Profound Query of the Day
- Thread starter doofer
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ferretmaster said:Does anyone want to try with a egg, bubblewrap and a model tower?
In year 4 (primary school) we did something just like that, There were people putting their eggs in huge boxes with tons of polystyerne. And there was my little box 10x10x10 cm with some foam in. My little egg didnt break whatever I did.
Visage said:
Very true, the problem is we don't really know what the maximum G is for a human, what would your idea be of a succesful landing, not breaking anything and walking away or just being alive?
How about sending an email to one of the bubblewrap companies? I dont really know how to word it but if omeone else worded one I would be happy to send it. Something along the lines of elasticity and compressability of a normal bit of bubblewrap for protecting something.
p4radox said:see Ninja edit
edit: lol
I still think you're using the wrong terminal velocity though, the terminal velocity of a person in a large ball falling is less than the terminal velocity of a person falling. I worked it out as 39m/s which is more accurate.
Amp34 said:I still think you're using the wrong terminal velocity though, the terminal velocity of a person in a large ball falling is less than the terminal velocity of a person falling. I worked it out as 39m/s which is more accurate.
So the new force by my calculations would be ((39^2)/2)*100=76050N.
Isn't that like having a 7.6tonne truck sitting on you? (or have I got this bit wrong?)
doofer said:As a result of quite a heated discussion in the pub last night..
If a human was to wrap oneself in layers and layers and layers of bubblewrap and got said bubblewrap at least a meter in thickness or more, would the human survive if they jumped off a sky-scraper type building of 20-30 floors (about 500ft), you get the picture, would massive layers of standard bubblewrap around you cushion the fall enough for you to survive hitting solid terra firma.
Discuss.
Wouldn't they suffocate first?
p4radox said:
Yeah something like that, although I stuck 150kg as the mass the bloke and bubblewrap so got 114000N.
I still cant work out how you can work out the g force on the body from that though.
I don't think the truck analogy is pertient to this question either, although it could be similar to a car crash as the force will only be on you for a split second so being hit by the truck, but at what speed.
Amp34 said:I don't think the truck analogy is pertient to this question either, although it could be similar to a car crash as the force will only be on you for a split second so being hit by the truck, but at what speed.
Well the truck analogy is only pertinent when the truck is stationary. I used the approximation of mavity as 10N/kg or 10ms^-2.
Is a "g" just 9.81N/kg? So 76050N is 76050/9.81=7752g. That's a hell of a lot, but I think I'm doing something wrong.
/goes to wikipedia
p4radox said:
Sorry, pertinent probably wasnt the right word, more like "quite right". Although having said that 7.5 tons isnt actually too bad...
Using F=mv^2 and using a car as an example you get:
F=2000x11^2
F= 246913N
(2000 is the mass of the car and 11 is 11m/s (40kmh) the car is going)
Which is way higher than my estimate and only slightly lower than your first one. And that is with a car going at 25mph. Not many people actually die from that sort of impact so it seems pretty accurate.
So to sum up (possibly
) If you surrounded yourself in a meter of bubble wrap on both sides and jumped off a 500 ft building it should be like being hit by something like a Vauhall corsa at 25mph.So now we done the maths who wants to test out the calculations?
EDIT: I lie, a 2000kg car would be in the range of a 5 series BMW, ie big exec cars.
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Visage said:
Not entirelly accurate. Actually not accrate at all. The distance he decelerates is completely immaterial (that is actually wrong because you can't really decelrate over a distance). The time it taks for him to decelerate is the key factor. He could decelrate 0.005m but if it takes 1000000minutes then he'l be fine.
The principle that covers this is impulse. If his rate of change of momentum is low enough then he'l be fine (as it's directly proportional to the force acting on him)N2S. I'm not sure of the exact physical properties of bubble wrap but if it's properties can be adjusted such that it's got just the right viscosity then he could definitely survive with 3m of it.
As for what actually kills you when you hit a hard surface, it's not the fact that your insides "are still moving while your body has stopped" - inertia. Its the reaction force that acts on you. Smashing your head on the pavement is *** same as someone smacking you in the face with a slab of concrete.
Inertial forces do contribute to some internal damage but if you think about it....imaging falling from a sky scraper and landing on your stomach! What would happen is your insides would hit the pavement through your elastic skin. Once again, the reaction of the pavement on your organs is what smashes them...
xsnv said:
Incorrect. Its the same mistake someone made above. There's no point discussing decellerating for for 10000000minutes if the object travells 1000m in that time, since you'll have hit the pavement (decellerating you VERY quickly) long before any 'gentle' force can stop you.
Incorrect. One of the first equations of motion (taught at A-level) is:
v^2 = u^2 +2as.
(v = final velocity, u = initial velocity, a = acceleration, s = distance).
If you stop over a distance of 3m (v = 0, s=3 )then you'd better hope that your initial velocity isnt too high, since you're gonna get splatted, since a = u^2/6 (accounting for sign changes due to measuring acceleration and speed in different directions).
Incorrect. People have been killed by g-forces long before they actually come to rest.
Again, incorrect, for the reasons outlined above.
xsnv said:
I'd say he's right, to be honest.
You say "He could decelerate 0.005m but if it takes 1000000 minutes then he'll be fine" which is clearly absurd if he's travelling at 120mph to begin with. The point being that the time in which he decelerates is directly related to the speed he starts decelerating at, in accordance with s=ut+1/2at^2. s is limited by virtue of the floor being stationary, we know u (huge) which means that t must be tiny.
