Deacceleration causing reverse of speed impact on the body of the frame causing reverse of shockwave.
Then the the speed of the molecules also increase
Answer:

Explanation:
speed of bat = 6 m/s
sound wave frequency emitted by bat = 30.0 kHz
as we know,
speed of sound (c)= 343 m/s



now frequency received by bat is equal to



hence the frequency hear by bat will be 29.98 Hz
I believe Isaac Newtons Law " Gravity ".