Profound Query of the Day

Visage · 17 Jan 2007 at 22:12

loopstah said:
So would 28 metres of bubblewrap be enough to stop you going splat then?

It would depend on the elastic modulous of the bubblewrap....in other words, how fast it slowed you down.

If it slows you down too quickly then the g forces would scramble your guts. If its slowed you down too slowly you'd smack the pavement at high speed, with scrambled guts being the result....

xsnv · 17 Jan 2007 at 22:14

daz said:
Draw a graph of v/t.

If you deccelerate from 54ms^-1 to 0 over a time of 1 second, assuming constant decceleration you'd travel 27 metres, i.e. you'd hit the pavement before you had finished deccelerating.

Agreed! but you're making one very very big assumption. Your assuming his velocity goes straight from 54m/s ot 0m/s! i.e he doesn't go through 30m/s or 10m/s. If that was true his deceleration would be infinite(as he went from 54m/s t0 0 m/s instantaneously). His speed doesn't remain constant at 54m/s and as such you can't use your argument.

You almost hit the nail on the head in your last post visage. It's the time thats crucial. Once again, the time it takes for him to decelerate is not dependent on his velocities as stated in your equation! It's dependent on the change in velocity per unit time.

p4radox · 17 Jan 2007 at 22:14

I think we've safely determined that there's no way of working out what would happen, short of trying it or finding out the Young's modulus of bubble wrap.

/heads to next "what if" thread...

Visage · 17 Jan 2007 at 22:15

xsnv said:
Agreed! but you're making one very very big assumption. Your assuming his velocity goes straight from 54m/s ot 0m/s! i.e he doesn't go through 30m/s or 10m/s. If that was true his deceleration would be infinite(as he went from 54m/s t0 0 m/s instantaneously). His speed doesn't remain constant at 54m/s and as such you can't use your argument.

He's not making that assumption at all.

xsnv · 17 Jan 2007 at 22:21

Visage said:
He's not making that assumption at all.

you can't draw a graph of v over t without knowing t. The time it takes for him to change from 54m/s to 0m/s isn't fixed. He's saying it is. the time to change from 54m/s to 0m/s in water is different to that in air and different to that in syrup. How would you model the effect of *** medium on a graph?! THEREFORE GIVEN U AND V YOU CAN'T FIX T

imagine the space between him and the floor wasn't 3m of bubble wrap but instead air or water or syrup. Would the deceleration be the same?

If you're both right then there's no point in crumple zones.

Actuallty, show me how he worked the distance out then...

xsnv · 17 Jan 2007 at 22:22

xsnv said:
You almost hit the nail on the head in your last post visage. It's the time thats crucial. Once again, the time it takes for him to decelerate is not dependent on his velocities as stated in your equation! It's dependent on the change in velocity per unit time.

Tbh by stating that you're agreeing with me so i don't see why we're arguing. You said he would be splatterd but I was saying not necessarily because if you can controll the time it takes for him to decelerate (by altering the properties of the material he's covered with) then you could make it possible to survive.

Visage · 17 Jan 2007 at 22:27

xsnv said:
you can't draw a graph of v over t without knowing t. The time it takes for him to change from 54m/s to 0m/s isn't fixed. He's saying it is.

imagine the space between him and the floor wasn't 3m of bubble wrap but instead air or water or syrup. Would the deceleration be the same?

If you're both right then there's no point in crumple zones.

Actuallty, show me how he worked the distance out then...

He clearly stated

If you deccelerate from 54ms^-1 to 0 over a time of 1 second, assuming constant decceleration you'd travel 27 metres, i.e. you'd hit the pavement before you had finished deccelerating.

Which part of this do you disagree with?

xsnv · 17 Jan 2007 at 22:29

I used 1s as an example to make a point. My whole point is GIVEN U, V AND S YOU CANT FIX T. As such the time it takes for him to decelerate which controls the "g" force on him can be altered.

Check out my ninja edit:

leaskovski · 17 Jan 2007 at 22:29

doofer said:
Profound Query of the Day...

Okay, how about this...

