A sound wave having frequency of 300 Hz is travelling in an unknown medium. Its wavelength is not known. It travels a distance equal to 150 times its wavelength in time t. The value of t is :
- A.0.5 s
- B.1 s
- C.1.5 s
- D.2 s
Correct answer
Explanation
The correct answer is A, 0.5 s. A wave moves forward by one wavelength in each cycle, so 150 wavelengths take 150 cycles, and 150 cycles at 300 per second take half a second.
Wave speed is v = fλ, where f is the frequency and λ the wavelength. The distance covered is 150λ, so the time is t = 150λ ÷ (300 × λ) = 150 ÷ 300 = 0.5 s. The wavelength cancels out, which is why it does not need to be known. Put simply, the time for one wavelength is one period, T = 1/f = 1/300 s, and 150 periods make 0.5 s. The medium does not matter either, because the frequency of a wave stays the same in every medium.
B is wrong because 1 s would cover 300 wavelengths, not 150.
C is wrong because 1.5 s would cover 450 wavelengths.
D is wrong because 2 s would cover 600 wavelengths.
Exam tip: one wavelength per period; distance in wavelengths divided by frequency gives the time.
Practice Questions
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2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
- A.Only one
- B.Only two
- C.All three
- D.None
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2. Carbon fibres once used cannot be recycled.
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- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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- A.Golgi apparatus
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- B.Onion, garlic, carrot
- C.Potato, onion, garlic
- D.Potato, garlic, turnip
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- A.equal to its weight.
- B.less than its weight.
- C.greater than its weight.
- D.zero.
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