PI Problems for the Physics Suite Edward F. Redish

Peer Instruction Problems:
Dynamics

 Problem Name Comments Source Demo (UMd) D1. Kicking This is a touchstone example. Students' intuitions about the impact of mass on acceleration, given a constant force, is often both correct and solid. However, they often fail to activate that intuition. It's useful to have this example to refer to, and to give them at least some examples where they all get it right -- if only to keep them off balance and thinking during every problem! EFR D2. Falling Bodies The classic examples. Many students know what the answer "is supposed to be" but don't really believe it. Worth climbing up onto the lecture table to do the demo. Doing the in-air demo first with two dense objects of signficantly different mass and then dropping a feather or a balloon helps the students see that their intuition (light objects fall slower) is correct in some sense, but needs to be refined to be both useful and consistent. EFR C4-34 (in air with dense objects) C4-33 (ball and feather in vacuum) D3. Normal Force I The simple situation of a book resting on a table is difficult for introductory students. They tend to associate the idea of force with will or intent. Anyway, how does the table know how much force to exert? (See J. Minstrell, The Physics Teacher, 20 (1982) 10.) EFR D4. Normal Force II Adding a second book makes the book-on-the-table problem even more confusing. Does the force of the top book "pass through" to the table? Following the problem and discussion with a careful free-body-diagram analysis is very useful here, especially if you also consider the case when you are pushing down on the top book. EFR D5. Friction I This is another surprise for many students. A careful free-body diagram analysis of the top block (after the discussion can be enlightening. Illustrates the importance of Newton's 0th law. EFR D6. Getting Going (Friction II) A second example on how -- in the Newtonian choice of how to analyze force and motion (see the Newton 0 paper) friction is actually the force usually responsible for most of our personal experiences of speeding up. I used to surpress this problem, as students find this idea very peculiar and difficult, but recently I have come to believe that making sense of this issue helps them dramatically to both understand what Newton is trying to say and to reconcile their own intuitions of everyday experience with physics. EFR D7. Shoot and Drop This is one that students find peculiar and surprising. Reconciling their expectations of how, for example, a bullet travels, can be a challenge for some students. This is a simpler version of the classic "monkey and hunter." That one is typically too hard for introductory students. They are surprised by the result but have a lot of trouble putting together all the pieces that are required to see what is responsible for the surprising result. You can tell them, but it's typically too much for them to integrate into what they know. If you do it, start with the gun horizontal, have them predict, and carefully discuss the result before tipping the gun upward. EFR C2-21(shoot-and-drop)C2-22(monkey and hunter) D8. Pulling a Block This is much harder for students than it looks. Introductory students, even those who have mastered the "constant velocity implies no net force" rule, often have trouble with pulling vectors into components at the same time. Worth some discussion. From the Mechanics Baseline Test (D. Hestenes and M. Wells, Phys. Teach. 30 (1992) 159) D9. Popping Cart A good, challenging problem to help students understand the independence of perpendicular motions. Once you have helped them see why this one works, consider two other cases: with the track tipped, and with the cart being pulled by a weight over a pulley. One works (catches the ball), the other doesn't. Why? EFR C2-26 D10. Tension Another classic. Students are very confused about the idea of scalar vs vector tension and about the idea of an inert, apparently unchanging object (the wall) exerting a force. EFR This demo can easily be set up with pulleys and spring scales. D11. Black Ics A useful problem for bringing the friction issue down to its practical, everyday (in some climes and seasons) implications. 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