An engine whistling at a constant frequency n0 and moving with a constant velocity goes past a stationary observer. As the engine crosses him, the frequency of the sound heard by him changes by a factor f. The actual difference in the frequencies of the sound heard by him before and after the engine crosses him is
1
\(\frac{1}{2} n_0\left(1-f^2\right)\)
2
\(\frac{1}{2} \pi\left(\frac{1-f^2}{f}\right)\)
3
\(n_0\left(\frac{1-f}{1+f}\right)\)
4
\(\frac{1}{2} n_0\left(\frac{1-f}{1+f}\right)\)
5
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