# Re: Bizarre coincidence, but saved by their helmets.

On 28 Sep, 18:29, Derek C <del.copel...@xxxxxxxxxxxxx> wrote:
On Sep 28, 6:25 pm, Marc <webtr...@xxxxxxx> wrote:

On 28 Sep, 16:57, Derek C <del.copel...@xxxxxxxxxxxxx> wrote:

Agreed, which complicates your problem in that there are multiple and
changing centres of movement and rotation.
So how do you end up with the statment "you will be thrown off head
first"? I would be interested in seeing your diagram showing the force
vectors....

So are you denying that you stand a good chance of being thrown head
first over the handlebars, if your bike stops suddenly at any
appreciable speed? This is due to inertia plus a rotational couple.

I'm not denying anything, I'm asking you to prove, using physics and
not "common sense" your assertion that "you will be thrown off head
first". Do you need more time for your diagram?

Newton's first law states that a body remains at rest or in a constant
state of motion until an external force acts upon it. A cyclist
travelling at speed is more or less in a constant state of motion, or
in everyday terms has inertia. His centre of mass will be above the
centre or road contact point of the front wheel if it's a conventional
bike. If the front wheel stops suddenly for any reason (i.e
experiences a force in the opposite direction to the direction of
travel), the bike will stop, but the cyclist, not being restrained,
will continue on forwards due to Newton's first law, or inertia, and
probably rotate around the front wheel as well. Gravity (another
external force) will pull him down to the ground. Generally the height
from which you fall and the suddeness of the stop are more significant
than the forward speed in causing head injuries. Sliding along the
deck once you have landed will only scrape off a bit more skin.

A simplistic view that fails to take into account the rotation of the
bike around the front wheel, the contact points of the rider and the
rigidity of the rider.

Not according to Tony Raving, who seems to think that the rotational
effect is non-existant!

Derek C

Please don't try create a sidetrack of strawmen. I would be interested
in seeing your diagram, complete with the vectors.
.

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• Re: Bizarre coincidence, but saved by their helmets.
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