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bdenny20

How do I explain the differences in freefall rates?

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Hi.

So I went up to my folks' place for Christmas. This is the first time my entire family has been together since I picked up skydiving. While I was talking to my brother about it, I mentioned that because I'm a larger and heavier guy, I've been having a little trouble flying with other people because...well, most of you fuckers are small and I have some difficulty slowing down.

Now, real quick. He comes from an scientific background. Chemistry, but that path included lots of courses from other areas of focus, including physics. He immediately goes off saying how that shouldn't be a problem, gravity effects everyone the same, fall rates should be equal, yadda yadda yadda you know the type.

I'm not from a scientific background, let's put it at that. I like flying airplanes and as of recent, jumping out of them. I wasn't able to convince him...so I pulled up a video of a recent jump with me an one other small chick. It shows us docked in a two way belly, and as soon as we separate, I take off like a rock.

He's perplexed. I know of the concepts of air resistance and how I'm still new and my form isn't spot on, but I'm not smart enough to come up with an educated response....How do I explain to non-skydivers that us fattys have a hard time keeping up with lighter peeps?

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Yes gravity affects everyone the same.

We'd keep on accelerating the same (alongside a feather and bowling ball) in a vacuum.

Air resistance basically works the same for everyone, but the total amount depends on our surface area facing the airflow. (Plus shape to some degree, streamlined or not. But none of us are streamlined like a wing)

However the ratio of air resistance to mass changes because the area to mass ratio changes as things get bigger or smaller. (The square-cube law.) Double the area of the side of a cube, and the volume is 4 times as much.

Or in less technical terms, you may only be 20% taller than some smaller girl, but you could be, who knows, twice her weight.

So at a given equilibrium speed, the force from the air might keep the small jumper from keeping on accelerating towards the speed of light, as the upwards force of the air matches the downwards force of her weight. (I won't get into minor issues of weight vs. mass vs. buoyancy.)

For you at the same speed, you will have a little more air resistance from a somewhat larger surface facing the wind, but a whole lot more weight, so you keep accelerating to a faster equilibrium speed.

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Imagine a guy with a given exit weight falling at a given speed. Simplistically, he is falling at his terminal velocity as determined by his weight and body configuration or surface area (disregarding aerodynamic shape, and other complicating factors). Now he deploys a round canopy. He is now falling at a new, much slower terminal velocity, also determined by a new surface area (even though the total weight is the same). As far as physics is concerned, he's still in "freefall" ...not freefall as we define it, but "freefall" nonetheless. Same physics at work, just different numbers plugged in. Skydivers in "skydiver freefall" adjust the numbers with jump suit design, addition of weights, and subtle changes in body configuration, eventually doing the latter subconsciously. Basically, you think you want to match fall rates or"go over there" and you just do it. Similar to the way a hockey player concentrates on playing the game and not on his skating.

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Take two pieces of paper. Crumple one into a ball, and don't crumple the other. Drop both from six feet, and see which one lands first.

Wendy P.
There is nothing more dangerous than breaking a basic safety rule and getting away with it. It removes fear of the consequences and builds false confidence. (tbrown)

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bdenny20

...so I pulled up a video of a recent jump with me an one other small chick. It shows us docked in a two way belly, and as soon as we separate, I take off like a rock....



My guess is that, while you were in the 2-way, you both were working to match fall rates. When you let go, you both probably relaxed a bit and she floated while you dropped away.

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it could have something to with the Mass to Surface Area Coefficient.
A short person who weighs in at 200 pounds AND has short arms... i.e. wingspan, will fall faster in a basic (flat ) freefall position, than a 200 pounder who is tall and with much longer arms and legs .....

