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Applying Newton’s Third Law of Motion in the Gravitron Ride



Co Authors:

Anthony J. Creaco
Department of Science
Borough of Manhattan Community College / City University of New York
acreaco@bmcc.cuny.edu

Owen A. Meyers
Department of Science
Borough of Manhattan Community College / City University of New York
omeyers@bmcc.cuny.edu

David A. Krauss
Department of Science
Borough of Manhattan Community College / City University of New York
dkrauss@bmcc.cuny.edu

Abstract:

A trip to an amusement park is the setting for this introduction to the concept of centripetal force in terms of Newton's laws of motion and vector quantities.  A student who is a physics major helps his friend understand the action-reaction forces that cause a body to stick to the wall of the Gravitron ride as it spins.  The case intentionally avoids the use of mathematics so that students will not get bogged down in solving equations while trying to internalize complicated concepts.  Instead, this introduction to the concept of centripetal force is intended as a jumping off point for more detailed discussions of the underlying mathematics. The case is appropriate for use in lower division undergraduate college or senior high school classes. It has been used with students in introductory physics classes for science majors and in non-majors general physics classes. It could also serve as the basis for an informal writing assignment in a writing intensive course.

Objectives:
  • Stimulate interest in physics in general.
  • Reinforce physics concepts and critical thinking skills.
  • Enhance understanding of Newton's laws of motion and the fundamental concept of centripetal force.
  • Emphasize that centripetal force on an object always has a net vector quantity towards its center of rotation and that the direction of an object's velocity is always tangential to its circular path.
  • Emphasize that centrifugal force is a reaction force to the centripetal force exerted by the object in motion as described by Newton's third law.
  • Apply the concepts of Newton's laws of motion and the fundamental concept of centripetal motion to explain a real-life scenario.
Keywords: Centripetal force; Newton’s laws of motion; vector quantities; action-reaction forces; gravitron
Topical Area: N/A
Educational Level: High school, Undergraduate lower division
Formats: PDF
Type/Method: Directed
Language: English
Subject Headings: Physics  
Date Posted: 6/14/2012
Date Modified: N/A
Copyright: Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

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I wonder if the case would be pedagogically more effective if the term "centrifugal force" were avoided. The term (although not the misconception) is foreign to many students, and I find that introducing the term is counterproductive because it gives emphasis to and reinforces the misconception I seek to eliminate.


Craig Buszka
Science
Montgomery HS
Skillman, NJ
cbuszka@mtsd.us
6/14/2012
Author's Response

Many instructors have different opinions on whether the concept of Centrifugal Force should be discussed because of the few cases it applies to in solving physics problems. I do teach this concept in my classes because it does reduce the confusion when the few cases such as the principles covered in this case study need to be explained. However, one can apply this case study without using the term "Centrifugal Force" and just replace it with the "Reaction force of the Centripetal Force" as per Newton's Third Law of Motion.


Anthony J. Creaco
Science
Borough of Manhattan Community College
New York, NY
acreaco@bmcc.cuny.edu
6/19/2012

I am a little confused by this case. I agree that the wall exerts a centripetal force on the boy. The physical nature of this centripetal force is the normal force that the wall exerts on the boy. I also agree that by the 3rd law the boy exerts a force on the wall, but argue that this force is completely irrelevant to understand the motion of the boy, as that force does not act on the boy, it acts on the wall. It is therefore relevant only for analyzing the motion of the wall, if we were interested in it. What prevents the boy from sliding down is a force that points up, which balances out the gravitational force down. That's of course the static friction force. The (maximal) static friction force is proportional to the normal force (the centripetal force), and that's why the spinning prevents the sliding, because of the friction force upward. The force of the boy on the wall, even if relevant for the motion of the boy (which it is not), is at any case horizontal, and therefore does not contribute anything to the net force in the vertical direction, and there will be no sliding only if the net force in the vertical direction is zero.


Lior Burko
School of Science and Technology
Georgia Gwinnett College
Lawrenceville, GA
burko@ggc.edu
12/10/2015
Author’s Response:
It is this exact argument that causes confusion among students in understanding of what is happening with the boy. Everything mentioned in this posting is true—the reaction force which is the force that the boy exerts on the wall is usually observed if one analyzes (and does a free-body diagram on) the wall and not the boy. This is why most professors do not discuss this reaction “centrifugal force”; no one usually analyzes the wall in this motion. However, the point of the argument is not from the viewpoint that the reaction “centrifugal force” is not acting on the boy, but the perspective that the boy is still exerting a force on the wall involuntarily which is the reason why the boy sticks to the wall. By not accounting for this force, the misconception that there is a centrifugal force acting on the boy gets assumed. Therefore, in summary, this reaction force is discussed not to observe as one of the many forces acting on the wall to analyze it, but the fact that it is this force that causes the boy to stick to the wall. In other words, the fact that the boy is exerting a force on the wall, and not that there is a force acting on the boy away from the center of the circular path, is the reason the boy sticks to the wall.


Anthony J. Creaco
Science
Borough of Manhattan Community College
New York, NY
acreaco@bmcc.cuny.edu
12/15/2015



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