Jim Greenfield replied to Jeff Root:
>> George Dishman wrote:
>> > (3) AND (4) ARE QUESTIONS, NOT REASONS.
>> Actually they're scenarios, which were described in order
>> to pose and answer the questions.
> Agree. George is running interference
I can't even guess what you mean by that remark.
>> Of course, a scenario doesn't cause Doppler shift, any
>> more than a question does.
>> The question is:
>> What causes the Doppler shift in scenario (3)?
> Make that question (a)
> Answer: Change in position at instant of firing
That's good. Since the position at the instant of
firing is the only thing that changes, that must be
what causes the Doppler shift in this scenario.
> Question (b)
> What causes the Doppler shift in scenario (4)?
Good question. I should have included it in my post.
> Answer: Increased bullet velocity
Okay, that's interesting. You say that the cause of the
Doppler shift in scenario (4) is different from the cause
in scenario (3).
> .........(for an extra 1/2 mark, the position change
> is incidental)
Yes. Scenario (4) has both a change in position and a
change in speed at the instant of firing.
Compare the amount of Doppler shift in the scenarios.
In my animation, the times between bullet hits on the
1) 15.0 frames
2) 15.0 frames
3) 7.5 frames
4) 10.0 frames
So the frequencies are:
1) 0.066 per frame
2) 0.066 per frame
3) 0.133 per frame
4) 0.100 per frame
Cases (1) and (2) have no Doppler shift. Case (3) has
the most Doppler shift, and case (4) has some Doppler
shift, but less than case (3).
Case (4) has exactly the same change in position that
case (3) has. Shouldn't that change in position cause
Doppler shift? Since the position changes by the same
amount in both cases, shouldn't it cause the same amount
of Doppler shift in both cases?
Why is the Doppler shift in case (4) less than that in
case (3)? The only difference between the two cases is
that case (4) also has an increase in speed. Somehow,
the increased speed in case (4) caused the amount of
Doppler shift to be less than it is in case (3).
> Question (c)
> Which of the two ACTUALLY caused the Doppler shift, and
> where is the evidence?
> Answer: (4), because it demonstrated increased kinetic
> energy delivered to the target, whereas (3) did not.
Again, (4) is a scenario, not a cause of Doppler shift.
Scenarios do not cause Doppler shift. If you mean that
a change of speed causes Doppler shift, then say so.
Sticking the word "ACTUALLY" into the sentence doesn't
accomplish anything. We agree that the Doppler shift in
case (3) is caused by the change in position. We haven't
quite figured out yet what is going on in case (4).
You say that the change in speed in case (4) causes the
Doppler shift. I point out that, if change in position
causes Doppler shift in case (3), then it must also cause
the same amount of Doppler shift in case (4).
But case (4) has less Doppler shift than case (3), even
though the only difference between the two is that there
is an increase in speed in case (4).
If an increase in speed causes Doppler shift, shouldn't
there be more Doppler shift in case (4) than in case (3)?
And if an increase in speed causes Doppler shift, why is
there no Doppler shift at all in case (2)?
-- Jeff, in Minneapolis