intertia vs electricity

[Journer]

so i'm curious which is faster, inertia or electricity...hopefully i am using the right terms.

lets take this example:
a car has a fuel shutoff sensor in it's rear bumper. it trigger when a pendulum passes a point which is hit when a car collides at about 30mph. the signal goes to the computer which shuts off the fuel pump.

the car is rear ended at 30mph. what happens first?

the cars fuel pump is shut off

the fuel in the car's lines lunges (term?) forward (because of the impact) and momentarily creates more pressure in the fuel rail (potential causing a bigger explosion/fire hazard, if a fire was present)

 

[BassBomb]

I think you mean momentum and not inertia

 

[herm0016]

the tiny amount of fuel in the line is not going to move enough to make a difference. once the pump is off the line is under no more pressure. it probably all happens at about the same time, moments in between are not long enough to matter.

 

[TuxDave]

So if we're dealing with a race condition between an electric signal from the back of the car to the front and the rate at which the front of the car (where the fuel is?) starts to lurch forward after impact

Electricty takes a finite amount of time to propagate (the speed of light roughly). Fortunately for you, relatively is on your side because the front of the car can never INSTANTLY start moving forward after an impact at the rear. (otherwise the front of the car has received information from the back of the car at a rate faster than the speed of light)

The propagation of the impact through the car (given the most rigid car imaginable) can also at best move at the speed of light. However since the medium of travel (the car body) is not perfectally rigid (due to the crumpling you see) you can conclude that the propagation is actually much slower than the speed of light.

Therefore I conclude that the electric signal triggering the turn off will get there first and then it becomes a race condition between how fast the computer can shut it off.

 

[Journer]

maybe i do mean momentum. now, maybe my example was bad, because too many factors are involved...so...lets try this

lets say you have two solid rocks back to back, you have a wiring running the same length.
you send a pulse down the wire at the exact MS the rocks are hit on one side with a hammer

does the other end of the rock vibrate before the electrical signal reaches the other end?
or does the signal reach the other end quicker than the energy transmitted through the rocks from the hammer?

tiny moments in time do matter. i think this is a valid question :/

 

[TuxDave]

Originally posted by: Journer
maybe i do mean momentum. now, maybe my example was bad, because too many factors are involved...so...lets try this

lets say you have two solid rocks back to back, you have a wiring running the same length.
you send a pulse down the wire at the exact MS the rocks are hit on one side with a hammer

does the other end of the rock vibrate before the electrical signal reaches the other end?
or does the signal reach the other end quicker than the energy transmitted through the rocks from the hammer?

tiny moments in time do matter. i think this is a valid question :/

Same idea applies. For the wire you're dealing with EM wave propagation on a wire which moves close to the speed of light. For the vibration, you can treat it as the compressive wave velocity through the rocks. (again it still takes finite time for vibrations to propagate through any medium and is at best limited by the speed of light no matter how rigid the rock is).

To provide some numbers to back up what I'm saying:

http://www.ndt-ed.org/Educatio...sics/elasticsolids.htm

Examples of approximate compressional sound velocities in materials are:

Aluminum - 0.632 cm/microsecond
1020 steel - 0.589 cm/microsecond
Cast iron - 0.480 cm/microsecond.

While the EM wave will propagate at roughly the speed of light (3*10^8m/s)

You can compare the two numbers. On a side note I would like to make sure we understand that the individual electron is not moving from one end of the wire to the other at that rate. It's more like the wave propagates through the wire at C making the electron at the far end tumble out.

 

[Rubycon]

IIRC the safeguard on an autocar is triggered by a lack of oil pressure. So if the engine stops while the ignition is on (stall condition) the low oil pressure will turn OFF the electric fuel pump in the tank.

Additionally unless the lines are suddenly crimped there will be no sudden spike in fuel rail pressure. (think of the old petrol filling stations that had those rubber hoses in the driveways when driven over - the air would trigger the bell)

In either case light moves much faster than sound.

