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22 Feb 2005, 11:53
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#1
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Banned
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Posts: 19,441
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Skydiving question
When your parachute opens do you actually get jerked back upwards or is it purely an optical illusion? I'm aware that to some extent at least it's an optical illusion which is caused by the rapid changes in velocity, however the subject was being discussed last night in the pub (where else?) and the opinion was put forwards that the force imparted into the parachute itself when it opens is sufficient to literally move you upwards. This occurred to me to be slightly ridiculous unless the parachute itself when it is expelled from your parachute pack (or whatever) somehow has enough force to cause both you and itself to accelerate faster than 9.8 m s^-2 (gravity yo) vertically up but frankly I've got no idea how parachutes actually work.
You'd imagine that studying mathematical science would help me in this area but I'm beginning to think that no college course has ever had a single practical application.
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22 Feb 2005, 12:02
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#2
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Registered User
Join Date: Jan 2005
Posts: 278
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Re: Skydiving question
You feel a rather large jolt, i wouldnt say you go upwards though.
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Last edited by ceres; 31 Aug 2011 at 06:48.
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22 Feb 2005, 12:09
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#3
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I am an idiot
Join Date: Feb 2003
Posts: 2,145
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Re: Skydiving question
Having no paraschuting experience and no real interest in mathematics, Im afraid I cant help you on this occasion Jonny.
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22 Feb 2005, 12:10
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#4
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Banned
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Re: Skydiving question
That's quite alright crashtester, your mere presence in this thread has immeasurably brightened my day.
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22 Feb 2005, 12:10
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#5
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dazed and confused
Join Date: Apr 2004
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Re: Skydiving question
afaik you simply slow down... a lot. you dont move up, this effect is caused by the camera man who is usually still in free fall, continuing to descend at the same rate while you slow down.
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22 Feb 2005, 12:12
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#6
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TashTastic
Join Date: Jul 2001
Posts: 1,354
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Re: Skydiving question
You dont jerk up, you just slow down very quickly. When 2 people jump at the same time (one of them filming) it looks like the guy who opens his parachute (not filming) goes up purely cos the other guy filming is still going very fast towards the ground.
I assume thats what your talking about
Edit: pwned by TheBerk
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22 Feb 2005, 12:17
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#7
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Banned
Join Date: May 2001
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Re: Skydiving question
I remember the counter-point slightly better now. It was that when you're at terminal velocity you're not actually accelerating and the force imparted by the parachute being expelled from your pack is sufficient to cause you to accelerate upwards very briefly.
I'm perfectly aware of the whole camera angle thing, I mentioned it in my initial post. My question goes slightly beyond that sort of mindless drooling spastication.
Hopefully.
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22 Feb 2005, 12:18
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#8
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Registered User
Join Date: Jan 2005
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Re: Skydiving question
The jolt is because the forces on your body have changed dramatically. Normally once you are in freefall (terminal velocity) your drag is equal and opposite to your weight. Once you open your parachute, the drag is increased vastly and so there is an overall force upwards until your speed slows down and drag equals weight again. However the force is not great enough to overcome the forces pulling you down (velocity, potential) so you only slow down. So you do get jerked back but you wont go back up, because at no stage does your speed drop to zero then increase upwards (relative to earth) thus you cant have travelled further upwards..
Okay that was waffle mixed with truth, but summing up. You only slow down when you open your parachute (which btw i think you just unravel it and the force of air opens it)
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22 Feb 2005, 12:19
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#9
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Twisted
Join Date: Nov 2000
Location: Down with the sickness
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Re: Skydiving question
Quote:
Originally Posted by TheBerk
afaik you simply slow down... a lot. you dont move up, this effect is caused by the camera man who is usually still in free fall, continuing to descend at the same rate while you slow down.
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That's what I always thought.
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In my sleep I grind my teeth.
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22 Feb 2005, 12:20
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#10
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Banned
Join Date: May 2001
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Re: Skydiving question
Quote:
Originally Posted by sniborp
The jolt is because the forces on your body have changed dramatically. Normally once you are in freefall (terminal velocity) your drag is equal and opposite to your weight. Once you open your parachute, the drag is increased vastly and so there is an overall force upwards until your speed slows down and drag equals weight again. However the force is not great enough to overcome the forces pulling you down (velocity, potential) so you only slow down. So you do get jerked back but you wont go back up, because at no stage does your speed drop to zero then increase upwards (relative to earth) thus you cant have travelled further upwards..
