I think at some point it’ll run into a philosophical issue over the fundaments of what ‘instantaneous’ could really mean. Is it possible to measure anything in duration that is an infinitesimal point in time? Measurement itself is an event - it needs to transition from ‘not-happened-yet’ to ‘has-happened’, which seems to me like it requires time to pass.
It came up only because the OP attempted to draw a distinction.
And when “velocity” is used for a scalar in a formal setting, it’s wrong. All that proves is that physicists make mistakes sometimes, too.
This is demonstrably incorrect. Actual usage by actual scientists and engineers in actual formal settings happily includes velocity as a scalar, routinely. You have no problem with the word “momentum” being both a vector and the magnitude of a vector. So to is it with “velocity”, and that’s generally much clearer (and typically preferred) over introducing some additional word for the magnitude of the vector (why do that?), even though common language offers a word.
While any scan of any literature corpus will show this, I’ll take a moment to demonstrate what I claim is demonstrable. I took from Physical Review Letters the ten most recent articles with the word “velocity” in the abstract – i.e., using the highest profile APS physics journal; in which there is no specific subfield represented (it’s general); from which I pull articles covering topics sufficiently connected to velocity that the word’s in the abstract; all of which are written by professional physicists; and peer-reviewed by professional physicists; and copyedited by professional scientific copyeditors to adhere to a strict style guide.
This yields the following usage types from the first 10 results:
Velocity as a scalar: 6 of 10 articles
Velocity as a scalar but it’s a 1D setting so it had to be: 1 of 10 articles
Velocity strictly as a vector: 1 of 10 articles*
*and this article was specifically about measuring velocity vector fields, so it had to be (and is most naturally always) vector.
One other article had a niche application of the word that made it not useful in this survey, and another article used the word only a couple of times in a passing way with insufficient context to determine (or care) about vector or scalar.
So in the determinable cases, scalar is far and away in routine usage. It’s just… how it is. As with “momentum”. And that’s fine. Yet, physics teachers will continue to insist for reasons that have nothing to do with physics.
I’d be real curious to see the OP come back and review the results of his handiwork and give us some feedback & course correction. We’re having fun without him as we always do, but I’m curious about his thoughts.
I’m very curious about this contention. I don’t see a way to construct any such device, much less “loads” of them.
We may be using different definitions of the relevant terms, and I’m unclear on yours.
The context of that statement was in reaction to @Mangetout’s interpretation of the OP. To rephrase that interpretation, the OP is asking not about deep philosophical questions of what it means to measure something but rather the much more straightforward thing of measuring speed via a quantity that depends functionally on instantaneous speed as opposed to measuring an average via a time series. There are the deep questions about how “instantaneous” is a mathematical ideal, how there are always practical limitations, quantum stuff, yada yada, but I think (after reading @Mangetout’s take) the OP is not asking anything like that. Just: what measurement approaches depend “directly” on (instantaneous) speed.
And, so, in that view, Doppler-based devices and spinning magnets exerting torque, as given in the OP, would count as measuring speed directly. So, too, would Pitot tubes (measured relative to the air) or even @Chronos’s co-moving reference object concept. Beyond these, though, I can see lots of mechanisms for measuring the speed of something based on other physical principles: other fluid dynamical effects, particle scattering, relativistic effects, induction effects, etc. Some such constructions could be (or even are) actually used in real devices, and others would be impractical but at least physically possible.
Even that is indirect because it is a measure of rotational speed, and that is used to deduce the vehicle speed based on the subsequent mechanical linkage.
That’s why I now have the take in the previous post. There are always such aspects that introduce practical imperfections in the measurement, so the OP must be (??) asking the question summarized in the previous post. Perhaps they will return to, as @LSLGuy says, give some feedback.
That’s a very clear exposition of what’s probably the OP’s question. Thank you. And yes, I also accept that measurement errors, calibration issues, friction, quantum BS, etc., are distracting quibbles from the OP’s (likely) notion of “directness”.
And at that rate (heh) I can buy that there are physical quantities which naturally vary in a predictable manner versus “instantaneous” speed. They may be linear, they may be quadratic, or exponential, or whatever, but the relationship is predictable and calculable.
Real world, there will always be inertia effects, hysteresis, etc., which amount to an integration of a changing value over some short time to “derive” a short-term average. But if all that happens in the natural physics world and we’re just watching & recording the outcome, we can call that “direct” for argument’s sake. My first post was arguing the opposite, that this natural integration was a forced indirection and further a forced non-zero time. Which is perhaps an interesting POV, but as you say, not germaine to what we now think the OP was interested in.
