On Wed, 10 May 2006 23:46:00 +0100 someone who may be "Adam Lea" <[email hidden]> wrote this:-
Quoted message said:
Quoted message said:
If it shatters that does not absorb very much energy at all. The helmet has not done what it is supposed to do and it has not protected the rider other then to a minimal extent.
Is it possible to say exactly how much the helmet protected the rider without knowing at which point the helmet shattered.
"Adam Lea" <[email hidden]> wrote: .... | Is it possible to say exactly how much the helmet protected the rider | without knowing at which point the helmet shattered. | | The article says that the cyclist was dragged for 15 metres. If the helmet | was being crushed for 14 of those 15 metres and only shattered in the last 1 | metre then it did protect the rider. If it shattered upon impact then it was | useless.
If you assume a basically deccelerating situation then the greatest force on the helmet is likely to have been at the initial contact, and since the helmet shattered there must have been enough force to do so so the initial force was strong enough to do that, so it must have shattered at impact.
If you assume a basically deccelerating situation then the greatest force on the helmet is likely to have been at the initial contact, and since the helmet shattered there must have been enough force to do so so the initial force was strong enough to do that, so it must have shattered at impact.
I'd accept 'probably', but not 'must'. I dropped a cup on a tiled floor once - it bounced, then smashed on the second impact.
I'd accept 'probably', but not 'must'. I dropped a cup on a tiled floor once - it bounced, then smashed on the second impact.
I've seen this happen a number of times. Does anyone have an explanation for it? Is it simply the angle at which each impact strikes the cup?
I wonder if it's the propagation path of the shock wave in the cup from the impact. If it's allowed (ie, hits in such a manner as) to circle longitudinally round the cup, it'll dissipate without breaking the cup because it just goes round and round until it runs out of steam. If the shock wave heads from rim to base, the geometry will accelerate it until it breaks the cup.
I'd accept 'probably', but not 'must'. I dropped a cup on a tiled floor once - it bounced, then smashed on the second impact.
I've seen this happen a number of times. Does anyone have an explanation for it? Is it simply the angle at which each impact strikes the cup?
I think it is a Douglas Adams effect it happens so fast that the first impact you don't expect it to smash. Then by the second your brain has caught up and so it smashes. All you have to do is believe it won't smash on the second impact.
I'd accept 'probably', but not 'must'. I dropped a cup on a tiled floor once - it bounced, then smashed on the second impact.
I've seen this happen a number of times. Does anyone have an explanation for it? Is it simply the angle at which each impact strikes the cup?
It might crack on the first bounce, then smash on the second. I too have observed this. The best one was instead of bouncing it span round at high speed for a second or so, just long enough for you to think you'd got away with it, and then disintegrated.
I think it is a Douglas Adams effect it happens so fast that the first impact you don't expect it to smash. Then by the second your brain has caught up and so it smashes. All you have to do is believe it won't smash on the second impact.
Or perhaps the cup threw itself at the floor and missed on the first attempt and was so surprised it worked that it forgot to repeat it for the second impact ;-)
-- Tony
"The best way I know of to win an argument is to start by being in the right." - Lord Hailsham
I wonder if it's the propagation path of the shock wave in the cup from the impact. If it's allowed (ie, hits in such a manner as) to circle longitudinally round the cup, it'll dissipate without breaking the cup because it just goes round and round until it runs out of steam. If the shock wave heads from rim to base, the geometry will accelerate it until it breaks the cup.
I've no idea, I'm just guessing.
We need a population study!
Do cups need helmets?
-- Don Whybrow
Sequi Bonum Non Time
"I have come to the frightening conclusion that I am the decisive element. It is my personal approach that creates the climate. It is my daily mood that makes the weather. I possess tremendous power to make life miserable or joyous. I can be a tool of torture, or an instrument of inspiration, I can humiliate or humor, hurt or heal. In all situations, it is my response that decides whether a crises is escalated or de- escalated, and a person is humanized or de-humanized. If we treat people as they are, we make them worse. If we treat people as they ought to be, we help them become what they are capable of becoming." -Johann Wolfgang von Goethe
I think it is a Douglas Adams effect it happens so fast that the first impact you don't expect it to smash. Then by the second your brain has caught up and so it smashes. All you have to do is believe it won't smash on the second impact.
Or perhaps the cup threw itself at the floor and missed on the first attempt and was so surprised it worked that it forgot to repeat it for the second impact ;-)
Reminds me of the description of testing a scramjet engine - fire the plane up to about 200km high using a rocket and then fly it into the ground.
The first attempt went up ok but was a failure on the way down. :-)
Tim.
-- God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t," and there was light.
Don Whybrow <[email hidden]> whizzed past me shouting
Quoted message said:
Richard said:
I wonder if it's the propagation path of the shock wave in the cup from the impact. If it's allowed (ie, hits in such a manner as) to circle longitudinally round the cup, it'll dissipate without breaking the cup because it just goes round and round until it runs out of steam. If the shock wave heads from rim to base, the geometry will accelerate it until it breaks the cup. I've no idea, I'm just guessing.
We need a population study! Do cups need helmets?
Back in about 1970, a friend's school had a population of "unbreakable" drinking glasses that could be dropped from a great height without them breaking - but only once. The second time they were dropped, even from a foot or so, they'd shatter dramatically. Both phases were put to good use by ingenious teenagers.
-- Sue ];(🙂
Bicycle helmets are really a bit of a scam. They make most cyclists slightly less safe but there's money in selling them.
I have always wondered why Giro a few years back were selling a packing case to store a helmet in. Maybe a study by Thompson, Rivara et al could be commissioned to see if they work (85% of the time).
David Hansen said:
On Wed, 10 May 2006 12:37:08 +0100 someone who may be Paul Boyd <[email hidden]> wrote this:-
Quoted message said:
Quoted message said:
So his helmet shattered - great, it was really useful then!
They're meant to do that.
Incorrect.
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It means the helmet absorbed the impact rather than the skull.
Incorrect. It means that the helmet absorbed very little energy and what it didn't absorb was passed on to the skull.
A helmet that had been crushed would be a different matter.
The mechanics are fairly simple. Next contestant please.