Okay that was waffle mixed with truth, but summing up. You only slow down when you open your parachute (which btw i think you just unravel it and the force of air opens it)
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Yeah I was thinking of terminal velocity as being 0 v instead of 0 a. What a silly goose I am! Thanks for your help internet.
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22 Feb 2005, 12:24
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#11
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dazed and confused
Join Date: Apr 2004
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Posts: 379
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Re: Skydiving question
Quote:
Originally Posted by sniborp
The jolt is because the forces on your body have changed dramatically. Normally once you are in freefall (terminal velocity) your drag is equal and opposite to your weight. Once you open your parachute, the drag is increased vastly and so there is an overall force upwards until your speed slows down and drag equals weight again. However the force is not great enough to overcome the forces pulling you down (velocity, potential) so you only slow down. So you do get jerked back but you wont go back up, because at no stage does your speed drop to zero then increase upwards (relative to earth) thus you cant have travelled further upwards..
Okay that was waffle mixed with truth, but summing up. You only slow down when you open your parachute (which btw i think you just unravel it and the force of air opens it)
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thats what i said, just without all the math and scientific jargon
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22 Feb 2005, 12:25
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#12
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Registered User
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Re: Skydiving question
i was typing it while you posted, so couldnt see
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22 Feb 2005, 12:28
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#13
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dazed and confused
Join Date: Apr 2004
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Posts: 379
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Re: Skydiving question
nah, 'tis ok, he didnt believe me anyway when i said you dont move up, your explanation was much better
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22 Feb 2005, 13:01
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#14
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Registered User
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Re: Skydiving question
it was kinda just logical talking. One thing still confuses me is the force your body has going downwards at terminal velocity, according to force = mass x acceleration there is no force. Granted its the net force, but i still cant convert it so that there isnt just the force of the parachute moving you upwards
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22 Feb 2005, 13:31
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#15
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Cynical Optimist
Join Date: Apr 2003
Location: Solihull / University of Warwick
Posts: 502
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Re: Skydiving question
Quote:
Originally Posted by sniborp
it was kinda just logical talking. One thing still confuses me is the force your body has going downwards at terminal velocity, according to force = mass x acceleration there is no force. Granted its the net force, but i still cant convert it so that there isnt just the force of the parachute moving you upwards
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Force downwards on your body:
Fd= ma = body mass x (accelaration due to) gravity
Force upwards on your body
Fu = air resistance on parachute
And obviously at terminal velocity Fd == Fu
No resultant forece -> no accelaration
Edit: gravity -> accelaration due to gravity
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22 Feb 2005, 14:09
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#16
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Last Of The Soul Reaver
Join Date: Oct 2000
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Re: Skydiving question
What is intresting, is the fact that, if there is the same amount of gravity pushing down as there is force pushing up, then you dont move... is that possible???
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22 Feb 2005, 14:13
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#17
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Registered User
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Re: Skydiving question
Quote:
Originally Posted by slick
What is intresting, is the fact that, if there is the same amount of gravity pushing down as there is force pushing up, then you dont move... is that possible???
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no you travel at constant velocity, 0 or otherwise
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22 Feb 2005, 14:14
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#18
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Registered User
Join Date: Jan 2005
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Re: Skydiving question
Quote:
Originally Posted by Toccata & Fugue
What happens in Monkey ball?
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same thing as with parachutes, but using semi spheres
notice the altitude on the right when you pop your ball, notice how the reading still goes down
good game tho
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22 Feb 2005, 14:15
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#19
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Last Of The Soul Reaver
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Re: Skydiving question
but if the velocity id zero, your not moving, are you?
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22 Feb 2005, 14:18
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#20
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Registered User
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Re: Skydiving question
if the forces are equal, then there will be no acceleration. E.g. A car travelling along a motorway at a steady 70mph is not accelerating, because the force of drag and the work done by the engine are equal and opposite
the velocity can be 0 and you can be moving, just depends what you take it relative to
remember: its all relative!
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22 Feb 2005, 14:22
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#21
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Last Of The Soul Reaver
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Re: Skydiving question
BUt if the force making you accelerate (gravity) is being slowed by another force (a parachute for example) and it cancels the downward force, gravity in this case, then the two forces are equal and so there is no movement.... a car stays at a steady 70mph because of the lack of friction. and the engine is constantly pushing forward, because if there was equal forces then the car would not move.
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You're just jealous coz the voices are talking to me!
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22 Feb 2005, 14:25
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#22
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Rawr rawr
Join Date: Dec 2000
Location: Upside down
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Re: Skydiving question
Quote:
Originally Posted by slick
but if the velocity id zero, your not moving, are you?
