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.

Re: Skydiving question
 

You feel a rather large jolt, i wouldnt say you go upwards though.
________
BodyCanela cam

Re: Skydiving question
 

Having no paraschuting experience and no real interest in mathematics, Im afraid I cant help you on this occasion Jonny.

Re: Skydiving question
 

That's quite alright crashtester, your mere presence in this thread has immeasurably brightened my day.

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.

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

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.

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)

Re: Skydiving question
 

That's what I always thought.

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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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thats what i said, just without all the math and scientific jargon :p

Re: Skydiving question
 

i was typing it while you posted, so couldnt see

Re: Skydiving question
 

nah, 'tis ok, he didnt believe me anyway when i said you dont move up, your explanation was much better

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

Re: Skydiving question
 

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

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???

Re: Skydiving question
 

no you travel at constant velocity, 0 or otherwise

Re: Skydiving question
 

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 ;)

Re: Skydiving question
 

but if the velocity id zero, your not moving, are you?

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!

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.

Re: Skydiving question
 

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.

Re: Skydiving question
 

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.

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....

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

Re: Skydiving question
 

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.

Re: Skydiving question
 

you just fall slower, but its so much slower it seems like youre going up, thats all.

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?

Re: Skydiving question
 

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

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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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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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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That is a very good point, it would probably (at the very least) tear your arms out and give you some nasty whiplash.

Re: Skydiving question
 

The people who "inspired" this thread need to be killed.

Re: Skydiving question
 

jonny and his friends?

Re: Skydiving question
 

yes

Re: Skydiving question
 

go ahead..

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.

Re: Skydiving question
 

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.

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.

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.....