We talk about canopy aspect ratio but seldom have I see described,,,, any concept of "freefall weight to surface area "
Such a ratio is seldom considered , let alone spoken about....

small surface area and small mass... might fall compatibly with a large surface area having Larger mass.... as long as a similar "balance ' is achieved. I.e. an " area to mass ratio "
By the same token, small surface area with Large mass... will Rocket while Large surface area and small mass... will float....( Relatively speaking )

Jumpsuits can increase or decrease surface area and weightbelts can increase mass.....

gravity IS gravity and we know that it is a Law...But we skydive in a non vacuum and so wind resistance and drag, also come into play.....as well as body position exposed to "the relative wind" All these things can increase or decrease Velocity...:)
jmy
A 3914
D 12122

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Thanks for the responses!

I got just what I hoped for, I knew there had to be some smart types floating around here.

I directed my brother to this thread, all seems to be understood now.

In case anyone was curious, I uploaded said video after I created the thread.

https://www.youtube.com/watch?v=Xhw8bOPy6Vk

Just shows me taking off like a rocket, and when we got closer that was all her. Left the belly and took a more streamlined approach to catch up.

The good news is that since that video was taken, I spent a few more weekends up at the Ranch and specifically sought out small jumpers who wanted to belly with me. By the end of those weekends, I was getting considerably better. Not perfect, but better. The second I took my focus off my body position though, I lost it and lost them in the process. I will be working in the tunnel a little bit on Long Island after I get back from Florida.

Thanks again!

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Yeah that screwed me up early on, too. Dude was putting weights on and I was like "But... But... Galileo..." Although it doesn't so much touch on that exactly, I really like this talk on the subject. You don't even need to fill in your own bits about wind resistance et al. This pretty much covers the rest.
I'm trying to teach myself how to set things on fire with my mind. Hey... is it hot in here?

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pchapman

...Double the area of the side of a cube, and the volume is 4 times as much.



Your maths are a little off. If you enlarge a cube by doubling (2x) the area of the sides of the cube, then you've increased the edge length by SQRT(2). Volume would go up by (SQRT(2)^3) or about a factor of 2.8. OTOH, if you meant to say that you've doubled the edge of each side of the cube (so increasing the surface area by a factor of 4) then the volume increased by factor of 8.

To get a 4 fold increase in volume of a cube, you would need to increase the edge length by a factor of ~1.59, resulting in an increase in the surface area by a factor of ~2.52.

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Divalent


Your maths are a little off.

To get a 4 fold increase in volume of a cube, you would need to increase the edge length by a factor of ~1.59, resulting in an increase in the surface area by a factor of ~2.52.



Dang it, thanks, should have just gone with twice the length, four times the area, eight times the volume.
Messed up the square-cube rule I quoted...

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Vt = sqrt((2 * m * a)/(Cd * rho * A)
where:
Vt => terminal velocity (m/s)
m => mass (kg)
a => acceleration (m/s^2)
Cd => coefficient of drag (dimensionless)
rho => air density (kg/m^3)
A => frontal area (m^2)

So anything that makes the numerator larger, increases speed.
- "m" increasing mass.
- "a" always equals "g" i.e. 9.8m/s^2 for freefall.

Anything that makes the denominator larger, decreases speed.
- "Cd": Changes with body position, e.g. bellyflying generates higher Cd than headdown.
- "rho": Increases with higher barometric pressure, lower altitude, or colder, drier air.
- "A": Increases with flatter body position, and/or bigger jumpsuit, especially a wingsuit.
"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

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Ask him if he thinks that you still fall at the same speed when your parachute is open compared to when it was closed. (although probably more scientifically correct if it was a round parachute as it's not a wing)

That should hopefully help him understand the surface area plays a pretty big role.

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your brother is not all that sciencey.
Tell him to drop a pen and a piece of A4 paper at the same time, then walk away and tell him to Google it.
You are not now, nor will you ever be, good enough to not die in this sport (Sparky)
My Life ROCKS!
How's yours doing?

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bdenny20

Ha!

Thanks for all the replies. I think we're on the same page now. Like I said, he's a engineer in the Chemistry department, not physics. But he fancied himself savvy in the area therefore questioned the whole idea. But we're straight now.



Now he will understand that it is because of all that drag that skydivers go back upwards when they open.....

:P



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