18-02-2009, 05:16 PM
Peter Wrote:Jerry1237 Wrote:Can I ask a naive question. Why do we take de-acceleration of a train to be 0.5m/s/s?
Other than because the examiners tell us to? Do you mean "why do the examiners pick 0.5m/s/s for deceleration?" or "what is the basis of 0.5/m/s/s?"
For the former, I guess it is because it has to be something and, while still requiring you to be able to calculate things, it does not make the sums especially hard.
For the latter, yes, a modern passenger train may achieve 9% or in some cases 12% g braking giving 0.8 - 1.1 m/s/s, but then you'd have to apply a different rate for the freight.
What did you want to use?
I do think that it is rather unrealistic for the exam seemingly always assuming acceleration = braking rate, freight = passenger, service = emergency. The metro layout tends to have different rates for accleration, service and emergency braking.
0.5m/s/s is of course particularly handy when working out braking distance as division by "2a" is very simple! trying to recall whether we were allowed calculators in my day of doing the exam; I am not sure that we were (though they did exist by then but only just). To put things in context, there was one electronic calculator in my school which only those in the 6th form could use kept locked away in a room in the science labs; it was the size of a two desktop PCs stacked vertically, used wirewound neon tubes for display and used "reverse Polish" notation: [Enter first number] xxxx, [Enter first number] yyyy, [+ or - or / or *] and that was it!
Probably IRSE exam set in stone in that era!
Probably the truth is that the examiners find that many struggle with the calculations as it is so they daren't make things harder.
PJW