Re: "Speeding" on bicycles (was: Re: Sustrans Blues)

Ian Smith wrote:
On 13 Jun 2006 02:59:56 -0700, The Luggage <alan.collier@xxxxxxxxxx> wrote:
Ian Smith wrote:
On 12 Jun 2006 08:49:23 -0700, The Luggage <alan.collier@xxxxxxxxxx> wrote:
[snip]
No, you're using data serived from one particular category of vehicle
- namely road-going vehicles, which have a mass of less than (about)
40 tonnes, and assuming that the data holds valid for all categories
of vehicle, including 2000 tonne trains. That is, your data DOES
quote type, implicitly - road-going vehicles, not 2000 tonne ones.

JOOI, are we having this discussion to score pedant points, or do you
really believe that a person is less likely to survive a 25 mph hit
from a goods train than a 75 mph hit from a railbus?

I think it is not obvious which is 'more likely' to kill you - I would
guess that you are almost certain to be killed by either. However,
you seem to believe it obvious that one is more lethal, and I'm
intersted to determine why.

Fairy nuff.
There are two parts to my argument. The first is that plenty of people
survive impacts with objects at 25 mph. Whether that object is a wall,
bus, car or train is largely irrelevant. I only put the train in
because it was easier to disprove the effect of mass (and kinetic
energy) when the mass could be increased by a factor of 30.

The second is the effect of momentum and energy tranfser I mentioned in
an earlier post. If you are hit by a vehicle that is significantly
heavier than you, the vehicle's velocity will not change much (the
momentum bit). The energy transferred to you will only be enough to
accelerate you to the velocity of the vehicle, regardless of how much
KE the vehicle has to start with.

Therefore for the train example, if you're hit by the goods train at 25
mph, a 100kg person will receive 6.23 kJ of energy in the impact. The
same preson hit by a railbus at 75 mph will have to absorb 56 kJ of
energy. Which of these is more surviveable? The 6.25 kJ impact from the
goods train, despite the fact that it has 3x more initial KE, which is
where we started.

Even at 10 mph, the goods train has more KE, and is highly likely to be
more surviveable than the 75 mph impact.

So far, it seems to be because you're extrapolating wildly from
inappropriate data (specifically, extrapolating two orders of
magnitude outside teh available figures for the parameter under
question when the data available covers only about one order of
magnitude). If that is the sole basis for your assertions, I can
confidently regard your opinion as worthless. However, if there had
been (or indeed, still is) some other basis behind your assumptions,
it would have been interesting.

The most important factors in whether you will be killed by an impact
are the speed of impact, and possibly the shape of the object you're
being hit by, not the mass or KE of the object. I don't consider the
flat front of a train to be significantly different to the flat front
of a bus or lorry, and plenty of people (though not all of course)
survive being hit by them at <30 mph

I'd already pointed to my other posts on the subject of momentum and
energy transfer, but have gone back through it above to show that there
is a sound mathematical reasoning behind by assertions.

TL

.

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