<h2>
Answer: Gravity force</h2>
If we approximate the orbit of the planets around the Sun to circular orbits with a uniform circular motion, where the velocity 
is a vector, whose direction is perpendicular to the radius 
of the trajectory; the acceleration 
is directed towards the center of the circumference (that's why it's called centripetal acceleration).
Now, according to Newton's 2nd law, the force 
is directly proportional and in the same direction as the acceleration:
Therefore the net force resulting from the movement of a planet orbiting the Sun points towards the center of the circle, this is called Centripetal Force which is a central force that in this case is equal to the gravity force.
Answer:
f" = 40779.61 Hz
Explanation:
From the question, we see that the bat is the source of the sound wave and is initially at rest and the object is in motion as the observer, thus;
from the Doppler effect equation, we can calculate the initial observed frequency as:
f' = f(1 - (v_o/v))
We are given;
f = 46.2 kHz = 46200 Hz
v_o = 21.8 m/s
v is speed of sound = 343 m/s
Thus;
f' = 46200(1 - (21/343))
f' = 43371.4285 Hz
In the second stage, we see that the bat is now a stationary observer while the object is now the moving source;
Thus, from doppler effect again but this time with the source going away from the obsever, the new observed frequency is;
f" = f'/(1 + (v_o/v))
f" = 43371.4285/(1 + (21.8/343))
f" = 40779.61 Hz
