terminal & sub-terminal??

[billvon 2,396]

>how do I tell if I'm still sub-terminal when I pull my ripcord???

If you exit from a slow aircraft, just pull within the first 3 seconds.

If you want to do it by ear, do it before the roaring of the wind gets really loud.

If you want to do it by sight, do it before you "belly out" and get face to earth.

If you want to do it by results, check and see if you had a really slow opening and the canopy sort of opened behind you. If yes, then you had a subterminal opening.

[kallend 1,623]

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Soooo
Back to the reason I posted this in the 1st place, how do I tell if I'm still sub-terminal when I pull my ripcord???

Deploy within 8 seconds of leaving the plane and you'll be going at less than 90% of terminal velocity (unless you are a feather).

[kallend 1,623]

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hmm i don't get much of the above although i just got back from work.....but i was always told by my (granted rather doppy) science teacher that a brick and a feather droped from the same height would eventually fall at the same speed? This never made sence to me though with wind resistance and all that.....hmmmmm

As someone who has been teaching college science and engineering for 30 years, it has been my experience that, for various reasons, many students completely misunderstand what they are told by their instructors, especially in math and science classes. I hope skydiving instruction fares better.

[Parrot 0]

For sub-terminal, pull after 10 secs
For terminal pull after 12 secs
I just passed those jumps on my way to the jumpcertificate

Heh, thats funny .

[SkydiveMonkey 0]

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For sub-terminal, pull after 10 secs

you'd be at terminal (or close to) after 10 seconds

____________________
Say no to subliminal messages

[kallend 1,623]

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For sub-terminal, pull after 10 secs

For terminal pull after 12 secs

I just passed those jumps on my way to the jumpcertificate

You passed subterminal, too

[Macaulay 0]

Basically, if there are no particles in the way, everything will fall at the same rate. If there are particles in the air, objects will be affected by those particles according to the objects' respective properties.

- Mac

[bobsoutar 0]

You reach terminal velocity (approximately 120 miles per hour if you are flat/belly flying) after 12 seconds in freefall. Anything less than 12 seconds is sub-terminal.

[nacmacfeegle 0]

"You reach terminal velocity (approximately 120 miles per hour if you are flat/belly flying) after 12 seconds in freefall. Anything less than 12 seconds is sub-terminal."

Unless you are exiting a Herc, or a 727, or an AN72. In which case you exit and spend the first couple of seconds slowing down!!!.Which is admittedly unlikely if you are still doing AFF consolidation jumps

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He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson

[bobsoutar 0]

Just trying to keep it simple for an AFFie! You're right though.

[nacmacfeegle 0]

No worries Bob, not jousting but jesting.....

--------------------

He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson

[Galen 0]

In short,
In a vacuum:
Every object, regardless of mass, accelerates at the same rate. That rates is 9.8 m/s/s or 32.2 ft/s/s here on earth. There is no terminal velocity because there is no column of air to support you.
The amount of damage you would do to the earth or site of impact if falling in a vacuum would depend upon your mass. that comes from the handy old equation f=ma posted eralier.
Guess college can come in handy.

Respect the Dolphin

[kallend 1,623]

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You reach terminal velocity (approximately 120 miles per hour if you are flat/belly flying) after 12 seconds in freefall. Anything less than 12 seconds is sub-terminal.

I guess you didn't read the rest of the thread.

If you want to feel what a subterminal deployment is like, don't wait 11.9 seconds before pulling. Or even 11. Or even 10.

[bobsoutar 0]

I did read the rest of the thread and was trying to give a simple answer to the guy who asked (ie without all the physics, vacuums, objects dropping off the Leaning Tower of Pisa etc. etc.)
For a sub-terminal opening the canopy obviously needs to be deploying and slowing you down before the 12 seconds are up. For an AFF qualifying jump the student is normally briefed to pull at 3-5 seconds (which is obviously sub-terminal but doesn't answer the original question).

[TheBile 0]

No one has brought gravity into it yet. All objects of mass have gravity. Strictly speaking, if you drop a feather and a cannonball at exactly the same on the moon, the cannonball will fall faster because it's mass generates a stonger gravitational field than the feather. The trouble is that gravity is such a relatively weak force that the difference between such objects and the effects it has on them is negligible, but it is still there.

Gerb

I stir feelings in others they themselves don't understand. KA'CHOW !

[nacmacfeegle 0]

Bile, sign up to Professor Kallend's 'Physics class for beginners' immediately......

Gravity is a force, which acts on objects of mass.

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He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson

[TheBile 0]

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Gravity is a force, which acts on objects of mass

Correct, but all objects of mass generate their own gravity. See that paperclip on your desk ? That generates it's own gravity, just like the Earth and the Moon. But because it's mass is so small, the gravity it generates is minniscule compared to that of the Earth, so ofcourse we don't notice it.
Ask Professor Whatsisface about it. He'll probably tell you that he teaches this in his 'Advanced Physics class for Intellectuals'.

