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F=MA , The Science of Safety!!!


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30 MPH is like falling out of the 3rd floor window. Of course, unless you actually fall directly into a wall, you will slide so the speed will scrub off along with whatever non-specific gear your slide upon.  

 

 

speed.PNG

Edited by Pard
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Ignoring horizontal movement I was trying to work out the speed your head is moving through the quarter arc of rotational fall in the event of a cut out. Very very roughly I came to the conclusion that if it took 0.5 seconds to go from upright to head on the ground then for a 6' person could expect their face to be mashed into the concrete at about 20mph vertical velocity. In reality it is probably higher as you will accelerate through the fall and my ballpark figure is for a fixed speed through the whole 1/4 rotation. I also didn't take into account pedal height allowing slightly more than a quarter rotation in the same amount of time.

I brought a full face helmet!

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24 minutes ago, WARPed1701D said:

Ignoring horizontal movement I was trying to work out the speed your head is moving through the quarter arc of rotational fall in the event of a cut out. Very very roughly I came to the conclusion that if it took 0.5 seconds to go from upright to head on the ground then for a 6' person could expect their face to be mashed into the concrete at about 20mph vertical velocity. In reality it is probably higher as you will accelerate through the fall and my ballpark figure is for a fixed speed through the whole 1/4 rotation. I also didn't take into account pedal height allowing slightly more than a quarter rotation in the same amount of time.

I brought a full face helmet!

 

That sounds reasonable.

Again, a very light weight person will put less force on their head than a larger person. But either case...not something you want to test!

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2 hours ago, Circuitmage said:

All of this safety talk has me thinking about the "real world" parameters involved here. Everyone is different, physically, and we are on different machines using them in different ways. So, it seems the best way to address this is with SCIENCE!

These EUC's are wonderful machines, capable of thrusting various size humans (M=MASS) at various speeds (A=ACCELERATION), resulting in some FORCE (F).

Specifically, the physics of safety has to do with Force. Here are some quick numbers, compiled, to aid in mitigating safety needs;

F=MA Comparisons (at 10 MPH / 16 KPH)

10 MPH (16 KPH) @ 100 LBS (45 KGS) = 203 Newtons

...

sorry for beeing picky - but f=m*a, as you wrote and not m*v. M times v is the momentum.

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13 hours ago, Chriull said:

sorry for beeing picky - but f=m*a, as you wrote and not m*v. M times v is the momentum.

I was wondering about this too... on the other hand, you will have very brief and strong "negative acceleration" as the pavement stops your body from whatever speed it was traveling at, but I doubt you can simply use the speed you were traveling before the fall? My physics is very, very rusty, but I think kinetic & potential energy plays a big role here, and the a (acceleration) -value of the equation is something whole other than 32kph or whatever (kph is unit for speed, not acceleration):

An unrestrained adult fall from standing has been shown to produce a
minimal force of 873 N which is more than enough to produce a skull
fracture.

"Fall from standing" would indicate travelling speed of 0km/h, so all the force comes from the kinetic & potential energy gained during the fall towards ground and then the sudden stop ("negative acceleration")?

 

Edited by esaj
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Yep, mixing speed and acceleration is not physics. Kinetic energy makes some sense. It is M * V^2 / 2. When you bump into vertical wall, you have pretty much the same stopping path at different speeds. It is something like M * V^2 / 2 = F * S, where F is the (average) force and S is the length of the path where you decelerate. That is why speed is so important in collisions, you need to pass all that kinetic energy and it grows like a square. Your gear makes decelerating path longer and force more constant. Also, it is not the force that matters, but pressure. If equal force is applied to whole your body (like every atom) - you won't be harmed by that, regardless of the force's magnitude. Gravity is an example of such force - when only gravity affects you (when you in in a free fall) - you are fine, also you are affected by M * g force. This is the magnitude of force your legs compensate when you stand. It's 980 Newtons for 100 kg person. Yet the skull is not breaking (also if the 100 kg person will stay on his head only, that might happen). 73 Newtons is just like 8 kg, but they can be applied to a small spot. You don't need a big force to put a needle into you. Also, gear makes area where the force is applied bigger (not only a small part of your skull). 