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Indeed.
But when you're falling the air resistance is depending on your velocity.
So don't come telling that you can hang in the air, because you don't.
If your velocity is 0, then the drag generated is 0, so gravity gets the whole of you accelerating you towards the ground.
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22 Feb 2005, 14:31
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#23
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Rawr rawr
Join Date: Dec 2000
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Re: Skydiving question
Quote:
Originally Posted by slick
BUt if the force making you accelerate (gravity) is being slowed by another force (a parachute for example) and it cancels the downward force, gravity in this case, then the two forces are equal and so there is no movement.... a car stays at a steady 70mph because of the lack of friction. and the engine is constantly pushing forward, because if there was equal forces then the car would not move.
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read up on Newton's rule of preservation of energy. You need energy to accelerate, you need energy to slow down (negative acceleration).
You do NOT need energy to maintain a speed.
In a 0-friction world, if you gave your car a push to 70mph, your car would keep traveling at 70 forever.
However, at 70mph you have a friction (air resistance, road friction, etc etc) of 10000 newton, your engine will need to generate 10000 newton to MAINTAIN 70mph.
In the parachute example, if the friction equals your falling energy, your velocity will remain equal.
At the moment you open the parachute, your friction increases, resulting in a negative acceleration.
As you slow down, friction decreases again, and the equilibrium of velocity vs friction will be maintained again.
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22 Feb 2005, 14:36
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#24
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Last Of The Soul Reaver
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Re: Skydiving question
what about zero gravity...... is that a plus force in your body, caused by soimething else....
You know, like when they put those guys in the plane, and they get it....
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22 Feb 2005, 14:40
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#25
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Registered User
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Re: Skydiving question
zero gravity is that, zero gravity...
gravity is a force, zero gravity = zero force thats why you arent pulled to the bottom of the spacecraft like you would on earth
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22 Feb 2005, 14:43
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#26
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Rawr rawr
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Re: Skydiving question
Quote:
Originally Posted by slick
what about zero gravity...... is that a plus force in your body, caused by soimething else....
You know, like when they put those guys in the plane, and they get it....
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What sniborp said.
And that what you describe as those guys in the plane, is the plane falling it an equal speed as the guys.
So the plane is falling around the guys, or the guys are falling inside the plane, whatever your point of reference is.
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22 Feb 2005, 16:01
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#27
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Next goal wins!
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Re: Skydiving question
you just fall slower, but its so much slower it seems like youre going up, thats all.
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22 Feb 2005, 16:42
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#28
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This is bat country
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Re: Skydiving question
if you jumped out of a airplane as it hit the ground, how fast would you need to jump upwards to avoid dying?
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22 Feb 2005, 17:25
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#29
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Cynical Optimist
Join Date: Apr 2003
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Re: Skydiving question
Quote:
Originally Posted by I am Idler
if you jumped out of a airplane as it hit the ground, how fast would you need to jump upwards to avoid dying?
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You'd need to jump up at the speed the plane is about to hit the ground at, less the maximum impact speed you could survive without dying.
Eg. if you can survive a landing of -5m/s and the plane's speed as it hits the ground is -20m/s, you would need to jump hard enough to give you an upwards velocity of 15m/s relative to the plane (or 5m/s relative to the ground)
Edit: see http://www.straightdope.com/classics/a1_205a.html
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22 Feb 2005, 20:56
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#30
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Mathamagician
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Re: Skydiving question
I'm fairly sure that you wouldn't move upwards, if you think about it in terms of how a parachute works, the idea that you move upwards would mean that the air beneath you must have an upwards motion also.
That might not be too clear, think about it this way. When you open a parachute, it slows you down by giving you an increased resistance to your downwards motion. This cannot excede your mass purely due to conservation of energy in a closed system. The parachute opening is, to my knowlage, not launched out with a force, I think it just flops out somehow, meaning no energy is entered into the system. Even if it is fired out, it would need to be kicked out with a momentum of something like 3632.2kgm/s in order to stop an 11st person (google tells me that 52m/s is not an unreasonable terminal velocity). I personally would say that a parachute weighs less than 15kg, meaning the parachute must shoot up with a velocity of 242m/s. Thats not going to happen.
The idea that it moves you upwards because you are in equilibrium is similar to saying that my having a parachute on the ground should make you fly, because you are in equilibrium.
I'm sure google will tell you more if you really care, but I recon that that's enough detail to shut someone in your local up.
