HKUGA College

Unit Test 5 (2021/22)

Physics

Question-Answer book
Time allowed: 35 minutes Form: 4

Name: Class (No.): ( )

GENERAL INSTRUCTIONS

1. There are TWO sections, A and B, in this Paper. You are advised to finish Section A in about 15 minutes.
2. Section A consists of multiple-choice questions, while Section B contains structural questions.
3. The diagrams in this paper are NOT necessarily drawn to scale.
4. When told to open this book, you should check that all the questions are there. Look for the words ‘END
OF PAPER’ after the last question.
5. No marks will be deducted for wrong answers.
6. ANSWER ALL QUESTIONS. You must mark the answers clearly; otherwise you will lose marks if the
answers cannot be captured.
Section A (12 marks). There are 8 questions. Fill in the answer in the box below the question number.

Q.1 Q.2 Q.3 Q.4 Q.5 Q.6 Q.7 Q.8

1. A water wave travelling in region P enters into region Q. If the wavelength in Q is twice that
in P, which of the following statements is/are correct?

(1) The speed of the water wave in P is twice that in Q.

(2)
(3) If the angle θ is increased, total internal reflection may occur.

A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only

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2. The diagram shows a stationary wave of frequency 6 Hz produced in a string of length
12 cm. P, Q and R represent three points on the string. Which of the following statements
is/are correct?

(1) The wave speed is 36 cm s-1.

(2) If P is momentarily at rest, R is also momentarily at rest.
(3) If the frequency is changed to 9 Hz, another stationary wave will be formed.

A. (1) and (2) only

B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
C

3. An interference pattern is produced by two coherent sources S1 and S2 each vibrating at

frequency 450 Hz. There are altogether 9 anti-nodal lines as shown. If the distances S1P and
S2P are 1.9 m and 1.3 m respectively, what is the speed of the wave emitted by the sources?

A. 45 m s–1
B. 90 m s–1
C. 180 m s–1
D. 270 m s–1
B

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4. The solid lines in the diagram represent the crests of the waves from two coherent sources S1
and S2. Which of the following statements is/are correct?

(1) If S1P = 12 cm, then S2P = 18 cm.

(2) If S1Q – S2Q = 6 cm, then the wavelength is 12 cm.
(3) If the wavelength is 8 cm, then S1R – S2R = 12 cm.

A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
B

5. A string is kept taut by a hanging mass m. When it is plucked at the middle, a sound is
emitted. Which of the following statements is/are correct?

(1) The wavelength of the stationary wave is 2L.

(2) The frequency of sound emitted can be reduced by reducing the vibrating length L.
(3) The frequency of sound emitted can be reduced by increasing the hanging mass m.

A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
A

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6. Two coherent sources S1 and S2 are vibrating in phase with each other. P is a point such that
S1P = 0.8 m and S2P = 1.4 m. If a destructive interference occurs at P, which of the
following wavelengths is/are possible?

(1) 0.12 m
(2) 0.4 m
(3) 1.2 m

A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
D

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7. Which of the following statements is/are true?

(1) The longest wavelength of sound waves is about 17 m.

(2) The longest wavelength of visible light is about 0.7 μm.
(3) Both sound waves and EM waves can be used to detect motion of objects.

A. (2) only
B. (1) and (2) only
C. (2) and (3) only
D. (1), (2) and (3) only
C

8. A HiFi system consists of loudspeakers of different size. Which of the following statements
is/are true?

(1) Smaller loudspeakers are used to display higher pitch because the diffraction of
sound is less for a smaller loudspeaker.
(2) Interference of sound from loudspeakers of different sizes will never occur.
(3) Speed of sound produced by smaller loudspeakers is usually lower than that
produced by larger loudspeakers.

A. (1) only
B. (2) only
C. (1) and (2) only
D. (2) and (3) only
B

END OF SECTION A

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Section B (18 marks) Answer ALL questions.

1. When the string of a guitar is plucked, a musical note is produced. Under the analysis using
an oscilloscope, the note is found to consist of many frequency components. Two
consecutive frequency components are 200 Hz and 300 Hz. Fig.a shows the trace of the
200 Hz component.

Fig.a Fig.b
(a) What is the time base setting of the oscilloscope? [2 marks]

(b) In Fig.b, draw the trace for the 300 Hz under the same time base setting. [2 marks]

(c) Explain why the fundamental frequency of the string cannot be 200 Hz. [1 mark]

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(d) Hence, determine the fundamental frequency f1 of the string. [2 marks]

(e) The length of the guitar string is 0.8 m. Calculate the speed of wave in the string.
[2 marks]

(f) If the tension in the string is increased, what happens to the fundamental frequency?
[2 marks]

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2. In an experiment, a diffraction grating with 3000 lines per cm is used to view a
monochromatic light source. A meter rule 1.5 m away is perpendicular to the line joining the
light source and the grating with the “0” mark on the line.

(a) Find the distance of separation between adjacent lines of the grating. [1 mark]

(b) A pin is moved on the rule starting from the “0” mark until it coincides with the first
order image. When this occurs, the reading on the rule is 22.7 cm.
(i) Find the wavelength of the light used.
[2 marks]

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(ii) If the pin continues to move, how many more coincidence will be encountered?
[2 marks]

(iii) Describe the changes in the coincidence if a light source with a lower frequency
is used. [2 marks]

END OF PAPER

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