What is the direction of the net force acting on the object at position a? hints what is the direction of the net force acting on the object at position a? upward downward to the left to the right the net force is zero?

Looking at the position A, we can actually see that velocity vectors connecting position A and its adjacent positions appear to have similar magnitude and in similar direction. Hence the acceleration is zero and similarly, the net force is also zero.

Net force is zero

Increasing until terminal velocity is reached

Explanation:

Provided the scream is a constant pitch at the source, Doppler effect will make the pitch increase as the velocity of the source towards the listener increases.