Which of the following correctly states the relationship between the observed…

PHYSICS · Single correct · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics

Problem

Which of the following correctly states the relationship between the observed frequency

f_\text{obs}

and the rest frequency

f_0

of a siren when the source moves directly toward a stationary listener in still air? (a)

f_{\text{obs}}=f_0

because the wavelength of the sound does not change in air. (b)

f_{\text{obs}}<f_0

because the speed of sound decreases for a moving source. (c)

f_{\text{obs}}>f_0

because successive wavefronts reach the listener in less time. (d)

f_{\text{obs}}>f_0

because the siren’s motion increases the speed of the sound waves it emits.