A sound wave travelling at a speed of 330 m/s produces 20 crests and 20 troughs in 0.1 second. The wavelength of the sound wave is:
- A.1.1 m
- B.3.3 m
- C.1.65 m
- D.2.2 m
Correct answer
Explanation
The correct answer is C, 1.65 m. Twenty crests and twenty troughs make twenty complete waves in 0.1 second, so the frequency is 200 Hz and the wavelength is 330 ÷ 200 = 1.65 m.
One complete wave, or one cycle, contains one crest and one trough (for sound, one compression and one rarefaction). Twenty such waves in 0.1 second means 200 waves in one second, which is a frequency of 200 hertz. The wave equation links the three quantities: speed = frequency × wavelength, so wavelength = speed ÷ frequency. Putting in the values gives 330 m/s ÷ 200 Hz = 1.65 m.
A is wrong: 1.1 m would need a frequency of 300 Hz.
B is wrong: 3.3 m would mean only 100 waves per second, which comes from counting the crests and troughs as ten waves in 0.1 second.
D is wrong: 2.2 m would need a frequency of 150 Hz.
Exam tip: v = fλ; one crest plus one trough is one wave, so count pairs, not peaks and dips separately.
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Show answer
A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
Read the full explanation →