View Full Version : Physics/Mechanics Geniuses

Just a Girl

05-03-2005, 03:29 AM

i don't suppose any of you know of Stokes Law/Force and could explain it to me could you?

i'm supposed to be doing Mechanics 2 coursework and, having not done Mechanics 1 or Physics, i'm at a slight disadvantage and have no idea what's going on. the internet is a big pile of crap and i can't find anything useful.

Bazza

05-03-2005, 03:33 AM

quoting a site:

An equation relating the terminal settling velocity of a smooth, rigid sphere in a viscous fluid of known density and viscosity to the diameter of the sphere when subjected to a known force field. It is used in the particle-size analysis of soils by the pipette, hydrometer, or centrifuge methods. The equation is:

V = (2gr²)(d1-d2)/9µ

where

V = velocity of fall (cm sec-¹),

g = acceleration of gravity (cm sec-²),

r = "equivalent" radius of particle (cm),

dl = density of particle (g cm -³),

d2 = density of medium (g cm-³), and

µ = viscosity of medium (dyne sec cm-²).

i do a2 physics, but i've never come across this before.

Just a Girl

05-03-2005, 03:48 AM

cheers, though i don't really understand that either. what's confusing me is that every site i've been on seems to have a different equation for it. and i don't understand any of them. and they all seem to need values for things that i don't know.

i hate mechanics and i hate physics. why do you like it? hmm? pure maths is much nicer.

Bazza

05-03-2005, 03:59 AM

i said i did physics, not enjoyed it. i never really know why i choose to do physics. i do maths as well, but switched courses, so don't have to do mech 2, hehe :D

Bazza

05-03-2005, 04:04 AM

ahh shit, got russian now, better go.

ta ta

Just a Girl

05-03-2005, 05:17 AM

ah we could choose between mech 1 and discrete in maths last year so i chose discrete because it had no coursework (i fucking hate coursework) but then this year i find out we're doing mech 2 in further maths. so now i really wish i'd taken mech 1. stupid school could have told me. bastards.

Gordie

05-03-2005, 07:02 AM

i dropped physics 12 :D

sKratch

05-03-2005, 07:30 AM

Stoke's Theorom is calculus. It allows you to evaluate the closed line integral F dot T ds as the double integral curlF dot n dS.

Endymion

05-03-2005, 07:38 AM

ugh...

the so-called stokes law is a solution to the navier-stokes equation for spherical particles in a viscous fluid. it boils down to F=6*pi*r*eta*v, where F is your stokes' force, pi is 3.14..., r is the particle's radius, eta is the viscosity of the fluid, and v is the particle's velocity. if the particle is falling under gravity, you can find it's terminal velocity using(2/9)r^2 g (rho1 - rho2)/eta, where rho1 is the particle density and rho2 is the fluid density.

Just a Girl

05-03-2005, 09:55 AM

that made sense. thanks!

Endymion

05-03-2005, 10:25 AM

any actual explanation of where that comes from would involve fancy calculus stuff, so i didn't bother. suffice to say, when you assume some stuff and simplify down to just a sphere falling in a uniform viscous fluid, you can actually solve it.

what machanics are you doing, anyway? the "normal" way to handle falling bodies in fluids is to assume some sort of collision rate with the fluid against the surface of the body, and get some sort of f=kv^2 sort of thing for the drag and integrate away.

Just a Girl

05-03-2005, 11:09 AM

i have to do a project on whether a crap golf player should pay for more lessons or buy a better golf ball. which leads to me doing time consuming experiments with golf balls, and then having to justify why i can ignore air resistance. which is why i asked about stokes force. yes, it really is as boring as it sounds.

Endymion

05-03-2005, 03:54 PM

stokes' force doesn't even apply because the golfball is pitted, you'd have to actually solve the navier-stokes equation...

[edit] or numerically approximate it... or empirically find the data you need.

I knew Endy would come to the rescue.

*imagines Endy in a Superman-suit with the II of "Intelligence Inside" on his chest*

sKratch

05-03-2005, 04:44 PM

But all his super powers would have to follow the laws of physics and that's no fun.

But all his super powers would have to follow the laws of physics and that's no fun.

*Endy saves a dame from a burning building*

*flies above the city*

*people look up & point, saying "is it an airplane, yadayada"*

Endy; "& -this- is what Newton meant with his theory of gravity."

Endymion

05-03-2005, 05:31 PM

newton's theory of gravity is so naive. einsteins isn't all that much better... i'm totally into process physics (http://www.scieng.flinders.edu.au/cpes/people/cahill_r/HPS13.pdf) now, yo.

[edit] =/ space got stuck in the url.

you totally ignored & ruined my joke.

*frowns & folds arms like a spoiled little girl*

sKratch

05-03-2005, 05:59 PM

four oh four

404

Endymion

05-03-2005, 07:04 PM

four oh four

404

fixed por tu.

JohnnyNemesis

05-03-2005, 07:06 PM

If Endy wasn't dominating this thread, I'd be looking out the window for the winged pigs.

you totally ignored & ruined my joke.

*frowns & folds arms like a spoiled little girl*

For what it's worth, I got a chuckle out of it!

*face brightens*

really??

JohnnyNemesis

05-03-2005, 07:12 PM

Really! *watches as your face brightens to almost dangerous levels just like the sun, which is incredibly beautiful but oh so bright*

Endymion

05-03-2005, 07:30 PM

Really! *watches as your face brightens to almost dangerous levels just like the sun, which is incredibly beautiful but oh so bright*

damn, that's HAWT!

sKratch

05-03-2005, 07:41 PM

But apparently my reasoning for why it's hot is all wrong :[

Freaking process physics...

I started reading the pdf a little (instead of studying for my calculus exam) and it's a little unsettling to read "Einstein is wrong," written just about as bluntly as that. Yeek.

Endymion

05-03-2005, 07:51 PM

<g> not that he was *wrong* wrong, just wrong.

Just a Girl

05-04-2005, 04:29 AM

stokes' force doesn't even apply because the golfball is pitted, you'd have to actually solve the navier-stokes equation...

[edit] or numerically approximate it... or empirically find the data you need.

it wasn't my idea, my teacher told me to do it. and he marks it. so that's ok

sKratch

05-04-2005, 07:29 AM

Would I be incorrect to associate this with analytical mechanics?

Endymion

05-04-2005, 08:14 AM

no, but it wouldn't be easy there either. you'd still have to use some mathematics software to crunch all the numbers for you.

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