Either way, he's screwed.
Here we go:
The equations of motion have nothing to do with this. It's the force that we're worried about not his final or initial velocities. You mentioned the fact that it's his change in velocity that is important (54m/s to 0m/s) right? Its not though because his mass has a lot to play. It's the change in momentum thats important not his change in acceleration.
Velocity or change in velocity cant kill anyone or have any physical effect because it's not a force! Once again his changing from 54m/s to 0m/s is immaterial. It takes a finite time for his velocity to change from 54m/s to 0m/s hence acceleration. Let's use figures. He can change from 54m/s to 0m/s in 1s hence his acc is -54m/s2. he could also change his velocity from 54m/s to 0m/s in 1000s in which case his acceleration is -0.054m/s2. multiply that by his mass to get the "G" force ... not a lot is it! the time it takes for his speed to change is very important and the bubble wrap can increase this significangtly.
The time it takes to decelerate is independent of the speed he's travelling at.have a look at visage's motion equation below. v and u are fixed but t is not. bubble wrap can increase t.
Accel is nothing without a mass to act on. What is the G-force on mass on 0.0000kg? you need mass to have a force. You are however entirely right with your equation. however note you have a "time" term in your denominator. this is what i'm trying to stress. That time term as proven by your equation is independent of "u". As such if you increase the time, you reduce the acceleration hence reducing the "G" force acting on him...
I'm not going to go on arguing but it's all about force and thats not linked in anyway to his velocity! The only equation that governs the magnitude of the force on a body is newtons second law (and it's many variations).
quick thought experiment. he's just about to hit the ground, remember he's wrapped in bubble wrap. This dissipates the energy and force of impact. It takes him longer to get to rest and as such the force acting on him is less.
It's the same principle crumple zones in cars work on. They take longer to deform hence reducing the force on the passengers
The equations of motion have nothing to do with this. It's the force that we're worried about not his final or initial velocities. You mentioned the fact that it's his change in velocity that is important (54m/s to 0m/s) right? Its not though because his mass has a lot to play. It's the change in momentum thats important not his change in acceleration.
Velocity or change in velocity cant kill anyone or have any physical effect because it's not a force! Once again his changing from 54m/s to 0m/s is immaterial. It takes a finite time for his velocity to change from 54m/s to 0m/s hence acceleration. Let's use figures. He can change from 54m/s to 0m/s in 1s hence his acc is -54m/s2. he could also change his velocity from 54m/s to 0m/s in 1000s in which case his acceleration is -0.054m/s2. multiply that by his mass to get the "G" force ... not a lot is it! the time it takes for his speed to change is very important and the bubble wrap can increase this significangtly.
vonhelmet said:
The time it takes to decelerate is independent of the speed he's travelling at.have a look at visage's motion equation below. v and u are fixed but t is not. bubble wrap can increase t.
Visage said:
Accel is nothing without a mass to act on. What is the G-force on mass on 0.0000kg? you need mass to have a force. You are however entirely right with your equation. however note you have a "time" term in your denominator. this is what i'm trying to stress. That time term as proven by your equation is independent of "u". As such if you increase the time, you reduce the acceleration hence reducing the "G" force acting on him...
I'm not going to go on arguing but it's all about force and thats not linked in anyway to his velocity! The only equation that governs the magnitude of the force on a body is newtons second law (and it's many variations).
quick thought experiment. he's just about to hit the ground, remember he's wrapped in bubble wrap. This dissipates the energy and force of impact. It takes him longer to get to rest and as such the force acting on him is less.
It's the same principle crumple zones in cars work on. They take longer to deform hence reducing the force on the passengers
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you're both using equations of motion which have no physical consequnce. You can't feel a velocity, and you can't feel an acceleration (theoretically anyway but it's practically impossible as you have a mass). If you didnt have a mass then you wouldn't be able to feel an accelaration. you however do feel a force because it acts on a mass.
Equations of motion don't have anything to do with force or momentum which is what would kill him
I dont want to start an argument or prove who's smarter...it's just this is a common mistake a lot of people make.
Give it some thought...the time it takes to decelerate (and his mass)is what determines the "G" force acting on him. Google the term "impulse" if you don't beleive me.
Force = mass x acc.
acc = (v - u)/t
U is fixed as his terminal velocity
V is 0
m - his mass is constant (unless of course he ***** himself...but that would be trapped by the bubble wrap so yes..constant
)
The only variable is time! Increase that (which is what the bubble wrap would do) and the "G" force acting on him is reduced. How much bubble wrap you need to reduce the time is a different question entirely...
Equations of motion don't have anything to do with force or momentum which is what would kill him
I dont want to start an argument or prove who's smarter...it's just this is a common mistake a lot of people make.
Give it some thought...the time it takes to decelerate (and his mass)is what determines the "G" force acting on him. Google the term "impulse" if you don't beleive me.
Force = mass x acc.
acc = (v - u)/t
U is fixed as his terminal velocity
V is 0
m - his mass is constant (unless of course he ***** himself...but that would be trapped by the bubble wrap so yes..constant
) The only variable is time! Increase that (which is what the bubble wrap would do) and the "G" force acting on him is reduced. How much bubble wrap you need to reduce the time is a different question entirely...
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daz said:
Exactly.
Its all very well saying 'if you decellerate over a long enough time period then you'll survive', but this omits the fact that you'll hit the pavement long before you've slowed down to the point at which that impact would not kill you.
