Doppler Effect Demo

This animation illustrates some aspects of the Doppler effect, comparing four cases in which a gunner shoots at a target and:

1) remains fixed
2) advances as he fires but always fires from same place
3) advances between shots
4) advances at constant speed as he fires

The gunners fire at a rate of one shot per 15 frames. They fire simultaneously, four shots each.  The guns are lined up side-by-side at the instant of the first shot.

Muzzle speed is 8 pixels per frame.  Gunners 2 and 4 advance at 4 pixels per frame as they fire, so the speeds of the bullets relative to the targets are:

1) 8 pixels per frame
2) 12 pixels per frame
3) 8 pixels per frame
4) 12 pixels per frame

Distances between bullets in flight:

1) 120 pixels
2) 180 pixels
3) 60 pixels
4) 120 pixels

When the first bullet from each gun hits the target, a timer is started (red) which counts frames.  The timer stops when the second bullet hits, and the elapsed time between bullet hits is shown in green.  The times are:

1) 15.0 frames
2) 15.0 frames
3) 7.5 frames
4) 10.0 frames

Frequency of bullet strikes is the reciprocal of these times, so cases 1 and 2 have the lowest frequency, while case 3 has the highest frequency.

The motion of the gun toward the target at the instant of firing in cases 2 and 4 cause increased bullet speed at the target.  There is no increase in speed in case 3.

The advancing position of the gun between shots in cases 3 and 4 cause increased bullet frequency at the target.  There is no increase in frequency in case 2, even though the bullets hit the target at higher speed.

This clearly demonstrates that the motion of the gun at the instant of firing determines the speed of the bullet at the target, but does not increase the frequency.  The advancing position of the gun between shots increases the frequency of the bullets at the target, but has no effect on speed.

Case 4 shows that the increased speed of the bullets caused by the forward motion of the gun at the instant of firing actually *reduces* the frequency from what it is in case 3, where the gun fires each shot from exactly the same distances as in case 4, but is motionless at the instant of firing.