So when you bump into things - the upper pretty much applies.

Falling from EUC is another thing and needs different model. Maybe something like friction force + reaction force on impact and then sliding when friction decelerates you. The impact is the most dangerous, it is where people break their bones (also sliding will tear your gear and possibly your skin). This is tricky to estimate, as there are many ways you can fall. However, if you slide after the fall - the impact influence is pretty much the same for different speeds (if you fall the same way). So there is likely some limit after some speed, when impact is the same.Then, you can try to stop at once and don't slide, so you'll get the friction force way more.

I think as the fall process is so dependent on the way you fall - you should learn how to fall. Then ideally it should be not much more then just falling and dirty clothes. Yet you are falling from a full stand, which is already not that easy, and there can be obstacles and side of the road can be way lower.

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Yeah, I was kinda thinking about this...and I did this really quickly thinking the math worked out. And I even had 2 physics courses in high school and a couple in college!

I am a bit rusty. But at least I remember F=ma being important!

So, technically F=m*(dV/dt). And we are talking about a sudden change in velocity over time, so F=ma applies, while mass is constant. Anytime you hit something your velocity change rate (deceleration) is what matters. See video in next response for nice summary of how gear helps reduce the dV/dt affect.

When falling from a stand-still, you are accelerating with gravity, until the point of impact, which is not insignificant with our gravity 9.8m/s/s (hence the 980 Newtons for a 100kg person). So, there is some consolation if you are not at the 100kg mark!

If you add on top of that any EUC velocity, you are simply adding the velocity of your EUC, plus gravity, which can be more and more significant the faster you go.

I think the concepts stated in responses above are also good points. Force on specific areas is they key (hence the video focusing on head injuries). I think the force of a bullet hitting your body is much less than the impact of a car crash, but many people can survive a car crash where as a single bullet can be fatal. It all depends on where you get hit and how hard, and can come down to where you get hit being important. 

Also, the idea of gear spreading out the impact damage is important.

On 8/4/2017 at 8:26 AM, ubertoad said:

...Also, it is not the force that matters, but pressure. If equal force is applied to whole your body (like every atom) - you won't be harmed by that, regardless of the force's magnitude. 

...

Falling from EUC is another thing and needs different model. Maybe something like friction force + reaction force on impact and then sliding when friction decelerates you. The impact is the most dangerous, it is where people break their bones (also sliding will tear your gear and possibly your skin). This is tricky to estimate, as there are many ways you can fall.

Edited by Circuitmage
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On 04/08/2017 at 1:30 PM, esaj said:

"Fall from standing" would indicate travelling speed of 0km/h, so all the force comes from the kinetic & potential energy gained during the fall towards ground and then the sudden stop ("negative acceleration")?

Free fall from 1.8m takes 0.6s with a final velocity of 21km/h. 

h = 1.8m
g = 9.81m/s^2
t = (2 * h / g)**0.5
v = g * t

Edited by Mono
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What kills (other than a bullet or a knife) is rapid (i.e. high values of) acceleration, i.e. change of speed, or, usually, deceleration, which is the same thing from the physics perspective. The problem with speed is that most objects around are at standstill and heavy. Hitting these heavy objects leads to rapid deceleration (often measure in g-force). The own weight is of rather minor relevance, though in case one hits a lighter object it's better to be heavy as this results in less deceleration.  

Then there is pressure (force per area). To achieve the deceleration, a force is necessary (force is acceleration/deceleration times mass), and the force is applied to an area on the body. High enough pressure disintegrates tissue or bones. The larger the force (e.g. due to rider weight) and the smaller the area (hard surface or even a rim or a blade) the worse. 

Edited by Mono
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