Is it possible to measure the instantanious speed? Like, when a car is moving along, to figure out its speed, you need to know how far it moved in a set time. How can you find how fast it is traveling at an INSTANT in time. I am stuck because it must have a speed but isn't speed defined as the ditance traveled in a certain amount of time? What if time = 0? Any thoughts?

nope
just a stoned giggle

calculus. or limits if you're the masochistic type. but those are just extremely good approximations. there's really no way to get perfect instantaneous speed, since there is no such thing as an infinitely small slice of time. I think the Planck time is about as close to the quantum unit of time as we have, and it's non-zero of course. so barring any further drolling on I'll just say "no"

-IggDawg

I actually read a new scientific theory in Popular Science a couple months ago. Perhaps time does equal 0. It doesn't exist. There is an infinite amount of still frames and our consciousness is passing thriough each of them. But the thing is, it doesn't even have to happen in order. For exampine, if in one frame you are falling, you'd think in the next frame you'd land, but perhaps in the next frame you're waking up, but IN THAT FRAME you have the memory of going to sleep the night before, and no memory of falling. So time would be 100% illusion...

Originally posted by Jimbothegreat
Is it possible to measure the instantanious speed? Like, when a car is moving along, to figure out its speed, you need to know how far it moved in a set time. How can you find how fast it is traveling at an INSTANT in time. I am stuck because it must have a speed but isn't speed defined as the ditance traveled in a certain amount of time? What if time = 0? Any thoughts?
It is not possible to build an instrument which measures instantaneous speed, but it possible to work with the concept theoretically.

- Warren

Originally posted by notme2000
I actually read a new scientific theory in Popular Science a couple months ago. Perhaps time does equal 0. It doesn't exist. There is an infinite amount of still frames and our consciousness is passing thriough each of them. But the thing is, it doesn't even have to happen in order. For exampine, if in one frame you are falling, you'd think in the next frame you'd land, but perhaps in the next frame you're waking up, but IN THAT FRAME you have the memory of going to sleep the night before, and no memory of falling. So time would be 100% illusion...
This is an example of why one should not read Popular Science to learn about metaphysics....

- Warren

The whole theory was very toungue in cheek, they didn't pass it off as credible or anything.

The heisenberg uncertainty priciple would say- no, you cannot know exactly the position and velocity.

The uncertainty principle is a perhaps a convenient way to describe the way that , as in a reel of film, an event (single frame) give you no exact information about the previous events (frames).
The new thinking is that time it self is a quanta ( 10 power -42 seconds) and that it is actually a vibrating string/membrane (?) and forms the space/time from which `particles` ( different vibrations of this) arise.

What do you think a speedometer on your car measures?

What do you think a speedometer on your car measures Movement. Without time, there is no movement, thus no speed. Let's say time froze as a car was going 200 miles an hour, when time continued would the car STILL be going 200 miles or would it stay stationary?

Originally posted by hlreed
What do you think a speedometer on your car measures?
It measures the number of times the wheel turns in a small interval of time. It measures an approximation to the instantaneous speed.

- Warren

If you are at x, then your velocity is dx/dt no matter how you are moving.
That is what a speedometer does.

Originally posted by hlreed
If you are at x, then your velocity is dx/dt no matter how you are moving.
That is what a speedometer does.
No real speedometer does this. Only an ideal one.

- Warren

Warren,
You must not like machines. You are one you know.

Originally posted by hlreed
Warren,
You must not like machines. You are one you know.
Uh what? Crack open an engineering book, skippy.

- Warren

Originally posted by hlreed
If you are at x, then your velocity is dx/dt no matter how you are moving.
That is what a speedometer does.

Oh yeah? What if you were moving in place, aka rotation ? Then x would not change, yet you would still have a velocity, measured in cycles per time unit.

Also, the spedometer only measures the wheels spinning. It wouldnt work in the case of skidding with your tires locked up even though the car is still moving.

I am assuming in this example you are not able to measure anything using a time interval?? since velocity is the rate of change of position (dx/dt) it would seem that you can not measure the speed...

what if you were able to measure the kinetic energy of the vehicle at time t = 0 (by destroying it??) if you could measure that then you would have the speed it was travelling at the time you measured it's kinetic energy??

Measure the air resistance. :bugeye:

Upon reading the replies since my post I'm going to have to go with the unceartainty thing. I hadn't thought of that.

But as far as getting the best working approximation which is all that matters anyways, I think calc would be your best bet. get an accelerometer and tally the frame by frame readings. then integrate. That's really the best way to go about it. if you tried to get a REAL instantaneous speed reading, you'd end up with some unsolvable equation due to quantum mechanics.

-IggDawg

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)?

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...

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.

Originally posted by cephas1012
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.

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.)

Originally posted by lethe
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.)

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.