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which is accurate? Camera, car spedo, sat-nav?
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Bengali
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PostPosted: Fri Jan 08, 2010 1:20 pm    Post subject: which is accurate? Camera, car spedo, sat-nav? Reply with quote

I notive that the speed on my Sat Nav is lower than on my car spedo. I assume that the Sat nav is tyhe more accurate because it calculates movement over the ground. Hills will bring in inaccuracy perhaps. My car speedo will vary as the tyres wear and their effective diameter and therfore circumference changes. Which should I use?
By the way; speed is the average of velocity, to be very simple and as it is mathematically incorrect to try to have an average of averages, how can specs register "average speed". Surely they measure SPEED. ie The distance travelled and the time taken( Miles per Hour)
Another thought. If speed is the problem in a particular place and it is a danger it should be stopped. If a camera is installed and it registers speeders it is not doing its job. The only successful cameras are those that don't earn. Those that earn are failing and a better method should be found. Perhaps we should have a man with a red flag walking in front. Why did that stop?
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GerryC
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PostPosted: Fri Jan 08, 2010 1:32 pm    Post subject: Reply with quote

Questions, questions. Laughing Welcome to the forum.

Satnavs calculate the speed based on the distance travelled between receiving timed position updates. Car speedos generally take it from some engine part. The important thing is that it must NOT show under-speed and can show over-speed upto 10% (I think). Tyre wear etc is an important part of this.

Which to use? Well, the speed limit is a speed you should not go over but can be under it. For this reason, I usually go by the car speedo.


Normal cameras capture the distance travelled in 1/2 a second (set time, variable distance). The difference with Specs is that it does this twice some distance apart (set distance, variable time). Knowing this distance and the time it took you to get from one to another gives the "average" speed through the section. This is to stop sudden braking for a camera then speeding up again.


I won't go into accident blackspots but the man with a flag ended when cars could manage to go at more than walking pace. Very Happy
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PaulB2005
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PostPosted: Fri Jan 08, 2010 2:08 pm    Post subject: Reply with quote

Quote:
By the way; speed is the average of velocity


No it's not. Speed is a measure of distance over time. Hence you use terms such as Mile Per Hour. No averages involved.

Velocity is a measure of Speed in a given direction. i.e. 50 mph in an easterly direction.

If an object travels at 50 meters per second in a northerly direction and then turns east maintaining 50 meters per second the speed hasn't changed but the velocity has.

Speed is NOT an average of velocity. Such a calculation makes no sense.

Everything else after that is based on this "average of average" error and is also wrong. SPECS measures your Average Speed.
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peterc10
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PostPosted: Fri Jan 08, 2010 3:46 pm    Post subject: Reply with quote

Wikipedia is our boring friend.

http://en.wikipedia.org/wiki/Velocity

Here

BTW how can I put in a hyperlink to a word in this forum?

Edit to add hyper link - Mike Alder
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mikealder
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PostPosted: Fri Jan 08, 2010 4:12 pm    Post subject: Reply with quote

Peter, click the edit button on your post and you will see what I did, I left the original there for you to spot whats different - Mike
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peterc10
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PostPosted: Fri Jan 08, 2010 6:07 pm    Post subject: Reply with quote

mikealder wrote:
Peter, click the edit button on your post and you will see what I did, I left the original there for you to spot whats different - Mike

Mike - do I have to type that lot out? I was assumimg I could just highlight text and hit a button (or right click) like I can on most other things.

I thought the URL thingy would work but that just replaces what I typed
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MaFt
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PostPosted: Fri Jan 08, 2010 6:19 pm    Post subject: Reply with quote

peter - simply type the following:

Code:
[url=http://www.LinkWebsiteHere.com]Link Text[/url]


you could also simply press the 'URL' button and add the = sign then paste the URL, add the Link Text and press the URL button again to close it off.

MaFt
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peterc10
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PostPosted: Fri Jan 08, 2010 6:30 pm    Post subject: Re: which is accurate? Camera, car spedo, sat-nav? Reply with quote

Bengali wrote:
it is mathematically incorrect to try to have an average of averages,

It is quite possible to average averages if you wish

Take a simple calculation like journey time. 6 people make the journey. Each has a different average speed (average because in each case their speed varies throughout the journey). You can then average those average speeds.

And don't even get me started on means, modes and medians ............ (because I will have to go up in the loft to look at my Uni notes - written on parchment with a quill pen) Wink
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mikealder
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PostPosted: Fri Jan 08, 2010 7:10 pm    Post subject: Reply with quote

I only type the [url= and then hit CTL V to paste the link URL in to the text followed by Here [/url]
Note I will have put the target URL on the clipboard usually with a double click to hichlight it and pressing CTL C

Then I double click the word "Here" and hit the B button to make it BOLD and repeat click the "Here" then use the Font Colour box to select the red (not really necessary as the forum code can do this on its own but old habits etc, etc With practice it takes only a few seconds - Mike
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AliOnHols
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PostPosted: Sat Jan 09, 2010 8:30 am    Post subject: Reply with quote

PaulB2005 wrote:
If an object travels at 50 meters per second in a northerly direction and then turns east maintaining 50 meters per second the speed hasn't changed but the velocity has.

If I remember my physics classes correctly, doesn't the change in direction also means that it is accelerating despite the constant 50m/s?
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M8TJT
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PostPosted: Sat Jan 09, 2010 8:52 am    Post subject: Reply with quote

Yes. The direction is changing, so the velocity is changing so there is an acceleration towards the instantaneous centre of the turn. This is usually called (wrongly) centrifugal force.
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DennisN
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PostPosted: Sat Jan 09, 2010 11:40 am    Post subject: Reply with quote

WOT?????

I woz just gonna answer the OP with "Yes" Confused Now I'm struggling to decide how many beans make five.
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MaFt
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PostPosted: Sat Jan 09, 2010 11:57 am    Post subject: Reply with quote

we're drifting off-topic here...
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BigPerk
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PostPosted: Sat Jan 09, 2010 12:17 pm    Post subject: Reply with quote

Yes, although it has to be said that it went off topic straight after bengali's own first paragraph Cool !

I remember seeing a thread a month or so back answering this same question - it seems to be a topic second only to 'why haven't I got life membership?' Razz
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GerryC
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PostPosted: Sat Jan 09, 2010 5:52 pm    Post subject: Reply with quote

Bengali, Please don't be put off by the comments on this thread - it's nothing personal. Sometimes, someone just says something that gets a conversation going. Please feel free to respond to the thread or any others that take your fancy. Very Happy

Gerry
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