Does a speedometer measure a scalar or vector quantity?

[zomgwtf]

This is a pretty elementary question but I had it on a quiz and it made me think... does a speedometer measure a scalar or vector quantity?

I answered that it measures a vector quantity my rational being that it is the instantaneous speed in a forward direction, always. It doesn't matter if the car is turning the measurement on the speedometer is always going to be instantaneous speed in the tangental direction right? It's not like it's a measurement over a given amount of time because instantaneous is taken with the limit of t approaching 0 right? I'm not entirely sure if I answered correctly though :tongue: but I still stand by my answer, it makes sense to me atleast.

 

[Astronuc]

Speedometer does not measure direction (orientation) only magnitude of velocity (speed).

 

[zomgwtf]

Speedometer does not measure direction (orientation) only magnitude of velocity (speed).

Isn't the direction assumed? I mean it goes positive forwards or positive backwards, that's it since it's instantaneous.

 

[lisab]

Isn't the direction assumed? I mean it goes positive forwards or positive backwards, that's it since it's instantaneous.

The direction can be north, south, east, west, even up and down a little bit. None of that is measured by a speedometer.

 

[Klockan3]

Isn't the direction assumed? I mean it goes positive forwards or positive backwards, that's it since it's instantaneous.

Think a bit, which direction is "forward"? Would the speedometer measure a different quantity if I went to the north or to the east?

 

[Proton Soup]

it's a scalar. but if you really want to confuse yourself, consider that it's not even measureing the actual speed of the vehicle, but the rotational speed of some element of the drivetrain. on my vehicle, the speedo cable attaches at the transmission.

 

[BobG]

it's a scalar. but if you really want to confuse yourself, consider that it's not even measureing the actual speed of the vehicle, but the rotational speed of some element of the drivetrain. on my vehicle, the speedo cable attaches at the transmission.

I second what Proton Soup said: It's a vector! Your speedometer actually measures angular velocity, which is a vector aligned with the rotational axis. :lol:

No, what is displayed on your gauge is a magnitude, regardless of the mechanics used to measure it (for one thing, more than one possible speed is possible for the same rotational velocity, depending upon what gear you're in).

Does your speedometer register a speed when you're going in reverse? (I've never thought to look, seeing as how I'm usually looking for a nice new Lexus to back into.)

 

[Anonymous217]

Take a speedometer and use it to measure the speed as you travel forward. Now use it to measure the same speed as your travel backwards. Is there any indication on the speedometer to differentiate direction? In other words, if you looked solely at the speedometer, would you be able to tell when you traveled forwards and when you traveled backwards?
To be more technical, the speedometer does not measure the angular velocity, but the magnitude of the angular velocity.

 

[S_Happens]

No, what is displayed on your gauge is a magnitude, regardless of the mechanics used to measure it (for one thing, more than one possible speed is possible for the same rotational velocity, depending upon what gear you're in).

Does your speedometer register a speed when you're going in reverse? (I've never thought to look, seeing as how I'm usually looking for a nice new Lexus to back into.)

I've never seen a case where the speedometer mechanism wasn't located on the transmission tailshaft (output), which means that it's rotational speed is fixed for any reading on the speedometer. If it was located on the input shaft somehow then the gear would need to be identified to determine actual vehicular speed, which adds quite a bit of complication with no real benefit.

I am not familiar enough with older eddy current style speedometers to comment on them, but certainly modern electronic speedometers can read magnitude in reverse exactly as they do forward.

 

[Loren Booda]

At worst, a speedometer measures a component of the velocity vector.

At the input, (e.g., the transmission tailshaft) the vector of angular velocity eventually manifests at the readout in a form (e.g., electromagnetic) usually foreign to the transmission but still conserving some semblance of the initial rotation.

Back in the day, my mechanic-acquaintance would use a drill to reverse mileage.