PI Problems for the Physics Suite Edward F. Redish

Peer Instruction Problems:
Momentum and Energy

 Problem Name Comments Source Demo (UMd) P&E1. Momentum is a Vector Inbtroductory students often confuse momentum and energy. They often fail to note that momentum is a vector and direction matters. Here's a warm-up problem to the next, harder one. This is pretty challenging in itself because it requires combining momentum conservation and Newton's third law, neither of which students are comfortable with at first. EFR P&E2. Knock it Down! This one is very challenging. I've tried it at half a dozen universities around the world and after instruction, typically 98% of students get this wrong. This problem activates an intuition that "the superball keeps its speed" but the clay ball "gives up all its motion to the block." The result is a big surprise for students -- after they have predicted and defended their answers. Working out why is a good step in helping them focus on momentum as a vector. (The need for Newton 3 in the explanation of the result also causes some confusion.) EFR C7-25 P&E3. Racing Billiard Balls A very challenging problem. This has lots of clues that tend to activate "compensation," especially for those students who know a little physics. (Psych majors tend to do better on this one than engineers!) Warning: We've found that if the demo is done with insufficient prediction and discussion, students will remember the wrong result a year later! Adapted from Dick Berg's Physics IQ Test C2-11 P&E4. Inelastic collision with PE 1 An interesting and challenging variant of the standard inelastic collision problem. Evaluation using energy and momentum just before interaction. From the Energy Concepts Survey (C. Singh and D. Rosengrant, Am. J. Phys. 71 (2003) 607) P&E5. Inelastic collision with PE 2 An interesting and challenging variant of the standard inelastic collision problem. Evaluation using energy and momentum just after interaction. From the Energy Concepts Survey (C. Singh and D. Rosengrant, Am. J. Phys. 71 (2003) 607) P&E6. Table Hockey One of the best problems for using and understanding the implications of the impulse-momentum and work-energy theorems. Adapted from the Mechanics Baseline Test (D. Hestenes and M. Wells, Phys. Teach. 30 (1992) 159) P&E7. Galileo's Pendulum The classic "pinned" pendulum. Fairly straightforward using conservation of energy. Adapted from Dick Berg's Question of the Week #108 C8-03 P&E08. The Force at Closest Approach This is one to see how many students are still confused about the basic idea of Newton's second law and the identification of forces. The trivial setup on velocity is to make sure the students are thinking first about the fact that the cart is not moving at this instant (but they have to realize that is isaccelerating). This primes those who think "no velocity means no force" to come up with the "only two balanced forces" answer. Others may correctly get the force from the cart on the right, but come up with some imaginary force to the right to "balance" it, since the first cart isn't moving. No answers are provided (no room on 1 page) but they are easy to draw from the class. EFR

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

Page last modified February 8, 2005