Physics of Skydiving Question

[UnusualAttitude 0]

This is something that's been bugging me for a while, so maybe someone who knows more about physics than I can explain...

We all know that famous experiment where Galileo dropped two objects of different weights and they fell at the same speed. In fact, as I understand it this is just a simple fact of Newtonian mechanics -- a heavier object is pulled faster by gravity, BUT it has more inertial desire to not move, so all in all, heavier things do not fall faster than lighter things.

Of course, if you're dropping a 100lb dumbell and a feather, the feather will fall slower due to air resistance issues...

I imagine parachutes also work something like this ;-)

At any rate, I know that in freefall people can wear weights, and they fall faster. I've seen it! But how can this be? It just changes their weight, but not their shape and therefore not their air resistance, right?

Jim who weighs 100 pounds, and Jim with the same shape who weighs 120 because he has a weight belt - shouldn't they fall the same speed according to the fundamentals of physics?

But, they don't -- so what do i have wrong?

[kelpdiver 2]

acceleration is the same. terminal velocity is not.

If the tower of Pisa were taller, Gallileo's experiment would have failed.

[tommythevegan 0]

Surface Area to Mass ratio.

http://www.straightdope.com/mailbag/mbugfall.html

[rehmwa 2]

Quote

.... We all know......-- a heavier object is pulled faster by gravity, BUT it has more inertial desire to not move, so all in all, heavier things do not fall faster than lighter things.....

"We all know"? "faster"?
I think many agree with the theory that we all have a constant proportional force of acceleration by gravity at the same free acceleration factor.

This has nothing to do with fall rate in a non-vacuum.



In any case, we accelerate (resistance will increase) until termnal velocity is set when resistance equals weight.

Resistance is a function of (friction, area presented to the wind, velocity, air density, shape)

If you consider all these things to be constant (same air, same suits, same body types and position (area and shape)) then weight will be offset by velocity (it's squared relation).

Jim with the weight belt falls faster.

the classic equation (I think is derived from a units analysis) is: Force = [(air density)/2*vel^2]*(area)*(friction/shape coefficient)]

Force must equal weight

In skydiving fall rate, we change our area and our coefficient by moving about (mostly shape). The balance responds by equalizing with Vel changes.

In your Jim scenario, all those are steady and we only change Weight and Vel responds


In other words, you're having a block by thinking of the acceleration term incorrectly instead of the steady state term correctly.

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[Trivial_Trekker 0]

150 pound person wiht a 20 pound weight will fall faster than with out the weight.

Terminal Velocity is when the falling object is in a state of equilibrium. Meaning it is not accelerating in any dircetion.

So when a 150 pounder reach TV his weight equals the force of drag imposed on his body by wind resistance.

A 150 pounder with a 20 pound weight (same body shape) will fall faster, because extra drag is needed to put him in a state of equilibrium.

i.e. the faster an object moves through the air, the more drag from wind resistance pushes back.

I hope this helped

[]DETE

[jakee 1,254]

Quote

At any rate, I know that in freefall people can wear weights, and they fall faster. I've seen it! But how can this be? It just changes their weight, but not their shape and therefore not their air resistance, right?

Balancing of forces!

A body at terminal has the same amount of force pushing it down (weight) as it does pushing it up (drag).

When you add more weight but do not change the shape or surface area the only way for the two forces to be balanced again is for the object to fall faster before reaching terminal.

Honestly, there are serious, serious issues with the way physics is taught in schools these days

Do you want to have an ideagasm?

[Andy9o8 0]

Quote

Honestly, there are serious, serious issues with the way physics is taught in schools these days

Oh, that's nonsense. There's this book, "The idiot's guide to physics." About 250 pages, and it's all anyone will ever need to know about physics. Ever.
The rest is just supersilious bullshit designed to create meaningless jobs for nerds.

[rehmwa 2]

Quote

The rest is just supersilious bullshit designed to create meaningless jobs for nerds.

Mmmmmmmm, tenure, (slobber)

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[jakee 1,254]

Quote

Quote

Honestly, there are serious, serious issues with the way physics is taught in schools these days

Oh, that's nonsense. There's this book, "The idiot's guide to physics." About 250 pages, and it's all anyone will ever need to know about physics. Ever.

Is it used in the classroom?

If not, I rest my case

Do you want to have an ideagasm?

[Andrewwhyte 1]

Quote

acceleration is the same. terminal velocity is not.

Not quite. Terminal velocity is a function of acceleration. Each object has their own acceleration function with acceleration reaching zero at a time and speed unique to the function. If this was not true we would not be able to make up any vertical distance on each other for the first twelvish seconds of the skydive.

Quote

If the tower of Pisa were taller, Gallileo's experiment would have failed.

Gallileo's experiment would have failed if he had conducted it; he did not. What he did was announce he was going to as an example of applying his new "scientific method" to the problem as opposed to simply applying logic to accepted principles. This was enough to get the church on his back. If I am not mistaken the experiment was conducted about ten years ago, demonstrating the flaw in his hypothesis (but not his method).

[UnusualAttitude 0]

Ok, I think that clears it up. So, roughly speaking, acceleration is the same, but the heavier person has a faster terminal velocity. I should have known that from my experience skydiving...but I think it was the whole "myth" about dropping two things of different weights from a tower that was confusing me.

Thanks guys.

[boyd38off 0]

If in air, the terminal velocity is a factor of the mass to surface area ratio. Increasing the mass without increasing the surface area means higher terminal velocity (think weight belt). Increasing the surface area without changing the mass reduces the terminal velocity (think deploying the parachute). This is also how skydivers change their fall rate relative to each other. Want to go slower? Get bigger... you still have the same mass, but you have added surface area, changing the mass to surface area ratio.

