SAT Subject Physics Practice Question 267: Answer and Explanation
38. As a bat flies at a constant speed of 0.04 V toward a large tree trunk (where V denotes the speed of sound), the bat emits an ultrasonic pulse. The pulse is reflected off the tree and returns to the bat, which can detect and analyze the returning signal. If the returning signal has a frequency of 61 kHz, at approximately what frequency did the bat emit the original ultrasonic pulse?
A. 56 kHz
B. 62 kHz
C. 68 kHz
D. 74 kHz
E. 78 kHz
Correct Answer: A
Because the bat flies toward the tree, the frequency of the waves as they hit the tree is higher than the frequency with which they were emitted by the bat. Then as these waves reflect off the tree, they are detected by the bat, which is still flying toward the tree, so the frequency gets shifted higher again. Therefore, if the returning signal is detected by the bat as having a frequency of 61 kHz, the original pulse must have been emitted at a frequency lower than 61 kHz. Only A is lower than 61 kHz.