Doppler Effect for Moving Source

Steps:

• Use Doppler formula for source approaching stationary observer: f' = f × (v / (v - v_s)), where v = 340 m/s, v_s = 40 m/s.
• Calculate shift factor: 340 / (340 - 40) = 340 / 300 = 17/15.
• Original frequency difference: 1000 Hz - 700 Hz = 300 Hz.
• Apparent difference: 300 × (17/15) = 340 Hz.

Why D is correct:

• The multiplicative Doppler factor scales the original frequency difference proportionally, per the wave equation for sound.

Why the others are wrong:

• A: Underestimates shift by ignoring full scaling of difference.
• B: Equals original difference, neglecting Doppler effect entirely.
• C: Approximates factor incorrectly, possibly using v_s/v ratio.

Final answer: D