Gerb

I stir feelings in others they themselves don't understand. KA'CHOW !

[FridgidAir 0]

How do weights effect Terminal?
OK not an expert, but I like to think of two things effecting fall rate (term vs. sub term)
1. Gravity. The earth's "pull" on an object. The heavier the Object, the greater the gravitational pull will be (not factoring in size)
2. Friction. Specifically in this case, the resistance the relative wind has against the falling object. It increases as the moving object falls faster.
To make it easy, Terminal Velocity is reached when these forces equal each other.
Soooo... Two objects of the same weight can have vastly different TV if the amount of friction is different (think head down vs belly to earth. Same weight, less friction to the relative wind)
AND two object of the same apparent size (belly to earth) can have vastly different fall rates (think of a tandem, where the weight is doubled and the friction is not much different from one person falling) This is how weights increase someone's fall rate and Terminal Velocity.
seems logical, but let me know if I've broken a law somewhereFridgidAir

[Cajones 0]

This exact discussion has taken place on another thread... Do a search.

The laws of physics are strictly enforced.

[fred 0]

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The laws of physics are strictly enforced.

But clearly not well-understood.

There's so much misinformation and misunderstanding in this thread... I find it frustrating to read.

The answer to the original question is simple. It takes an average person about 12 seconds to reach terminal velocity from an average airplane at an average elevation/temperature/pressure/humidity.

Gravity is a force that acts between any two objects of mass. As with all forces, there is an equal attraction between the two objects of mass, and this force is directly proportional to the mass of the two objects and inversely proportional to the distance between them. (equation is: F=G (M[1]*M[2]) / R^2, where F is the force, the M's are masses of the two objects, and R is the distance between them). In the end, it's not even this simple, since most objects with mass also have volume and you get into some pretty complex calculus to determine the 'exact' force of attraction.

So, given that M[1] is the earth, and M[2] is a skydiver and assuming the difference between 0AGL and 10,000ft AGL is insignificant, we find that a skydiver and the earth have a mutual attractive force of approximately M[2]*9.8 m/s^2.

Note that "Weight" is a measure of force, not mass. Thus, I exert a force of about 210 lbs (approximately 938 N) toward the earth. In the absence of any wind resistance, this force does not matter. F=MA is our friendly equation here, and we can establish that because of the relationship between F and M, A remains constant for the (simplified) case of earth's gravitational field. (Altitude will affect the value of A, but it will remain constant for any given M at that altitude).

Then we have drag. I've never been exposed to the actual mathematics behind it, so anybody is welcome to fill in details here. The drag force is proportional to the square of the velocity and a reference area, and is affected by a number of other factors (air density and viscosity, etc, etc). It exerts a force against whatever is causing the drag.

So as we fall through the air, we exert a (mostly) constant downward force (in my case, 938 N), while drag causes an opposite upward force. After leaving the airplane, my speed increases rapidly because my downward force is much greater than the force caused by drag. But drag catches up exponentially, and as I accelerate, it causes greater and greater upward force until the point where the drag force is exactly equal to the force due to my mass. At this point, I have reached terminal velocity.

By changing the reference area presented to the wind (and thus subject to the drag force), I can increase or decrease the force provided by drag, and the process repeats itself. Either I slow down until the forces match, or I accelerate until they do.

---

I don't expect that many of you read this far, but here's a nice example that I enjoyed from my physics textbook. Numbers may have been changed to protect the innocent.

Inner city cats have a habit of falling off of fire escapes and window ledges. Over the years, information has been collected and compiled from various NYC veterinary hospitals about the height of the fall and the extent of the injuries. An interesting trend appeared.

As heights increased to about 8 stories, the injuries to the cats increased proportionally, as would be expected. However, above 8 stories, injuries actually decreased. Thus, a cat who fell from the 11th story had a much better chance of survival than one who fell from the 7th.

So why is this? It has to do with how animals sense motion. We don't feel velocity, but only acceleration. This is why we get that stomache feeling when we go on a rollercoaster but thankfully don't feel the spin of the earth (or, if you were in a closed room falling at terminal velocity, you would have no idea that you were actually falling).

So, during the first 8 stories, the cat feels the acceleration, get's frightened, and curls up into a tiny ball. But after falling for 8 stories, the cat reaches terminal velocity. At this point, no longer feeling the acceleration, the cat begins to relax, and actually spreads out. This increases it's apparent area, thus increasing drag, and reducing it's terminal velocity. Voila, a slower falling cat has fewer injuries.

[kallend 1,623]

Quote

Quote

The laws of physics are strictly enforced.

