Can you notice Doppler effect, if both the observer(or listener) and the source of sound are moving with same velocity in the same direction.

What is the apparent change in frequency when source and observer move with same velocity in the same direction?

Two parallel beams fo positions moving in the same direction will

In case of a moving source of sound approaching an observer.

In an SHM, can velocity and displacement be in the same direction?

The following particles are moving with the same velocity.The particle with maximum momentum is :

When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day ?