Life's journey is not to arrive at the grave safely in a well-preserved body, but rather to skid in sideways, totally worn out and shouting,
".... holy crap....what a ride!"

[Cashmanimal 0]

everything does fall at the same speed neglecting air resistance, aka in a vacuum. I am sure there are some videos I am too lazy to find. One that sticks out in my mind is a feather and a quarter being dropped at the same time in a vacuum, and both hitting the ground at the same time.

The most basic way to explain your confusion i think would be to point out that speed due to gravity is a multiple of the acceleration of gravity. Imagine gravity as a person pulling you down by a string attached to your waist in freefall. Heavy people fall faster because their weight causes gravity to pull harder, lighter people fall slower because gravity does not pull as hard. It is the multiplier coming into effect (acceleration of gravity is approx. 10 meters per second. 10 meters per second multiplied by a person weighing 100 pounds is not pulling as hard as 10 meters per second pulling on a person weighing 200 pounds).

Air resistance definitely comes into play as well. That whole "Every action has an equal and opposite reaction" thing holds quite true in skydiving. When you jump out of the plane, you accelerate. But not for very long. As soon as air resistance is pushing up on your body as hard as gravity is pulling down (equal/opposite) you have reached terminal velocity. This is why low-profile people (skinny) can wear weights and fall at the same rate as a high-profile (fat guy) wearing baggy clothing/jumpsuits.

I am sure i somehow repeated what other people said, and am fully aware that you said it had been cleared up. But, can't deny an opportunity to flaunt at least a sample of this so-called education of mine.

It's all fun and until someone loses an eye... then it's just a game to find the eye

[strop45 0]

an object increases its speed when it has unequal force acting on it. For a skydiver, gravity provides one force and drag an opposing force. When the force due to drag is equal to the force due to gravity the objects speed remains constant, i.e. it is at terminal velocity.

In the absence of air and therefore drag, a feather fails at the same rate as a brick. Add some air and the story is different.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[toronto_bill 0]

Don't listen to them. None are right.

Gravity is a 'Push' not a pull.

Its the stars pushing, not the earth sucking.

http://www.keelynet.com/gravity/wright.htm

I dont call it 'Jumping' I call it getting shoved.

[iamsam 0]

all you need to take from this thread - FFFF








Fat Fuckers Fall Faster

but what do I know

[champu 1]

This thread is fascinating. Even the correct answers aren't presented very well.

So far jakee's answer wins. (sorry rehmwa, you're killing the audience.)

[happythoughts 0]

Want to make it interesting?

Think of two things of equal weight. A person with their container closed and the same person with their parachute inflated.

speed = (mass times the acceleration of gravity) minus drag.

mass can be increased with weights.

drag is air friction. in a vacuum, there is no air friction.

things that increase air friction:
-shape of objects (open/closed parachute). size.

-presentation. turn a sheet of plywood on its side or lay it flat. it can lay on a column of air or slide through it.

-surface friction as air passes over it (nylon vs cotton).

Formations start to fall slower because the column of air under them cannot get around them as easily.

Different shaped formations fall at different speeds with the same people in their same gear.

[jakee 1,254]

Quote

This thread is fascinating. Even the correct answers aren't presented very well.

So far jakee's answer wins. (sorry rehmwa, you're killing the audience.)

Thanks. Sometimes I think ya just gotta sacrifice precision for understandibility. Get the basic concept out there and then worry about the details.

Do you want to have an ideagasm?

[kallend 1,619]

www.iit.edu/~abet/physics.zip

An entire Powerpoint presentation on the physics of skydiving...

...

The only sure way to survive a canopy collision is not to have one.

[Steve_Lewis 0]

What's all this talk of "vacuum"? There are virtual particles everywhere. If there are no particles there then "there" isn't anywhere.

Tee hee.

[kallend 1,619]

Quote

Quote

The rest is just supersilious bullshit designed to create meaningless jobs for nerds.

Mmmmmmmm, tenure, (slobber)

ARe you, perchance, typing that on a computer containing semiconductors, dielectric, piezoelectric, photonic and magnetic materials?

...

The only sure way to survive a canopy collision is not to have one.

[jakee 1,254]

Quote

Quote

Quote

The rest is just supersilious bullshit designed to create meaningless jobs for nerds.

Mmmmmmmm, tenure, (slobber)

ARe you, perchance, typing that on a computer containing semiconductors, dielectric, piezoelectric, photonic and magnetic materials?

Nah you don't need all that stuff. I made mine out of a few lengths of scrap two by four, an empty picture frame, three bent nails, the pendulum from an old mantelpiece clock and a hampster going round in a wheel.

Just last week I upgraded by broadband connection by attatching a second piece of string between the two cups. Amazing what you can learn from a 'Boys Own' manual.

Do you want to have an ideagasm?

[Tink1717 2]

It always amazes me about how easily physics people forget about the atmosphere. Even when said person seems to remeber about it, as soon as they get past the first orpeation, the atmosphere disappears from the thought process. The reason why heavier people fall faster than light ones who are the same size has to do with the amount of mass for surface area. For the same surface area, the object with the greatest mass will have the higher terminal velocity.

Skydivers don't knock on Death's door. They ring the bell and runaway... It really pisses him off.
-The World Famous Tink. (I never heard of you either!!)
AA #2069 ASA#33 POPS#8808 Swooo 1717

[jakee 1,254]

Quote

It always amazes me about how easily physics people forget about the atmosphere.

No, I think that the problem is how easily non physics people forget about the atmosphere

Do you want to have an ideagasm?