Students will be challenged to apply recent learning about forces and multiple charges.

Forces can cancel ; a net force can be zero even if there are multiple forces on an object.

Today's lesson is challenging in two ways. First, the schedule is shortened to enable a school-wide assembly to happen. Second, I am attending a professional development meeting and am out of the building. The lesson needs to be one where students can function without my direct guidance.

To that end, I have created a couple of interesting opportunities for students to work independently of me. First, they work in teams competing as teams against one another while solving challenging problems in the areas of thermodynamics and electrostatics. Then, students return to the investigation of forces and work that we've begun, with a chance to collect more data or, perhaps, to create graphs or other materials needed for the final assessment. So, though I won't be in class, their time is valued and validated.

25 minutes

Earlier in the year, I created randomized teams of students to compete in occasional challenges called "Pride Points" events. These are challenge problems that require transfer of skills or concept and which are novel in some way. The results of these events count only toward a team's total points - student grades are unaffected. Today I ask my students, via my notes left with a substitute, to assemble into their Pride Points teams and work collaboratively to address the two problems on the Pride Points handout.

The first problem is a calorimetry problem which reverses the usual procedure: the water is hot and a cold piece of brass is dropped into the hot water bath. Students need to read the graph carefully to glean the important information (initial temperatures, final temperature, etc.), then go through their familiar analysis to establish the specific heat of the brass. Successful teams are not thrown by the reversal of roles and recognize the standard nature of the solution.

The second question is completely novel, though we have just recently practiced the colinear charge problem. The novelty here is that, given a net force of zero, a distance needs to be discovered. Again, this is a reversal of the typical problem we've considered - normally, we start with distances and discover the net force. This problem also features a heavy dose of algebra, though some judicious choices can make it simpler to deal with (waiting a while before inserting specific values can help in seeing that one of the charge values is common to both sides and, therefore, can be eliminated).

I choose to give this problem while away as it is my policy, due to the competitive nature of these events, to provide very little assistance during these problems. My substitute need not have any insight into Physics and, in fact, it's probably best if he or she has none! When student teams are finished, they submit their best work to the substitute. I assess the work when I return.

35 minutes

During this segment of class, students are given some time for final team work on their "Work as Area" investigation. As they have used the equipment and materials before, they are familiar with everything and should be primed to work independently. In addition to collecting data, students may choose to create the graphs necessary (force versus distance graphs) to compute the amount of work done overcoming their resistive materials (springs, elastic bands, etc.). If time allows, students may even begin writing their reports.

My strategy during this segment, and throughout this entire lesson, is to leverage the sense of independence we've been creating throughout the year. In principle, this task (and the Pride Points task) is one that can be managed without my direct oversight. Overall the timing of this section is very student-driven. Some students invest more time into the Pride Points segment and have less time for continuing lab work. Others move into the lab work a bit earlier and dedicate more time to that part of the lesson. All students, however, are expected to use the time to their advantage until the end of the period.