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Buffalo State PHY 690 - Air resistance is neglected

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Andrew SannesPhy 690Air resistance is neglected in introductory kinematics problems to help simplify models of motion and allow students to see the underlying mechanics of motion, specifically freefall. While this approach is appropriate a simple demonstration involvinga piece of paper and a textbook will open up a discussion about freefall that can enrich the depth of understanding for students and allow them to go beyond simply stating “all objects fall at the same rate in a vacuum.” The demonstration has been video taped and isavailable at the following URL: http://physicsed.buffalostate.edu/pubs/PHY690/ Sannes2004AirRes/Videos/8Mar05/. If video projectors are available the demonstration can be shown directly to the students and paused at the appropriate place for discussion. If no projectors are available then the video can serve as a guide for the instructor who can very easily perform the demonstration “live.” The only materials required are a table,a book and several pieces of paper. The video is an elaboration of a demonstration noted by Ed and Rosea van den Berg in the Physics Teacher1.The video consists of a series of four drops in which a 1000 paged book is dropped at the same time as a piece of paper that has approximately the same surface areaas the book. The book is therefore approximately 1000 times as massive as the piece of paper. After giving the class this initial information the instructor can then proceed with the book drops.The first drop demonstrates the importance of surface area. Holding up a piece ofpaper and a physics textbook (if you’re into modeling then finally your textbook will have a function) tell your students that both objects will be dropped from the same height at the same time and ask “which will reach the ground first and why?” After students have voted, most will say the book will land first, crumple the paper into a ball and dropAndrew SannesPhy 690both objects. It is best to practice this procedure a few times to develop a good technique. Students will groan as you crumple the paper and may cry “foul.” Do not be dissuaded simply proceed with the drop and address their concerns afterward. After both objects fall to the ground at the same rate you can begin a discussion of why this occurredand why merely changing the surface area had such an effect on the outcome of the drop. After the surface area issue has been addressed drop both the paper and book but do not crumple up the paper. The paper will fall gently to the floor following a sort of zigzag motion and the book will fall straight to the ground. At this time it would be appropriate to have the class discuss the forces at work on both objects and in both situations (paper crumpled and flat). While the forces acting on the flat paper are best described by chaos theory students can simplify this by looking just at the first few moments of freefall. Students should realize and point out that the surface area in some way determines the drag force on an object. The crumpled paper had a smaller drag forceassociated with it because its surface area was smaller. To help guide the discussion ask questions such as “why did the book fall faster than the flat piece of paper?” Have students break up into groups and whiteboard a free body diagram of both objects just after the moment of release. This should help them to see that the magnitude of the drag force is similar for both objects but that the weight force is 1000 times greater on the book than on the paper. This point may cause trouble for some students because they may reason that if the force acting on one object is 1000 times greater than the force acting on another object how could they fall at the same rate as the objects do when the paper is crumpled up. Students often forget that the weight force is not equivalent to an object’s acceleration due to gravity. By having students whiteboard both situations theyAndrew SannesPhy 690will uncover misconceptions that were previously hidden by the statement that ‘all objects fall at the same rate.’ The third drop is done by placing a piece of paper on top of the book. Ask the students “which will reach the ground the first and why?” After the class whiteboards theforces present on both the paper and the book students will reveal there understanding of freefall. Students that reason correctly will notice that the drag force acts only on the book and therefore the paper is allowed to accelerate downward without the impediment of the upward drag force. Some students may draw a normal force acting on the paper; however, asking them to defend their diagram will force them to address this misconception. The fourth drop consists of placing the paper underneath the book. Many studentswill think that the paper will fly out from underneath the book. It is important that they are directed to make a free body diagram of the piece of paper and analyze the forces acting on the paper; i.e., the weight force of the paper itself, the force of the book pushingon the paper and the drag force acting upward on the paper.Showing the video or doing the demonstration is not enough. According to a study by Gunstone and White, of 131 university students who predicted that an iron sphere and a plastic sphere of the same diameter would take an equal amount of time to travel the same distance “only 7 referred to personal experience as the basis of their knowledge; 21 invoked Galileo as an authority. Most of the 131 simply asserted that all things fall at the same rate.”2 For students to extend their understanding beyond a reliance on authority they must carefully analyze the various forces acting on the objects to gain a complete understanding of how freefall and air drag relate. After analysis ofAndrew SannesPhy 690these four situations instead of simply reciting that ‘all objects fall at the same rate’ students will understand that the rate at which an object falls depends on the relationship between the forces acting on the object.1. R. F. Gunstone and R. White, “Understanding Gravity,” Sci. Educ. 65, 291-299 (1981).2. Ed and Rosea van den Berg, “Two-Minute Impromptu Demos,” Phys. Teach. 36, 356 (Sept.


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Buffalo State PHY 690 - Air resistance is neglected

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