PI Problems for the Physics Suite Edward F. Redish

Peer Instruction Problems:
Rotational Motion

 Problem Name Comments Source Demo (UMd) R1. Centrifugal Force Students are very confused by the terms "centrifugal" and "centripetal" force. Many think it is a new kind of force, rather than just a name for a direction of an already familiar force. This problem, given after a lecture on the force needed for uniform circular motion, may show some surprising results. The discussion after the student answers have been collected can be very valuable. EFR R2. Bugs on a Wheel The concept of angular speed is difficult for students, even without the complicating issue of vector direction. This problem helps students struggle with separating the idea of angular speed from linear speed. A lot of interesting variants of this are possible. (Follow up asking about linear speed, net force, etc.) From the ConcepTest collection in Mazur's Peer Instruction R3. Ball in a Partial Ring The results here are surprising to both student and teacher. Students often think the ball remembers its curved motion -- and they even see it curve in a demonstration. See a detailed discussion with suggestions for how to handle this in E. Redish, Teaching Physics with the Physics Suite, Chapter 7, p.131. Discussed in A. Arons, A Guide to Introductory Physics Teaching. One of the FCI questions (D. Hestenes, M. Wells, and G. Swackhammer, The Physics Teacher 30 (1992) 141. D1-32 R4. Torque 1 The basic idea for the definition of torque. A trivial and intuitive lead in to the more complex R5. EFR R5. Torque 2 Some students' intuition on this is excellent. I have been able to get students to articulate the principle governing the rule of balance from this example and from that to generalize and get the students to begin to see how that the definition of torque is intuitive after all. EFR Easy to set up. R6. Balancing What? This problem is challenging, requiring combining the torque idea with the idea that the gravitational force on an object acts as if it were all applied to the center of mass. In this example, students find that non-intutive since part of the mass is on one side of the pivot and part on the other. Working this through in detail in whatever way they find natural is useful for helping students make sense of the idea of center of mass. (Showing them the calculation is just about useless.) From the ConcepTest collection in Mazur's Peer Instruction R7. Rolling A challenging problem, worth considerable discussion. By this point in the class, students may have developed the intuition that falling bodies (even ones falling down an incline) fall independent of their mass so they expect everything to be the same. How this plays out allows a useful discussion of the interaction of many different ideas and principles. EFR D2-02 R8. Rolling vs Sliding A nice example as a lead-in or follow-up to the detailed discussion rolling with R7. From Dick Berg's Question of the Week #11 D1-61

Work supported in part by a grant from the US National Science Foundation.

Maintained by Edward F. Redish
Comments and questions may be directed to redish@umd.edu