A 2.50 MHz sound wave travels through a pregnant woman's abdomen and is reflected from the fetal heart wall of her unborn baby. The heart wall is moving toward the sound receiver as the heart beats. The reflected sound is then mixed with the transmitted sound, and 78 beats per second are detected. The speed of sound in body tissue is 1500 m/s. Part A Calculate the speed of the fetal heart wall at the instant this measurement is made.

the speed of the fetal heart wall at the instant is 0.0325 m/s

Explanation:

Given the data in the question;

f₀ = 2.50 MHz = 1.80 × 10⁶ Hz

f  = 78 beat per sec ( Hz )

V = 1500 m/s

the speed of the fetal heart wall at the instant this measurement is made = ?

now, if v is the magnitude of hart wall speed, V is speed of sound and f the frequency the heart receives( and reflects), f₀ is original frequency and f, is reflected back;

so

heart is moving observer and device is stationary source

f = ((V + v)/v )f₀

Heart is moving source and device is stationary observer

f' = (V/(V-v ))f

Beats

f = f₀ - f' = f₀ - f₀( V+v / V-v ) = f₀( 2v / V-v )

so we solve for v

v = V( f / ( 2f₀ + f )

so we substitute

v = 1500 ( 78 / ( (2×1.80 × 10⁶) + 78 )

v = 1500 ( 78 / ( 3,600,000 + 78 )

v = 1500 ( 78 / 3,600,078 )

v = 1500 ( 2.1666 × 10⁻⁵ )

v = 0.0325 m/s

Therefore,  the speed of the fetal heart wall at the instant is 0.0325 m/s