SELECT threadtitle FROM threads WHERE author="doofer" INTO CURSOR curPantsThreads

Sorry couldnt resist after a days crappy meeting

Visage · 17 Jan 2007 at 22:29

xsnv said:
Tbh by stating that you're agreeing with me so i don't see why we're arguing. You said he would be splatterd but I was saying not necessarily because if you can controll the time it takes for him to decelerate (by altering the properties of the material he's covered with) then you could make it possible to survive.

OK....lets say you could create a material that would slow you down from 54m/s to zero nice and slowly, say over 10 seconds. That would be survivable, right?

How far does a body, under constant deceleration from 54m/s to 0 over 10s (i.e at 5.4 m/s/s) travel during its deceleration?

In other words, how far above the pavement would decelleration need to start in order to avoid hitting it?

Visage · 17 Jan 2007 at 22:30

xsnv said:
I used 1s as an example to make a point. My whole point is GIVEN U, V AND S YOU CANT FIX T. As such the time it takes for him to decelerate which controls the "g" force on him can be altered.

Absolutely - you can take as long to decelrate as you want....as long as you're not concerned about the rapidly approaching pavement

xsnv · 17 Jan 2007 at 22:36

Visage said:
OK....lets say you could create a material that would slow you down from 54m/s to zero nice and slowly, say over 10 seconds. That would be survivable, right?

How far does a body, under constant deceleration from 54m/s to 0 over 10s (i.e at 5.4 m/s/s) travel during its deceleration?

In other words, how far above the pavement would decelleration need to start in order to avoid hitting it?

I can't answer that question because you've overspecified it.

Lets start on common ground. We'l model the situation.

I'm proposing that THE ONLY VARIABLES GIVEN ARE:

U - his terminal velocity
S - distance which is 3m ie thickness of bubble wrap
V - His final velocity
M - His mass

My point is this. With that information which is all we're given, calculate the force acting on him.

Visage · 17 Jan 2007 at 22:38

xsnv said:
I can't answer that question because you've overspecified it.

Lets start on common ground. We'l model the situation.

I'm proposing that THE ONLY VARIABLES GIVEN ARE:

U - his terminal velocity
S - distance which is 3m ie thickness of bubble wrap
V - His final velocity
M - His mass

My point is this. With that information which is all we're given, calculate the force acting on him.

From v^2 = u^2 +2as, a = (v^2-u^2)/(2s)

From F=ma,

F = m(v^2-u^2)/(2s).

OK?

p4radox · 17 Jan 2007 at 22:39

xsnv said:
I can't answer that question because you've overspecified it.

Lets start on common ground. We'l model the situation.

I'm proposing that THE ONLY VARIABLES GIVEN ARE:

U - his terminal velocity
S - distance which is 3m ie thickness of bubble wrap
V - His final velocity
M - His mass

My point is this. With that information which is all we're given, calculate the force acting on him.

F=M[(V^2-V^2)/2S] :confused:

xsnv · 17 Jan 2007 at 22:40

You can't because WE need to specify either Acceleration or Time. There is no equation or method on this planet you could use to deduce A or T from the information given. A or T controll the Force acting on him and if we can manipulate A or T we can manipulate Force F. If we manipulate F to 0.001N which I can do then he'l survive.

I'm enjoying this btw. Nice to have an intellectual argument once in a while.

xsnv · 17 Jan 2007 at 22:41

p4radox said:
F=M[(V^2-V^2)/2S]

lol what is S?

xsnv · 17 Jan 2007 at 22:42

scratch that...rewriting my post as mad a stupid mistake!

p4radox · 17 Jan 2007 at 22:43

xsnv said:
lol what is S?

I thought you specified S as the displacement to bring the person to a halt?

I'm not trying to argue with you. I'm genuinely trying to correct myself here. You're worrying me - I thought I was okay at this kind of stuff...

xsnv · 17 Jan 2007 at 22:44

My bad...should't have used that as an example. It cant work as the deceleration is not constant ie

daz · 17 Jan 2007 at 22:48

T is specified by the force required to kill a human. If you know that 20G kills, you can then work out the minimum time that you will be required to stop in.

I don't understand why people memorise all these formulae, I just draw a little v/t graph and work from there.

Using the example of 20G, we get 0.27 seconds as our time... then just by looking at the area of the graph we see that in 0.27 seconds, a constant decceleration will mean you travel ~7 metres.

So before taking into account any material's properties, if you want to stop sufficiently, you should have >7 metres of material.