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Simply Harmonic Jello – Fun Physics for Thanksgiving November 23, 2010

Posted by admin in : Acoustics (ACOU), Condensed Matter and Materials Physics (CMMP), History, Policy and Education (HPE), Physics Education Research (PER) , add a comment

Jello is fun and delicious any time of year, and everyone has seen it “wiggling” and “jiggling”.  With a simple stopwatch and counting the frequency of the wiggles, serving jello brings up a special opportunity to work a physics experiment into your snack and dinner menu.

Those wiggles and jiggles can be described as simple harmonic motion, i.e., the force causing the displacement (motion) is proportional to the displacement itself,  F = -kx .

Consider a square block of wiggling jello on a flat plate.  If the jello is set into vibrating motion by a shear force that acts on the top of the jello, static friction will keep the bottom of the jello fixed in place on the plate.   The displacement (or deformation) of the top of the jello due to the shear force is some distance,  x . This displacement divided by the original dimension is called the shear strain.

From Giancoli, Physics for Scientists and Engineers

If you measure the wiggling rate, i.e., count the number of back and forth excursions per unit time, this frequency can be related to the a physical property of the jello called the shear modulus.

The shear modulus,  G relates the shear force,  F , and shear strain,  \frac{x}{h}   by  

 G = \frac{Fh}{Ax}    or F = \frac{GAx}{h}

where   A is the area of the top of the block.

Because the center of mass oscillates with half the displacement of the top,

 F=\frac{1}{2} k_e x ,

and the effective force constant is given by

 k_{e} = 2\frac{ F}{x} = \frac{2GA}{h} .

The frequency of the vibrations for any simple harmonic oscillator is

 f =\frac{1}{2 \pi} \sqrt{\frac{k_e}{m}}

where  m is the  mass oscillating object, in this case the piece of jello.  The piece of jello can be weighed directly (converting from weight to mass) or given by the density of the jello multiplied by its volume  m= \rho Ah .

So the wiggling frequency of jello is        \frac{1}{2 \pi}  \sqrt{\frac{\frac{2GA}{h}}{\rho Ah}} or  \frac{1}{2 \pi h}{\sqrt{ \frac{2G}{\rho}} .

Thus the shear modulus of jello can be determined from the measured vibrational frequency by  G= 2 \rho ( \pi  f h)^2  .

You can try this experiment at home and even study how the shear modulus changes with how you make the jello, i.e., with water, vinegar, juice, soda, or alcohol. And you can investigate how temperature changes the shear modulus.

Post your results here as a comment.   Check back for updates and useful data.


Units? When doing any calculation in science it is important to keep in mind the units of the factors in used in the equations.  The units have to be consistent throughout, and the final derived units of your calculation should be consistent with quantity that you are trying to calculate.  It is easy to mix up units if you make length measurements using English units, and mass measurements in the metric system for example.   Even when using the metric system throughout, one could easily make the mistake of mixing CGS units with MKS units.  Always check your units.

The density of jello? Understanding what jello is and how it is made is an interesting lesson in biochemistry, particularly protein structure and function.

The more general name for jello is gelatin.  (Jell-0 is a brand name for the foodstuff – edible gelatin – that has become synonymous with the food itself.) Gelatin is made from the connective tissue proteins of cows or pigs. It is made first by breaking down the cellular structure of the connective tissues.  Then collagen proteins from these tissues are isolated, denatured and subsequently rendered to a powdered form.  Sweeteners, flavoring agents, dyes and other additives are added to this powder to make the familiar gelatin dessert.  To make jello you have to add boiling water to the powder which dis-aggregates the proteins.   Cooling the mixture re-aggregates the proteins.   The final jello mold will be a complex solid mixture of proteins, water, air, and chemical additives.

This leads us to consider the density of jello, which like the biological tissue from which it comes, is mostly water.

Water’s density is  1 \frac{g}{cm^3} = 1000 \frac{kg}{m^3} .  So the density has to be close to water.  But the various additives result in partial molar volumes that contract or expand the total volume.   The final volume depends on the thermodynamic nature of the additives and their relative concentrations.  So while it is easy to think that in any given volume of jello there are constituents that are heavier than water, and that the density should be greater than  1 g/cm^3 , the complex mixture of additives could result in the overall density being less than  1 g/cm^3 .  The most prudent thing to do is to take a well measured cube of jello, calculate its volume (or use volume displacement), weigh it, then calculate its density.

Reported densities for  jello have ranged from  0.98 - 1.3 g/cm^3 (with sugar-free variants being on the low end), while for scientific gelatin (without all the food additives) the density has been reported to be  1.3 g/cm^3 .

Inclusiveness in Physics Education January 7, 2010

Posted by Acoustics (ACOU) Section Chair in : Acoustics (ACOU) , add a comment

As the national demographics project a shift towards a majority minority US population, a 7% minority representation in the Science, Technology, Engineering, and Mathematics (STEM) population may be viewed as an indicator of a systemic failure. While gender-equity trends are very encouraging, those for African Americans, Hispanic Americans, and Native Americans remain stagnant.

As a member society of the American Institute of Physics (AIP), the challenges facing the acoustics community reflect those in other fields of physics. Throughout physics, as promising intellectual talent is lost to higher-compensating professions, extra emphasis should be placed on effectively nurturing those inspired by positive role models to mitigate this pipeline leakage.

