Grade 121 January 2008
PHYSICAL SCIENCES - REVISION EXERCISE
Time: 3 hoursMarks: 150
QUESTION 1
Various possible answers are provided for each question. Write the letter only of the correct answer next to the corresponding number.
1.1The pitch of a blaring hooter on a fast-moving minibus taxi travelling at a constant velocity of x m.s–1 changes as it passes an observer. If the frequency of the sound on the minibus taxi is y, and the speed of sound in air is z m.s–1, the frequency the observer hears as the taxi travels towards him is …
A
B
C
D(3)
1.2Astronomers analyse the spectrum of light from a star. They have found that the light from other galaxies moving away from us has a red shift. Which of the following statements are FALSE?
ALight shifts to red since red has the lowest frequency of visible light.
BLight shifts to red since red has the longest wavelength of visible light.
CRed shift is due to the Doppler Effect of electromagnetic waves.
DAn increase in red shift implies that galaxies are receding slowly and that the universe is expanding. (3)
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QUESTION 2
Say whether the following statements are true or false. If the statement is false, give the correct statement.
2.1The Doppler Effect refers to a change in the pitch of a sound produced by an object moving at a velocity less than the speed of sound. (2)
2.2Ultrasonic waves reflected from red blood cells to determine the speed of blood flow is an application of the Doppler Effect. (2)
2.3If a source of sound moves faster than the speed of sound, the waves “pile up” producing a softer sound. (2)
2.4Frequencies in ultrasound are high so that the bigger waves produced can detect smaller objects. (2)
2.5When sources of light waves move away from an observer, the waves detected by the observer have a longer wavelength than those emitted. This application of the Doppler Effect is known as red shift. (2)
2.6A shock wave is the result of many constructive interference patterns on a large number of wave-fronts. (2)
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QUESTION 3
Complete the following statements by filling in the missing word or phrase.
3.1The phenomenon describing the change in pitch of a hooter on a fast-moving mini-bus taxi is known as ______. (2)
3.2People are talking on one side of a doorway. When the sound waves reach the doorway they spread out. This phenomenon is known as ______. (2)
3.3The light from stars in distant galaxies was found to have a wavelength longer than expected and is observed to be red shifted, allowing astronomers to calculate the speed of galaxies travelling ______. (2)
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QUESTION 4
Consider a police car, siren blaring, moving from rest towards the right. Both observers are on the sidewalk (pavement).
4.1When the police car is stationary, describe the sound heard by each observer.(2)
4.2As the police car moves between the two observers, describe the sound heard by each observer. (2)
4.3What is the name given to the phenomena described in 4.1 and 4.2?(2)
The siren of the police car at rest emits a dominant frequency of 1 600 Hz.
4.4What frequency will you hear if you are at rest and the police car moves towards you at 25 m.s–1 ? (4)
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QUESTION 5
The predominant frequency of the siren of a fire engine is 1 800 Hz when at rest. A driver in a car approaches the fire engine with a speed of 30 m.s–1 and eventually passes the fire engine.
5.1How does the pitch of the siren change, with reference to the driver in the moving car, when:
(a)the car moves towards the fire engine?(2)
(b)the car moves away from the fire engine?(2)
5.2What is the name given to the phenomenon described in 5.1?(1)
5.3What is the frequency of the sound detected when the driver moves towards it?(5)
[10]
QUESTION 6
Study the following diagram and answer the questions below.
6.1If the frequency of ultrasound used by a sonar system is 40 kHz, what is the wavelength of ultrasound if the speed of sound in water is 1 500 m.s–1? (4)
6.2If there is a time of 0,1 s between a pulse being sent and received, what is the depth of the water below the boat? (4)
6.3What time would be recorded by the sonar receiver if the boat was above a shoal of fish
10 m below the surface?(4)
6.4The speed of ultrasound in air is about 350 m.s–1. Why does ultrasound travel much faster in water than it does in air? (4)
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[TOTAL: 60 marks]
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