Maelochs

Awesome! Thanks!

badger1

Sir Bedevere:
...and that, my liege, is how we know the Earth to be banana shaped.

King Arthur:
This new learning amazes me, Sir Bedevere. Explain again how sheep's bladders may be employed to prevent earthquakes.

cyccommute 

I don't see any thing that is phrased "belligerently".

Here's your answer. Let's assume a 50kg rider traveling at 9 m/sec (32 kph or 20 mph). If we assume that the time to stop takes 0.1 second, the impact is 4500N. If we extend the time to 0.2 seconds...which is a very long time for sliding at 20 mph...the force of the impact is 2250N. For the metrically challenged, that 1100 pounds of force and 500 lb of force. Neither is going to leave the rider in great shape.

The time of the initial impact is fleeting. You aren't going to be sliding on pavement for more than fractions of a second. You'd slide longer in your ice scenario but there is still a lot of force put on the body at 200 mph. At 100 mph on pavement, the stopping time would take longer but the force on the body would be significantly larger.

At 200 mph ice isn't going to frictionless. Nor would it be all that smooth. A human body dropped from 2 feet at 200 mph is going to go through significant deceleration, probably lots of tumbling and end up broken and battered. Same thing happens at 100 mph. Look at the injuries that can result from a relatively short drop from a bicycle at only about 20 mph.

wphamilton

That's not even close, sliding or rolling, and at least an order of magnitude off for rolling.





You're getting there, realizing that the "200 mph" part is entirely separate from the "2 ft fall" part. ALL of the horizontal momentum is taken up by friction and by whatever rises from the pavement. NONE of the horizontal momentum is exchanged from the impact.


Now, to test your idea of "significant deceleration" and the tossing about, how much of your speed is reduced in say, 1 second, sliding on the flat, nearly frictionless, wet ice? Almost none? That means your change in momentum is also "almost none", which means you wont have those violent forces throwing you about.

63rickert

All accidents are different. The examples in my mind were, first, a riding companion maybe ten feet in front of me, stayed with bike, catapulted straight into car with his head, facial injuries, fractured skull, brain injuries, cervical vertabrae injuries, spinal injuries. Repetitive major surgery. Extended rehab. Full recovery, yay. Same happened to me couple years later. Very low car, an old MGA, would have gone over. And it was a steep hill. Catapulting over that car the landing zone was maybe 20, 30 feet lower. Like jumping off high board into an empty pool. Narrow little street, wasn't going that fast. Then neither was my friend. Got off bike, no problem. Bike catapulted over car, clattered down hill. Cosmetic damage only.

A much earlier incident I changed course so hard succeeded in pretzeling a Cinelli 1/A stem. Basically screwed bike into ground. Some other damage to bike. Again no injury to me. No way I could decipher if a rear exit off the bike would have worked better on that occasion.

Rear exit is not something most would even think of. Surprisingly easy once tried.

Oh, I found store stock for that large helmet. It fits. First one that fits in ten years. I have looked before. So thank you again.

63rickert

Going over is easiest. Works quite well. Done it, three occasions. Does not injure riders in the pile either. What is required is a lesson. Once you see someone else do it seems very simple. And is.

noglider 

I'm laughing at myself, embarrassed.

wphamilton

You can laugh at me for even getting into this. Nobody here wants a physics lecture from me, least of all me.

mr_bill

Does anyone want yet another Aikido on Ice lecture?

-mr. bill

noglider 

OK, back on topic, as I said earlier, rehearsal is a very useful tool. It doesn't have to be a close-to-realistic simulation. I was amazed at how well it worked, because I didn't develop the technique for that reason.

wphamilton

Well yeah, that would at least be entertaining, and on topic of "crash strategy", What we probably don't want is more sniping from the peanut gallery.

63rickert

High speed bike crashes on ice. Lots of them. There are other vids from other cameras, this one is good enough.


Mostly they get up and keep riding.

FiftySix

Seems to be a huge piece of ice in the video on post 44.

FiftySix

Mostly, especially low side slides.

----------------------------

I've fallen off my motorcycles at speeds ranging from 0 to 70 mph with dirt bikes and street bikes. 0 mph were comical fall overs. All others were above 25 mph. My physical well being depended on if a part of my body gained traction on touching the ground causing me to tumble, or if I landed into something soft or hard.

Personally, I got lucky and only lost skin, got heavily bruised, and one bell ringer of a concussion* with a helmet on. Friends of mine suffered broken bones, head injury, one collapsed lung, loss of skin from road rash or burns, etc.

*one out of 5 concussions that I know of.

mr_bill

But “Physics!”

-mr. bill

indyfabz

Bravo, OP!

livedarklions

I suspect a lot of the argument comes from differences in speed and other riding habits as well as knowledge of our own reaction times. We can probably trade anecdotes for years and never get to any kind of conclusion.

Really happy the helmet fits you. We lucked out that they had one in stock at an lbs in Amherst, MA. My son was about as surprised as you that we found one that fit.

bruce19

Well, if you believe that everything is energy and the First Law of Thermodynamics states that energy can neither be created nor destroyed, then....existence has no beginning and no end. So there.

wphamilton

Before it's a completely dead thread, does anyone still disagree with this?

"the added speed is parallel to the ground, and doesn't add anything to the vertical impact of the fall, right? The impact you feel from the speed is added by deceleration from friction, and whatever rises from the surface, including "concave up" shaped road grade. On a flat and smooth road, you could fall at 100 mph and that fall is not the danger. Getting part of your body wrenched back, or sliding into something, is where the danger is."

indyfabz

Depends.

base2 

I have no disagreement with this whatsoever. In fact, I posted a video of a couple of Saudi's getting out of a car, on the freeway, at speed even(!) Just to show that with out the deceleration component (or change in velocity vector) no harm is done.

livedarklions

So let me get this straight--if I'm lying on the ground and someone fires a bullet at me from a gun 4 feet above and 15 feet away, I'll be fine because I'll only be hit with the energy of the 4 foot drop?

base2 

Totally fine because it missed you by 4 feet.

Even if it hit you (gasp) you'd still only suffer injury in relation to the difference in momentum imparted.

If it barely made contact (shallow velocity vector) it wouldn't impart that much energy & would do so over a large moment. You'd end up with a cut or less depending on how shallow it made contact and the path of the bullet little affected & nothing more.

If it made a direct hit (perpendicular vector) all of the energy would be imparted in a very short moment over a very tiny area. You'd have a tiny enterance hole & catastrophic amount of damage caused by absorbing a huge amount of energy in a very small amount of time.

This whole thread has circled the drain endlessly regarding rate of decelleration (or more directly rate of energy change) being the cause of injury. It's why cars have crumple zones & bullet proof vests do their thing. The force is distributed over a larger area for a longer time to reduce the force to a rate that can safely be imparted by the occupant.

tagaproject6

:

wphamilton

No, as described above there are two components to the forces you'll feel. One, the momentum due to the fall. Two, the momentum due to the velocity. The bullet's momentum will result in piercing your skin and possibly other organs.


In falling, you have the same two components. Momentum due to falling is the same. But the effect of the linear velocity (ie, change in momentum) is distributed over time. The greater the time interval, the less force you feel. That's the main point of rolling, the other point being avoiding abrasion and localized forces on your skeleton and soft tissue. And the reason why sliding is better than hitting the ground and never moving. The longer it takes to stop, the less force there is.


If you meant that as a counter-example, do you see why it fails?