I see potential ambiguity in my post for stating "gross weight about 3kg" when I may have more accurately said "gross mass about 3kg" and a 3kg extinguisher still has a mass of 3kg when it is accelerated or decelerated at 20g. I could have included the word "mass" to be clear I wasn't talking about force. In terms of adhesion vs shear, it depends on the direction of the crash, but more likely it will be multiple impacts and directions that are neither completely working against the adhesion or shear resistance.
It could still ruin your day if it hit you which I think we agree on? If not we will start on how you denote your units
Whilst we're here, are you suggesting that the maximum damage done to the head from a 3kg extinguisher in a 20g crash is the equivalent to a reasonably slinky 60kg woman (or well dieted man) standing statically on a man's helmeted head? It isn't, the 3kg mass becomes a projectile that has been accelerated to a velocity proportional to the integral of force with respect to time. There is then a potential impact with the head (+- helmet) where it gives up most of its considerable kinetic energy through deceleration but the penetrating injury depends on the surface area of the presenting part of the projectile. That is why we see considerable skull fractures from accident victims from relatively trivial objects on the rear parcel shelf. Not a Scooby Doo toy, but try a wooden tissue box on the rear shelf like some of our older friends, or an umbrella with a pointy end. Or a fire extinguisher with pointy bits could end up with bits of brain on it, or bowel or aorta if it penetrates your relatively soft underbelly. The idea that it can only do 60kg worth of damage in a 20g impact is falsely reassuring.
Thanks for that john. it is what I was suggesting but knowing vernon, I'm sure he'll argue.
When considering such things it's much better to consider energy rather than forces.
The KE that must be dispersed is proportional to the square of the velocity and the hardness and presenting surface then becomes crucial.
They say a dog hits the back of your seat with the force of a charging elephant but the dog is still going to crumple and and hence absorb some of the energy as it deforms. A fire extinguisher will not.
I love a good discussion and am delighted to be corrected, awaiting Vernon's thoughts with interest as my physics is rusty, although my applied physics to penetrating injuries is part of the job and I did my pre hospital trauma and life support training under my former colleague who runs the medical centre at Knockhill. Some people will just look at the extinguisher tie down setup and dislike it or fail it (no disrespect to Barry P) and others will think it looks OK from giving it a tug, but it is relatively trivial appearing things like this that can cause injuries in an otherwise well considered situation.
Afternoon guys .
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