Dedicated to the strengthening of the teaching of introductory physics at all levels, The Physics Teacher provides peer-reviewed materials to be used in the classrooms and instructional laboratories. It includes:
Innovative physics demonstrations; New ways of doing lab experiments; Ideas for presenting difficult concepts more clearly; Suggestions for implementing newer technology into teaching; Historical insights that can enrich the physics course and Book and film reviews.
Special features include Physics Challenge Solutions, Fermi Questions, and Figuring Physics.
Every physics teacher wants to give his or her students the opportunity to learn physics well. Despite these intentions, certain groups of students—including women and underrepresented minorities (URMs)—are not taking and not remaining in physics. In many cases, these disturbing trends are more significant in physics than in any other science. This is a missed opportunity for our discipline because demographic diversity strengthens science. The question is what we can do about these trends in our classrooms, as very few physics teachers have been explicitly prepared to address them. In this article, I will share some steps that I've taken in my classroom that have moved my class in the right direction. In the words of Nobel Prize-winning physicist Carl Wieman and psychologists Lauren Aguilar and Gregory Walton:
“By investing a small amount of class time in carefully designed and implemented interventions, physics teachers can promote greater success among students from diverse backgrounds. Ultimately, we hope such efforts will indeed improve the diversity and health of the physics profession.”
A well-known physics demonstration is to pull a tablecloth rapidly from under some crockery without disturbing the crockery. An interesting question is whether the same result can be expected if the crockery is replaced by a ball, given that the ball might roll backwards on the tablecloth. Theoretical and experimental results are presented showing that the result depends on the acceleration of the tablecloth. If crockery is at rest on a tablecloth, and if the tablecloth is pulled slowly, then the crockery will move at the same speed as the tablecloth. With a rapid pull, the crockery appears not to move at all. In fact, the crockery will move a small distance horizontally since the coefficient of sliding friction is not zero. The secret of success is to minimize the horizontal impulse, not by reducing the friction force but by reducing the time over which it acts.
Kinetic energy and momentum are indispensable dynamical quantities in both the special theory of relativity and in classical mechanics. Although momentum and kinetic energy are central to understanding dynamics, the differences between their relativistic and classical notions have not always received adequate treatment in undergraduate teaching. It is shown that the contrast between these relativistic and classical quantities can be presented in a straightforward manner and with a minimal level of (undergraduate) mathematics.
Water has the unusual property that it expands on freezing, so that ice has a specific gravity of 0.92 compared to 1.0 for liquid water. The most familiar demonstration of this property is ice cubes floating in a glass of water. A more dramatic demonstration is the ice bomb shown in Fig. 1. Here a cast iron flask is filled with water and tightly stoppered. The flask is then cooled, either by leaving it outdoors in winter or by immersing it in a cryogenic fluid, until the water freezes. As the water freezes and expands, the pressure inside the flask increases dramatically, eventually becoming sufficient to fracture the metal walls of the enclosure. A related, but much less familiar, phenomenon is the explosive fracturing of small waterdrops upon freezing. That waterdrops can fracture in this way has been known for many years, and the phenomenon has been described in detail in the atmospheric sciences literature, where it is seen as relevant to the freezing of raindrops as they fall through cold air. Carefully controlled experiments have been done documenting how the character and frequency of fracture is affected by such variables as drop size, rate of cooling, chemistry of dissolved gases, etc. Here I describe instead a simple demonstration of fracture suitable for video analysis and appropriate for study at the introductory physics level. Readers may also be interested in other characteristics of freezing and fragmenting waterdrops, for example, charge separation upon fracture and the appearance of spikes and bulges on the surface.
I think most physics teachers would agree that two important components of a proper solution to a numerical physics problem are to first figure out a final symbolic solution and to only plug in numbers in the end. However, in spite of our best efforts, this is not what the majority of students is actually doing. Instead, they tend to plug numbers into formulas without considering the physical meaning of the equations, then frequently take the result and plug it into the next formula—a strategy known as “plug-and-chug.” In this chain of calculations, frequently physical insights are lost. If teaching problem solving is proving ineffective, maybe it is possible to steer students onto the right path by posing the problems in different ways?