[blog posts; previously published online]
(published 1 December 2010)
If I drive from home to the University of Surrey and you want to know where I am at a given time on my journey then if you know my speed and what time I left home you should be able to calculate roughly where I have got to. (Bear in mind that the queue at the Costa coffee at Exeter Services can be desperately slow, so perhaps factor in a random variable for that). However, if you want to know where I am exactly at any given time, you will not be able to know how fast I am going at that point because in effect you have to stop time at the moment you want to know where I am, in order to accurately calculate my position. And similarly if you want to know how fast I am travelling (relative to the road, I believe) you will have to accept that my position at that time will remain approximate, since calculation of my speed requires that I am moving past any given point as you catch me on your speed camera.
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