Re: [Phys-l] Linear Air Drag

["LaMontagne, Bob" <RLAMONT@providence.edu>]

One of the in-class mini-labs that I do with my General Physics students here at Providence College is to have them drop steel balls with diameters ranging from 1/16" to 1/2" in steps of 1/16" into 2 liters of Pantene Pro V clear shampoo.

BC is correct that drag is not a simple function of speed. The relevant drag formula in this case is Resistance = C r v, where C is a constant and r is the radius of the sphere. The terminal speed becomes proportional to r^2. In a single one hour lab the students can plot log(v) versus log(r) and verify the relationship. It's a nice break from the usual class routine. The smallest sphere takes about 8 minutes to traverse the fluid, the largest about 12 seconds.

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard Cleyet [bernardcleyet@redshift.com]
Sent: Tuesday, November 03, 2009 1:11 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Linear Air Drag

The Reynolds number for water droplets (e.g. in clouds) is within the
limit for viscous (Stokes') drag.

However, BW is quite correct (I pray!) for the usual projectiles.
Intermittent updraft in a cloud is the only case ICTO where Peter's
work would be applicable.  Well, one could w/ a submerged spring gun
fire a bb up a cylinder of oil.  Remember to include buoyancy!

bc must write to the "The Physics Teacher".

p.s. v^3 if extreme.   Drag is not a simple function of speed.

On 2009, Nov 02, , at 19:19, Brian Whatcott wrote:

> >
> Hmmm. I'm puzzled by the concept of linear air resistance.
> Air drag for projectiles is a function of v squared.

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