The response time of the electronic circuit and electromechanical relay would be in the order of tens of milliseconds real time. A solid state relay would trim a few tens of ms off the total time given the original scenario.

 

[Journer]

cool, looks like i got the answer i was looking for

 

[Journer]

ok, now i have another question on measuring the speed of vibrations or whatever it should be called.

ok...in the above post, tuxdave mentions the speeds of waves through different objects. well, obviously something that quick has to be measured by electronics. but, wouldn't there be a delay between the time the vibration reached the meter and the time the meter actually felt it? now, of course, this would be bajillionths (yes, bajillionths ) of seconds, but some amount of time non the less. wouldn't that mean that any speed measured by a device is off, even though it is extremely minute, it is off a statistically unimportant amount of time.

 

[Stiganator]

Nothing is perfect. No calculation is ever perfect. At 5 orders of magnitude above or below, there is no difference for any non nanotech/physics type application.

 

[TuxDave]

Originally posted by: Journer
ok, now i have another question on measuring the speed of vibrations or whatever it should be called.

ok...in the above post, tuxdave mentions the speeds of waves through different objects. well, obviously something that quick has to be measured by electronics. but, wouldn't there be a delay between the time the vibration reached the meter and the time the meter actually felt it? now, of course, this would be bajillionths (yes, bajillionths ) of seconds, but some amount of time non the less. wouldn't that mean that any speed measured by a device is off, even though it is extremely minute, it is off a statistically unimportant amount of time.

I guess you could setup an experiment that'll negate that offset. So you attach a similiar meter at the start point and end point and record times.

The first meter will have: time you hit it + time it takes to get to the meter from the bar + time to process it

The second meter will have: time you hit it + time it takes to get across the bar + time it takes to get the the meter from the bar + time to process it

Then when you subtract the two you get "time it takes to get across the bar".

But yeah I agree precision is never perfect in most experiments.

 

[PottedMeat]

Originally posted by: Journer
ok, now i have another question on measuring the speed of vibrations or whatever it should be called.

ok...in the above post, tuxdave mentions the speeds of waves through different objects. well, obviously something that quick has to be measured by electronics. but, wouldn't there be a delay between the time the vibration reached the meter and the time the meter actually felt it? now, of course, this would be bajillionths (yes, bajillionths ) of seconds, but some amount of time non the less. wouldn't that mean that any speed measured by a device is off, even though it is extremely minute, it is off a statistically unimportant amount of time.

Yes. Using TuxDave's steel number: .589cm/us = 5890m/s - if you were to put a sensor at the end of a 2 meter steel bar and hit the end, it would take 0.00034 seconds to propagate to the sensor. If the sensor is connected with a 2 meter cable and you assume the signal propagates at ~0.5*speed of light, the signal takes 0.00000000667 seconds to get to the end of the cable.

 

[Journer]

wtf are you people, quantum physicists or something. why the hell havent i studied this in college yet :/

lol, anywho, the information is interesting. i always figured no calculation could be exactly perfect, i mean, in the end we are limited by our human senses and whatever 'senses' we can read with technology (i.e. infrared)

 

[Crusty]

Originally posted by: Journer
wtf are you people, quantum physicists or something. why the hell havent i studied this in college yet :/

lol, anywho, the information is interesting. i always figured no calculation could be exactly perfect, i mean, in the end we are limited by our human senses and whatever 'senses' we can read with technology (i.e. infrared)

I'm guessing you haven't taken any physics classes yet

 

[Journer]

Originally posted by: Crusty

Originally posted by: Journer
wtf are you people, quantum physicists or something. why the hell havent i studied this in college yet :/

lol, anywho, the information is interesting. i always figured no calculation could be exactly perfect, i mean, in the end we are limited by our human senses and whatever 'senses' we can read with technology (i.e. infrared)

I'm guessing you haven't taken any physics classes yet

no..don't think i'm going to either. my gpa can't handle any more bad grades :/ although it is really interesting