From: Tony Raven on
Ben C wrote:

>> So its equivalent to holding your body horizontally 15" above the
>> floor and letting it drop.
> Not exactly I don't think.
> There are two main reasons why holding a whole person up by their feet
> is different to just dropping their head on its own: (a) their centre of
> mass will be much higher than 376mm (but only a bit higher in the case
> of the severed head), and (b) the whole person weighs more than just the
> head.

Yes but in your vertical drop scenario during the initial impact during
which the head is decelerating on impact the body can be assumed as a
rigid body dropping vertically 21mm so my estimate holds. Thereafter
the whole body will crumple, roll on the head and flex at the neck to
dissipate the rest of the potential energy but at that stage the
critical impulse to the skull is over so the potential energy of the
body is irrelevant to first order. Your assumption is that the body
continues to pile drive the skull downwards AND do the unlikely thing of
concertinaing itself vertically as it does so.

> Dropping them from a horizontal orientation fixes (a) but not (b) (and
> there are some other complications since the neck is not rigid).

It depends on the neck not being rigid in that posture so the body hits
the ground but the head continues downwards by flexing the next until it
too impacts the ground. The flexible coupling means in this case you
can to first order consider the head as falling quasi-independently of
the body.

> So it would be
> equivalent to quite a bit less than 21mm.

Which would make it even less severe.


" I would never die for my beliefs because I might be wrong."
Bertrand Russell
From: JMS jmsmith2010 on
On Sun, 23 May 2010 06:57:15 +0100, Tony Raven <traven(a)>

>Derek C wrote:
>> Looking at it another way, wearing a cyclehelmet increases the
>> distance your head has to fall to cause a skull fracture by a factor
>> of at least four (source TRL). Worth having I believe!
>Another piece of junk science from that report given meaning by you far
>beyond what is claimed.
>What TRL claim, and all that they claim, is an un-helmeted head will
>suffer a skull fracture in a fall from >15" (376mm) onto a flat surface
>and that with a helmet it will be 5ft (1.5m) (actually the best case
>from the 8 helmets they tested).

Hello, hello, hello - it's that demon for accuracy and common sense Mr

Surely you mean >150" - and is that 15m the best case?)


6,000 deaths a year or 16 a day are caused
by people tripping up on pavements : Tony Raven

From: Roland Perry on
In message <40e8v55ohlqbt3ek4ldcov5tq1c5rgl2i7(a)>, at 20:13:14 on
Wed, 19 May 2010, JMS <jmsmith2010(a)> remarked:
>>I will, but only when I get home and have the book in front of me.
>Are you not home yet?

No, on my travels. Currently en-route to India for two weeks, spent last
week in Geneva; so you'll just have to be patient.
Roland Perry
From: Derek C on
On 22 May, 13:32, %ste...(a) (Steve Firth) wrote:
> Sniper <Sniper8...(a)> wrote:
> > The best that can be said of cycle helmets is they may save some minor
> > injury...
> Ah yes, that old chestnut.
> the best that can be said of helmets is that compelling cyclists to wear
> helmets reduces the incidence of death among cyclists by 45-50%. Given
> that the root cause of death in cycling accidents can be both head
> injury (most commonly) or trauma to the body, that figure is quite
> astonishing, it shows that a simple preventative measure drastically
> alters post-accident outcomes.
> See: "The Effectiveness of Bicycle Helmets:A Review", Dr. Michael
> Henderson [ISBN 0 T310 6435 6]
> Cue the psycholists whining that Guy Chapman's completely made up
> statistics trounce any medical study.

These studies might prove useful:

They show that cycle helmets reduce injuries to the skull, brain, and
upper face by significant factors. Lower facial injuries are still
common, so helmets would be be better if they included face and chin
guards. Hard shell helmets provide better protection than soft shell

Derek C
From: Peter Clinch on
Derek C wrote:

> These studies might prove useful:

Well, they might, but many of them are from a source that's been
responsible for some /highly/ questionable work in the area.

As you'd know if you read the originals with a critical eye, rather than
stopping at the nice sounding headlines.

Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net p.j.clinch(a)