This becomes apparent when dealing with an atom emitting a photon and that photon being (later) absorbed by another identical atom. The exchange of a photon causes the emitting atom to experience a force and change in momentum and the absorbing atom to experience an equal and opposite force and change in momentum but at a slightly later time. So force reciprocity is not present at all times, but conservation of momentum always is (since the photon carries momentum).

I am currently engaged in a very interesting discussion with some very good physics minds on Physicsforums.com about the third law in the context of centripetal force and the proper characterisation of the “reaction” to a centripetal force. Since in a rotating system, there is only centripetal acceleration, my view is that the third law pair to a centripetal change in motion/momentum of one body in the system is an equal but opposite centripetal motion/momentum of the rest of the system. The other view is that the “reaction” pair to the centripetal force on a body in the system is a “centrifugal reaction force” of that body on the system. The suggestion being that this is a real force not to be confused with the “fictitious centrifugal force” but does not result in any “change in motion” at all. I am opposed to using the notion of a real centrifugal force as there is no real centrifugal acceleration. There is even a Wikipedia page on this “reactive centrifugal force” at http://en.wikipedia.org/wiki/Reactive_centrifugal_force , which I think is completely misconceived and mostly wrong. But I seem to be a voice in the wilderness on physicsforums on this issue.

I would encourage you to keep up this interesting blog.
Thanks again.

The real problem with getting them to stay in physics was explained by a current college junior at a large university: On most nights, his non-science major friends would drop by his room to see what was up and get confused that he was “working.” He had to explain over and over, while they had their evenings free, he had to work. Now it’s not quite the same for my former students in small liberal arts colleges… even the dance majors there apparently have lots to do outside of class meeting time. But the perceived difficulty of remaining in good academic standing when you are in the sciences chases some students off. I hope we have helped our students develop a little more backbone than that.

In response to your question about why we should “force” students to take algebra and physics, I should clarify my milieu: there are no students at my school who are not already deeply committed to going to college. Most see themselves in the future doing very intellectually challenging work. This is not a public school, nor is it a haven for the idle rich. The mission of the school is to live in the service of others, and, by golly, that’s going to take some serious preparation. So my starting point for convincing any student to take physics is quite different than that of other physics teachers.

The second item above is exactly what we are trying to do with standards based grading. Our feedback is very specific to the clearly identified learning goals. We are getting better and better at giving students guidance in how to “practice” physics on their own. As the year goes on, more and more this practice takes place outside the classroom, at whatever pace or urgency the student chooses (self-regulation/self-assessment could be thought of as yet another meta-curriculum that we teach).

Your third item is the one we have to work on. Before girls even enter our classroom, they run a gauntlet of pictures of old white men. An alumnus gifted the school his collection of pictures of Nobel laureates whom he has personally met. We need to balance that out with pictures of all of the fabulous female scientists we’ve had visit for our Crump Physics Lecture Series. And we need to think more about the problems we give… I can’t remember where, but somewhere recently I read a nice paper about unintentional male bias in physics problems– change the problems slightly, everyone does better.

(1) Teachers should explicitly teach students that academic abilities are expandable and improvable in order to enhance girls’ beliefs about their abilities. Students who view their cognitive abilities as fixed from birth or unchangeable are more likely to experience decreased confidence and performance when faced with difficulties or setbacks. Students who are more confident about their abilities in math and science are more likely to choose elective math and science courses in high school and more likely to select math and science-related college majors and careers.

(2) Teachers should provide students with prescriptive, informational feedback regarding their performance. Prescriptive, informational feedback focuses on strategies, effort, and the process of learning (e.g., identifying gains in children’s use of particular strategies or specific errors in problem solving). Such feedback enhances students’ beliefs about their abilities, typically improves persistence, and improves performance on tasks.

(4) Teachers can foster girls’ long-term interest in math and science by choosing activities connecting math and science activities to careers in ways that do not reinforce existing gender stereotypes and choosing activities that spark initial curiosity about math and science content. Teachers can provide ongoing access to resources for students who continue to express interest in a topic after the class has moved on to other areas.