This activity is designed to illustrate an application of resistive forces in the introductory physics curriculum with an interdisciplinary twist. Students are asked to examine images of riverbed boulders after a flood and estimate the water flow that was needed to push the boulders downstream. The activity provides an opportunity for students to gain an understanding of the physics of natural phenomena with applications to environmental science and environmental engineering, to gain a stronger appreciation for physical modeling and estimation, and to become environmental detectives!
References
1.
The town of Estes Park in Colorado was severely damaged by the Lawn Lake flood, and a summary of events can be found at http://www.reporterherald.com/ci_21071062/1982-flood-changed-downtown-estes-park.
2.
A description of the Lawn Lake flood by the National Park Service can be found at http://www.nps.gov/romo/planyourvisit/upload/flood_2009.pdf.
3.
Courtesy Rocky Mountain National Park.
4.
Courtesy Rocky Mountain National Park.
5.
Any riverbed image could be used in this activity, but the enormity of the boulder in the figure chosen tends to leave a lasting impression.
6.
Image by Jill M. Pestka, used with permission.
7.
D.
Halliday
, R.
Resnick
, and J.
Walker
, Fundamentals of Physics Extended
, 10th ed. (Wiley
, New York
, 2013
), p. 130
. 8.
K.
Takahashi
and D.
Thompson
, “Measuring air resistance in a computerized laboratory
,” Am. J. Phys.
67
, 709
–711
(Aug. 1999
). 9.
R.
Cross
and C.
Lindsey
, “Measuring the drag force on a falling ball
,” Phys. Teach.
52
, 169
(March 2014
). 10.
W.
Derski
, R.
Izbicki
, I.
Kisiel
, and Z.
Mróz
, Rock and Soil Mechanics
(Elsevier
, New York
, 2012
), p. 100
. 11.
A more detailed estimate, not assuming a perfectly spherical shape, gives the weight of the boulder a value of over an astounding 400 tons.
12.
The entire activity can be repeated for the flood of 2013 using Fig. 2. The largest rocks shown at the alluvial fan in Fig. 2 (the rocks far right and far left in the figure) yield estimated weights ranging from about 2000–7000 N, or about 400–1500 lb, producing results that are typical of severe floodwaters of about 4–5 m/s, or about 9–12 mph. These sorts of severe floodwaters are capable of washing passenger vehicles downstream.
13.
For example, incorporating buoyancy into the problem [for a completely submerged rock, this amounts to changing ρR→ρR−ρW in Eq. (3)] reduces the flow speed needed to move the boulder by about 20%, whereas altering the shape of the rock from spherical to one that is flatter with smooth edges influences the mass estimate, Bernoulli-like lift forces, rolling behavior, drag coefficient, and cross-sectional area. The result for the latter three is an increase in the flow speed needed to move the boulder.
14.
M.
Mawhinney
, M.
O'Donnell
, J.
Fingerut
, and P.
Habdas
, “Measuring drag force in Newtonian liquids
,” Phys. Teach.
50
, 173
–175
(March 2012
). © 2015 American Association of Physics Teachers.
2015
American Association of Physics Teachers
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