Therefore, in an effort to advance the discussions from diversity to inclusion in the science of sound and noise, the Acoustical Society of America (ASA) committees on education in acoustics and diversity in acoustics:


are co-sponsoring a special session on diversity issues in acoustics education to be held at the joint ASA/Noise-Con meeting in Baltimore, Maryland:


This special session will be held at the Baltimore Marriott Waterfront (conference room Dover C) from 8:40am on Thursday April 22nd, 2010, with invited speakers intended to expose a wide range of viewpoints followed by a panel discussion to identify efforts that the AIP, and all its member and affiliated societies, should take to foster a culture of inclusiveness among their students and professional members.

The list of invited speakers include Dr. Catherine O’Riodan, Vice President of the AIP Physics Resources Center, to describe existing AIP programs to work with students and to reach the general public. Dr. Rachel Ivie, Assistant Director at the AIP Statistical Research Center, will reveal the latest statistics and trends on academic degrees and employment in acoustics. These figures will be compared against those in other scientific and engineering fields.

In a research study with the National Society of Black Physicists (NSBP) and the National Society of Hispanic Physicists (NSHP), University of Maryland psychology Professor Sharon Fries-Britt examined the perception of the interactions of underrepresented STEM students with faculty. The findings of this study indicate that their interactions with faculty in the classroom and in advising sessions are critical. When those interactions are positive, students benefit tremendously. However, in many instances, they are negative and the interactions can cause barriers to their engagement in the learning process and in how they feel about pursuing science.  Several examples will be shown of unhelpful comments and attitudes that have been experienced and that inadvertently discourage students from pursuing higher academic degrees. An awareness of sensitivities is essential in increasing their retention rate.

Dr. Theodore Hodapp, American Physical Society (APS) Director of Education and Diversity will describe a new program that aims to significantly increase the number of underrepresented members receiving doctorate degrees in physics. He will also share ideas for potential partnerships and efforts that we can take within our communities, universities and workplace.

Prof. David Bradley will describe joint efforts by the Vassar College Physics and Astronomy Department and the Bronx Institute at Lehman College to establish a hands-on, inquiry-based acoustics workshop series for urban, low-income, ethnic minority students from New York City public high schools. Since today’s iPod generation is strongly attracted to music, acoustics represents an attractive gateway into the world of physics. Therefore, the described partnership exemplifies solutions that promise to fill the physics pipeline with increasing number of qualified underrepresented students.

Dr. Daryl Chubin, Director of the American Association for the Advancement of Science (AAAS) Center for Advancing Science & Engineering Capacity, will focus on the legal climate for increasing participation of underrepresented groups in physics education and profession. An understanding of the legal climate is paramount to the development of effective and legally sustainable diversity and inclusion programs.

Howard Ross is one of the nation’s leading diversity training consultants and a nationally recognized expert on diversity, leadership and organizational change. Howard is past chairman of Leadership Washington and a former director of the Greater Washington Board of Trade. He also was the 2007-2008 Visiting Professor of Diversity for Bennett College for Women in Greensboro, North Carolina. In an effort to find strategies to improve the way organizations are addressing diversity, he conducted extensive research that lead to the need for three major paradigm shifts in diversity efforts:


“These include a movement from the classic United States-based approach which focuses too heavily on race and gender and an assimilation model of diversity, to one that incorporates a deep understanding of Globalism and the impact of major changes in population demographics around the world, global business, and interactive communication and networking. A shift from the “good person/bad person paradigm” of diversity which has developed and permeated a corrective mindset about diversity; a “find them and fix them” approach which escalates the “us vs. them” way that people approach the issue and makes it more, rather than less difficult to address. We have to move away from the event-based way we have approached diversity, a pattern that has given us many specific activities, but not enough emphasis on systems thinking and culture-based change, to one that is strategic, systemic, and culture-based.

The wide range of perspectives in this special session promise to feed into a lively panel discussion that harnesses the information shared by these invited speakers into solid inclusion programs for implementation by the ASA and other AIP member and affiliated societies. An open invitation is extended to attend and become part of the conversation and to the solution to this national challenge.

Acoustics For The iPod Generation January 28, 2009

Posted by Acoustics (ACOU) Section Chair in : Acoustics (ACOU) , add a comment

All acoustics (ACOU) events at the annual joint NSBP/NSHP conference are outreach efforts sponsored by the Acoustical Society of America (ASA) through its Committee on Diversity in Acoustics. These events include musical and hands-on acoustics demonstrations, technical session, and an acoustic poster competition.

The American Institute of Physics (AIP) is very concerned with the low percentage of under-represented minority students and professional members. How best to attract current and future generations of students to Physics?

Today’s students, known as the iPod generation, may be attracted to Physics by leveraging their musical passions. Acoustics demonstrations and tutorials explaining the physical mechanisms responsible for the unique sounds of musical instruments offer very tangible means to help them comprehend many physics concepts such as stationary waves and frequency shift.

Acoustics is more than music. Acoustics subfields also include atmospheric, underwater, structural, and architectural acoustics. Acoustics subfields span beyond physics with subfields such as biomedical ultrasound, animal bioacoustics, speech communication, psychological and physiological acoustics, audio engineering, noise control, and signal processing.

Professors interested in teaching an acoustics course may start by reading an Acoustics Today article by Ilene Busch-Vishniac and Jim West entitled “Acoustics courses at the undergraduate level: How can we attract more students?” available as a free download here (http://www.nsbp.org/en/art/?29).

Acoustics resources include:

Acoustics.org (http://www.acoustics.org)
Discovery Of Sound In The Sea (http://www.dosits.org)
Acoustics Tutorials & Demos (http://www.gmi.edu/~drussell/Demos.html)
Education & Career Resources (http://asa.aip.org/map_education.html)
Student Project Resources (http://asa.aip.org/resources.html)

Do you know of another source of acoustics resources? Please share it here with a comment to this blog.