terminal velocity MATH question

roid · Post by **roid** » Thu Dec 01, 2005 4:25 am

for a sphere of:
radius 1.6meters (5.2 feet)
weight 180kg (397 pounds)

what would be it's terminal velocity (earth @ sea level)?

from my quick calculations i got 16.6m/s (60kph)
But i don't know too much about Reynold's numbers and all that, which supposedly go kinda bonkers when you are dealing with spheres (ie: drag coefficient of sphere drastically changes as velocity gets faster).

some other person got 26.1m/s, but i don't know if they are right or not.

can anyone capable of this kinda math try it out and tell me what they got? it'd be most helpful.

(i'm basically trying to figure out the terminal velocity of a manned Zorb ball)

Mobius · Post by **Mobius** » Thu Dec 01, 2005 3:03 pm

Interesting question, but very hard to answer.

26 m/s seems high to me. That's 93 kph.

I do know a stalled paraglider descends at around 10 m/s, with what looks like a bundle of washing above your head.

Given a Zorb ball is 3.2 metres in diameter, and larger than a stalled paraglider wing in cross-sectional area, I would think that 16 m/s sounds more correct.

FYI: 5 m/s will break a leg. 10 m/s will generally kill you on impact, or at the very least, drive your legs up into your abdomen, and your tibias get forced through your feet and 10 cm into the dirt. It's very nasty.

dissent · Post by **dissent** » Thu Dec 01, 2005 5:21 pm

roid wrote:(i'm basically trying to figure out the terminal velocity of a manned Zorb ball)

Hey, going on a trip, Roidy?

fyrephlie · Post by **fyrephlie** » Fri Dec 02, 2005 1:46 am

math is dumb ... you dont use it in real life.

easy answer... don't reach terminal velocity in a manned zorb.

roid · Post by **roid** » Fri Dec 02, 2005 7:59 am

i'm still hoping someone here can check my numbers.

dissent wrote:
roid wrote:(i'm basically trying to figure out the terminal velocity of a manned Zorb ball)
Hey, going on a trip, Roidy?

i could do eventually. it's a "parachute replacement" invention idea i'm working out some rough feasability numbers for.

I'm basically gonna start with the design for the Zorb ball and work from there.
now i gotta:
- estimate the cushioning force of the Zorb ball hitting the ground at that speed.
- work out if the subsequent landing G force exerted on a human body housed within the Zorb would be dangerous.
- design onwards from there.

fyrephlie wrote:math is dumb ... you dont use it in real life.

easy answer... don't reach terminal velocity in a manned zorb.

maths is rather important when you are dealing with a life and death situation like this. You'll find it hard to get much more "real life" than calculating whether or not you will die.

not to mention cheaper than stealing (as they don't sell them) a Zorb ball, putting "BUSTER" inside the ball and then chucking it all outof a flying plane.

ccb056 · Post by **ccb056** » Fri Dec 02, 2005 6:22 pm

this is exactly what monkeys can be used for

jakee308 · Post by **jakee308** » Fri Dec 02, 2005 6:36 pm

roid wrote: it's a "parachute replacement" invention idea i'm working out some rough feasability numbers for.

nasa did something similar for mars lander. perhaps they've got speed/mass/volume tables for earth gravity. i didn't look too hard but this article is specifically about the cushioned landing

http://www.nasa.gov/vision/universe/rob ... rbags.html

fyrephlie · Post by **fyrephlie** » Sat Dec 03, 2005 12:59 am

roid wrote:
fyrephlie wrote:math is dumb ... you dont use it in real life.

easy answer... don't reach terminal velocity in a manned zorb.
maths is rather important when you are dealing with a life and death situation like this. You'll find it hard to get much more "real life" than calculating whether or not you will die.

not to mention cheaper than stealing (as they don't sell them) a Zorb ball, putting "BUSTER" inside the ball and then chucking it all outof a flying plane.

man ... everyone caught the serious bug lately. boo!!

actually, a parchute replacement idea sounds very interesting. i seem to remember seeing projects like this in that past (such as NASA). have you looked into any work that others might have done, perhaps you can get a feel for your numbers that way.

imo it doesnt really sound feasible. the shear size and materials required would be tough to work out, but i put it to you to prove me wrong. since this is clearly not my area, i will bow and and lurk. sorry to upset you. :*(

SuperSheep · Post by **SuperSheep** » Sat Dec 03, 2005 11:08 am

This what your looking for?

http://hyperphysics.phy-astr.gsu.edu/hb ... i2.html#c5

roid · Post by **roid** » Fri Dec 09, 2005 12:34 am

i tried websites like that one Supersheep, but i wasn't too confident because it didn't mention anything about the changing Reynold's numbers that you get with a sphere. I wasn't sure if it had ignored Reynold's number issues to simplify the exersize.

It seems that the "Reynold's Number" for a particular object indicates whether the fluid/gas flowing past it is laminar or turbulent. This depends (this is especially the case with a sphere) on how fast the sphere is traveling.

ie: when the sphere is traveling at low velocitys, it's Reynold's number is low, it has a rather laminar (smooth) airflow around it. But if the sphere is going fast, the airflow is completely different - with vortexes and eddy currents forming - therefore showing that objects (especially spheres) have changing properties of aerodynamics which are dependant on the speed of the object.
(I guess this is all ULTRA important when dealing with scramjet engine designs, but that's another story)

Jakee, thx for the suggestion regarding the Mars Rover Missions. But those spacecraft are 900kg (+/- 100kg), that's approx 8x the weight in my project. Mars has different gravity, air density and weather. The airbags on the Rover missions are not used to slow the Rover's descent (traditional parachutes are used for that), they are just used to cushion that final impact. There were no Humans onboard so the landing could be of much higher G forces. And finally, the airbag configuration on the Rover lander is not spherical like the Zorb is.

It may help in estimating the cushioning ability of the Zorb for a landing though. But not useful in figuring out the terminal velocity of a Zorb (plus human) falling at low altitude in Earth atmosphere.

Actually, there is something else that maybe someone could help me with:

Does anyone have a recording of the Jackie Chan Movie "Armour of God II" / "Operation Condor" / "Fei ying gai wak" ?

http://www.imdb.com/title/tt0099558/

because there is a segment in that where Jackie Chan uses a ball similar to a Zorb, and rolls off a cliff with it. I'd like to see it.

terminal velocity MATH question

terminal velocity MATH question

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