Chronology |
Current Month |
Current Thread |
Current Date |

[Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |

*From*: Rondo JEFFERY <RJEFFERY@WEBER.EDU>*Date*: Tue, 6 Feb 2001 14:53:58 -0700

Bob Muir and others,

We do the experiment of dropping coffee filters and finding their terminal velocities with a sonic ranger in our intro labs. It is part of a larger lab on freely falling bodies, and the students aren't actually asked to calculate the drag coefficient, just see if the relation between mass and vt is linear or non.

A couple of years ago I did a more careful experiment to see if it would be good to add to our lab. The results were presented at our local (Idaho-Utah) AAPT section meeting. An article came out in TPT (March 1999, p.181) about the same time doing a similar thing with muffin cups, so I never bothered to submit my results for publication. However, since you asked for numbers, and no else seems to have come forward yet with any, let me report what I got.

I was able to get multiple mass values by cutting a filter into quarters, then adding pieces to one filter, giving 1, 1.25, 1.50, etc. times the mass of one filter. That gave 10 data points, up to 3.25 mass units. A plot of v-squared vs. mass is acceptably straight. With a drag force of: F-drag = mg = c_2 vt^2 , I found a value of c_2 of about 6.6x10^-3 from the slope of the line.

The coefficient Griffin gives for a sphere is 0.22(diam.)^2, with the diameter in meters. The coffee filters we used vary from about 0.08m at the flat end, up to about 0.13m at the top. Spheres of these diameters should have coefficients of 1.4 to 4.0 x10^-3. Thus, the coefficient we found is a little bit higher. But then again the filters are not spheres, so you would expect more drag than for a sphere.

Hope this report is useful to you.

Rondo Jeffery

Weber State University

Ogden, UT 84408-2508

ps. I know Bob invited responses off list, but I would be interested to know if any others have results similar to ours.

Has anyone determined drag coefficient in terms of cross sectionalmuirrob@UNCG.EDU 02/06/01 08:05AM >>>

area for coffee filters, i.e., c1 and/or c2 in the expressions:

Fdrag = -c1*v and Fdrag = -c2*v*v?

Fowles says that for spheres c1 = 1.55e-1 times diameter and

c2 = 0.22 diameter squared.

I expect these to be incorrect for something shaped like a coffee

filter.

My students are going to be studying this soon, so I'm interested in

having some independent results to compare with.

Feel free to respond off list.

Thanks,

Bob

----------------------------------------------

Bob Muir muirrob@uncg.edu

Physics & Astronomy 336-334-3255

UNC Greensboro, NC 27402-6170

----------------------------------------------

Life is NOT a dress rehearsal.

The trouble with reality is -- it's never the

way you imagine it! -- Moira (For Better or

for Worse)

- Prev by Date:
**Re: Charged disk; was electrostatic ...** - Next by Date:
**Re: Charged disk; was electrostatic ...** - Previous by thread:
**air drag** - Next by thread:
**Re: air drag** - Index(es):