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22 Feb 2005, 21:02
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#31
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Mathamagician
Join Date: Aug 2001
Location: At the very edge of existance
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Re: Skydiving question
Quote:
Originally Posted by JamMak
You'd need to jump up at the speed the plane is about to hit the ground at, less the maximum impact speed you could survive without dying.
Eg. if you can survive a landing of -5m/s and the plane's speed as it hits the ground is -20m/s, you would need to jump hard enough to give you an upwards velocity of 15m/s relative to the plane (or 5m/s relative to the ground)
Edit: see http://www.straightdope.com/classics/a1_205a.html
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This was done by the mythbusters with a lift in freefall. Although the concept is sound, it is in practice impossible to jump up fast enough.
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22 Feb 2005, 21:33
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#32
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Registered User
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Re: Skydiving question
Quote:
Originally Posted by vampire_lestat
That might not be too clear, think about it this way. When you open a parachute, it slows you down by giving you an increased resistance to your downwards motion. This cannot excede your mass purely due to conservation of energy in a closed system. The parachute opening is, to my knowlage, not launched out with a force, I think it just flops out somehow, meaning no energy is entered into the system. Even if it is fired out, it would need to be kicked out with a momentum of something like 3632.2kgm/s in order to stop an 11st person (google tells me that 52m/s is not an unreasonable terminal velocity). I personally would say that a parachute weighs less than 15kg, meaning the parachute must shoot up with a velocity of 242m/s. Thats not going to happen.
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The g-force that would occur if the parachute was launched with that velocity and the change in velocity of yourself would undoubtably kill you
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If one person is in delusion, they're called insane.
If many people are in delusion, it's called a religion.
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22 Feb 2005, 21:36
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#33
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Mathamagician
Join Date: Aug 2001
Location: At the very edge of existance
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Re: Skydiving question
That is a very good point, it would probably (at the very least) tear your arms out and give you some nasty whiplash.
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22 Feb 2005, 22:07
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#34
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so f*cking zen
Join Date: Jan 2003
Location: Hitting Bottom
Posts: 8,499
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Re: Skydiving question
The people who "inspired" this thread need to be killed.
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22 Feb 2005, 22:19
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#35
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Registered User
Join Date: Jan 2005
Posts: 3,174
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Re: Skydiving question
Quote:
Originally Posted by Dace
The people who "inspired" this thread need to be killed.
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jonny and his friends?
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If one person is in delusion, they're called insane.
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22 Feb 2005, 22:22
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#36
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so f*cking zen
Join Date: Jan 2003
Location: Hitting Bottom
Posts: 8,499
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Re: Skydiving question
yes
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On a long enough timeline, the survival rate for everyone drops to zero.
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22 Feb 2005, 22:31
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#37
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Registered User
Join Date: Jan 2005
Posts: 3,174
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Re: Skydiving question
go ahead..
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If one person is in delusion, they're called insane.
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22 Feb 2005, 23:02
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#38
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:alpha:
Join Date: May 2002
Location: London, UK
Posts: 7,871
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Re: Skydiving question
Surely the only reason it looks like the parachutist is "going up", is because the cameraman hasn't opened his/her parachute yet?
So therefore the person being filmed slows down dramatically, while the camera person is still freefalling. Then they open their parachute too, and they descend at the same speed.
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22 Feb 2005, 23:06
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#39
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Aardvark is a funny word
Join Date: Sep 2002
Location: I'm No Nino Rota
Posts: 5,923
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Re: Skydiving question
Quote:
Originally Posted by JonnyBGood
question
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i've never seen someone skydive and go _up_, but going almost horizontally backwards instead of down i've seen, which makes sense (parachute opening behind you). The jerking motion going backwards, combined with the dramatic loss of speed, is what i assume creates such an illusion.
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23 Feb 2005, 00:13
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#40
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Banned
Join Date: May 2001
Location: Further to the right
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Re: Skydiving question
Dear god guys the question has been answered. Talk about beating a dead horse until it starts to decompose and it's molecular structure breaks down and the atoms are split and you're still there dropping a grand piano on it from a great height.
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23 Feb 2005, 19:09
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#41
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home wrecker
Join Date: Jan 2001
Location: The other side of the galaxy ;)
Posts: 1,041
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Re: Skydiving question
never mind the physics stuff.....
the interesting thing is that when the chute opens you actually feel like you're going faster....
(presumably because you get used to the free fall speed and wind resistance and when it changes you register it but hey go nuts with plausible thoughts)
either way for the split second when I pulled the cord I certainly didn't feel like I went up.....
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