple harmonic oscillator?
Use v max equation. Doubling amplitude, reducing mass to one-fourth of the original, and doubling frequency will all double the max speed.
1. (11-Q) Is the acceleration of a simple harmonic oscillator ever zero? If so, where
Yes, momentarily at equilibrium. There is no mass or force here.
22. (11-Q) If a string is vibrating as a standing wave in three loops, are there any places you could touch it with a knife blade without disturbing its motion?
Yes. If you touch the string at any node, you will not disturb the motion.
4. (11-Q) If a pendulum clock is accurate at sea level, will it gain or lose time when taken to high altitude?
g will decrease and period will increase. So, it will lose time.