But clearly not well-understood.

There's so much misinformation and misunderstanding in this thread... I find it frustrating to read.

The answer to the original question is simple. It takes an average person about 12 seconds to reach terminal velocity from an average airplane at an average elevation/temperature/pressure/humidity.

Gravity is a force that acts between any two objects of mass. As with all forces, there is an equal attraction between the two objects of mass, and this force is directly proportional to the mass of the two objects and inversely proportional to the distance between them. (equation is: F=G (M[1]*M[2]) / R^2, where F is the force, the M's are masses of the two objects, and R is the distance between them). In the end, it's not even this simple, since most objects with mass also have volume and you get into some pretty complex calculus to determine the 'exact' force of attraction.

So, given that M[1] is the earth, and M[2] is a skydiver and assuming the difference between 0AGL and 10,000ft AGL is insignificant, we find that a skydiver and the earth have a mutual attractive force of approximately M[2]*9.8 m/s^2.

Note that "Weight" is a measure of force, not mass. Thus, I exert a force of about 210 lbs (approximately 938 N) toward the earth. In the absence of any wind resistance, this force does not matter. F=MA is our friendly equation here, and we can establish that because of the relationship between F and M, A remains constant for the (simplified) case of earth's gravitational field. (Altitude will affect the value of A, but it will remain constant for any given M at that altitude).

Then we have drag. I've never been exposed to the actual mathematics behind it, so anybody is welcome to fill in details here. The drag force is proportional to the square of the velocity and a reference area, and is affected by a number of other factors (air density and viscosity, etc, etc). It exerts a force against whatever is causing the drag.

So as we fall through the air, we exert a (mostly) constant downward force (in my case, 938 N), while drag causes an opposite upward force. After leaving the airplane, my speed increases rapidly because my downward force is much greater than the force caused by drag. But drag catches up exponentially, and as I accelerate, it causes greater and greater upward force until the point where the drag force is exactly equal to the force due to my mass. At this point, I have reached terminal velocity.

Actually it is nor exponential, it is a square law as you stated earlier.

The velocity approaches terminal according to an arctangent function assuming everything (drag coefficient, reference area, air density) stays constant.


***
By changing the reference area presented to the wind (and thus subject to the drag force), I can increase or decrease the force provided by drag, and the process repeats itself. Either I slow down until the forces match, or I accelerate until they do.

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A point that seems to be overlooked in a lot of these answers is the rate at which terminal velocity is approached. As I stated above, the function that describes this is the arctangent. Practically, this means that you reach about 90% of terminal in about 8 seconds, and slowly creep up on TV thereafter.

So if your goal is to experience a subterminal opening, don't assume that an 8 second delay will feel detectably different than a 40 second delay.

To get an opening that feels distinctly different, you need to be deploying within about 4 seconds.

jk - physics professor.

[Push 0]

Drag depends on your acceleration and surface area (assuming all the other hojillion variables remain constant as you're falling. Actually, one probably doesn't - the air is turbulent as you're stabilizing. The exact effect of this needs a physics professorAs far as the skydivers affecting the earth, unfortunately the "rock your world" expression is not applicable in this case, sorry
How will you know you're at terminal? You can actually feel it if you're stable. Have you ever jumped on a mattress? Same feeling, only a little less pronounced. It's almost impossible to explain, but you will know it when it happens. Just look for it.

[kallend 1,623]

Dorm-room physics is always interesting.

[billvon 2,396]

>All objects of mass have gravity. Strictly speaking, if you drop a
>feather and a cannonball at exactly the same on the moon, the
> cannonball will fall faster because it's mass generates a stonger
> gravitational field than the feather.

Incorrect. They will both accelerate towards the moon at exactly the same speed. Now, if that cannonball masses as much as the moon, the moon will move towards _it_ and thus decrease the time to impact, but the acceleration of both the cannonball and the feather will remain the same (ignoring the attraction between the feather and cannonball of course.)

[cgross 1]

You guys are somewhat right.. (I have a major in Atmosheric Physics) It is all reletive right. Yes every object does have its' own mass and therefore gravity, but when we are discussing objects like people or cannonballs or feathers, it really doesn't matter. Obviously the numbers are so small (1x10 -10) that they can be neglected. Even large objects like mountains could be neglected when comparing it to the entire earth.
So, in a vacuum, objects will continually accelarate at the same rate (10m/s2) until impact, and TV will never be achived, since there is no drag to slow accelaration. Also 2 people that have the same shape and mass will have the exact same drag coefficient, and will therefore fall at the same rate. However several things can dramatically alter the rate of accelaration. body possition, clothing... etc.
For people that fall faster than their partners, steps can be taken to match speeds. Wear baggier cloths, arch harder, give slower people weights...