# Instantanious speed

Discussion in 'Physics & Math' started by Jimbothegreat, Jan 17, 2003.

1. ### myhrRegistered Member

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22
If we do not count quantum effects, why can't we measure the instantanous speed using some magnets and coils and reading a voltage (e.g. in a speedometer)?

3. ### letheRegistered Senior Member

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2,009
yeah, i think mechanically there is no way to measure speed.

but e+m provides a few ways to measure exact instantaneous speed, right? like radar guns measuring doppler shifts? or in theory, you might be able to measure the deflection of the car in a magnetic field?

or does that just move the measurement question from one place to another...

5. ### cephas1012Registered Senior Member

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161
lethe,

you could not use doppler shift to get instantaneous speed. If you have a radar gun and are mesauring the speed of a car and it is changing its velocity then you would get a varying frequency difference. So then the problem would become finding instantaneous frequency. Good thought though.

I have one way to measure intstantaneous speed. Have a constant velocity. heh.

7. ### letheRegistered Senior Member

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2,009
i think i see what you mean. so to know exactly the velocity using doppler shift, you would have to have an instantaneous radar pulse, right? any finite length radar pulse would have rate of change associated. so we just move the problem of measuring instantaneous quantities from the car to the radar gun.

but what about magnetic deflection? i still think that would work (although it s kind of ridiculous. i don t know about your car, but mine is electrically neutral.)

8. ### cephas1012Registered Senior Member

Messages:
161
Ya, something like that. As for the magnetic thing I am not even sure, I dont know enuogh about it yet.

However, I do have a real idea for measuring instantaneous velocity now. Its kind of theoretical but maybe it would work...

If you know the acceleration of your car for how hard your pushing down the pedal and you can make an equation for it then you just have to integrate I think. Then you have velocity. Of course, this would only work from a dead stop and accelerating at the same rate or some rate you have a "perfect" matching equation for...so it wouldnt be highly useful.