As to an old-fashioned analog electric speedo I’ll agree that the meter movement driving the needle is directly measuring shaft RPM, by the above definition of “direct”. It’s a bigger jump to say that’s measuring vehicle speed.
Every measurement system has measurement error. So you will never get a perfect recording of speed.
Furthermore, speed is not an absolute but must be measured in relation to something else. So if you want to be precise you wouldn’t say “What is the speed of that car”, but “What is its speed with respect to the surface it’s driving on?” To measure that, you pretty much need to measure the change in distance over time or the doppler shift of waves, so an ‘instantaneous’ method is not really possible.
Direct measurement is certainly possible, and done all the time. You could do it with a camera looking down at the road surface, or with LIDAR, or with magnetic encoders on a shaft, whatever. If you need super accuracy you can attach one of those external wheels that measure speed very accurately. Indirect methods would include timing the vehicle between two marks a known distance apart, as aircraft speed control does.
To be fair, I believe they insist because velocity is often one of the first vector quantities that students learn about (they also often insist on the term displacement as different from distance). They also need to support the idea that acceleration is a change in velocity and that change could be one of direction.
Yes one can easily get far to pedantic about it, but as I said I can the justification for trying to differentiate these in the beginning of learning physics.
Agree completely.
Said more cynically … When overcoming the lies taught to children last year, it’s important to implant a fresh set of lies this year to be beaten out of them next year.
I’m fine with a high school physics classroom adopting a convention that “velocity” will be used to refer to a vector. The problem is that the convention often gets delivered as being a universal definition of the word. You can very easily have both, i.e. a convention used in the class and also adequate clarity that it is in no way a universal convention. Otherwise, you just sow incorrect pedantry, which is no good for anyone.
(Edited to add: This case is different from simplifying physics and then complicating it later. “Hiding” non-ideal conditions, relativity, quantum mechanics, etc., is fine and doesn’t lead to routine clashes with reality. Telling students that “velocity” can only be a vector is incorrect the moment they step out of the class room and have a conversation with, well, anyone or read any common or, as noted above, formal literature.)
I would just take the barometer to the nearest policeman and say “Dear Mr. Policeman, I have here a fine barometer. If you would tell me the speed of this car by using your radar gun, I will give you this barometer.”
For the sake of argument.
What would be an example of a “direct” measurement?
Direct would mean timing the car over a given distance. Timing rotation rate of the wheels. You get a direct reading of the quantity. No fiddle factors or conversions between units - especially the latter.
Indirect is anything that is deriving the speed measure from a consequence of the speed in more than one step. So indirect might include, say, increased air pressure in a pitot tube. Or something as fancy as measuring the fuel consumption and deriving speed from known aerodynamic and efficiency properties of the car.
If you get a number that isn’t a speed and there is a conversion that isn’t just a unitless scalar, it wasn’t direct. A pressure or a rate of fuel consumption isn’t a speed. So the measure is indirect.
IMHO there is a lot of wiggle room in modern science here. Doppler radar yields frequency that is a consequence of the car’s speed, but also the transmitted frequency, with the frequency shift being converted to a speed. That would count as indirect. But unpack the mathematics and you are really still measuring the distance travelled over time. So I would argue for direct. YMMV 
That would be an indirect measurement. You could hit the policeman on the head with barometer and take his radar gun and do it yourself for a direct measurement.
I mean that hypothetically because hitting policemen on the head with barometers and/or stealing their radar guns might be illegal in some jurisdictions.
The OP has never been back to this thread to provide any clarity on what “direct” and “indirect” mean to him. Different folks upthread have very different definitions of that distinction.
How else can we measure the car’s speed with a barometer? Some ideas come to mind:
- Measure out 100 barometer lengths along the road, and time how long it takes the car to pass them. We now know the car’s speed in barometers per second.
- If the car is going uphill or downhill, we can measure the rate of altitude change with the barometer. If needed, we can measure the slope of the road by seeing how many barometer-lengths it takes to rise or fall a single barometer length.
- Use a piece of cardboard to construct a primitive pitot tube, using the barometer to measure the pressure.
- Figure out the coefficient of friction of the barometer with the road. Drop it out the side and measure how long/far it takes to slide to a stop.
- Attach the barometer to a string, and dangle out the window. The angle it forms with vertical due to airflow will relate to the car’s speed.
- Toss the barometer out the window towards a stationary person. Develop a model relating the amount of pain they experience with the car’s speed.
I’m sure we can come up with many more ideas.
Attach a microphone to the car. Drop the barometer from the moving car and record the noise it makes as it impacts the ground. Do a similar drop from a a stationary car and perform an FFT analysis and comparison of the two sound spectra. The difference is Doppler shift and can be converted to a horizontal speed / velocity by straightforward math.