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Grandpa’s Flying Hammer

Rotation Around the Center of Mass


Author(s)

Joel Hernandez
Department of Science
Borough of Manhattan Community College / City University of New York
jhernandez@bmcc.cuny.edu
Canan Karaalioglu
Department of Science
Borough of Manhattan Community College / City University of New York
ckaraalioglu@bmcc.cuny.edu

Abstract

Jane, a freshman in college, is chatting with her neighbor, a retired physics college professor, when she suddenly sees a flying hammer thrown by her grandfather. Jane is puzzled by her observation that as the hammer rotates in the air, there appears to be a point located in the head that does not rotate. Guided by her neighbor, Jane first analyzes the physical system and comes up with a simplified model to simulate its behavior. She then applies physical laws to the model and reaches the important conclusion that the model system rotates about a point that coincides with the center of mass. She then applies this conclusion to determine the location of the center of rotation of the real system (the hammer), and thus explains her experimental observation. The case is appropriate for an undergraduate physics of mechanics course and is ideally used right after teaching the topic of rotational kinematics of rigid objects. The case was developed for students who are required to take algebra-based physics; however, it easily can be adapted for an arithmetic-based or calculus-based physics class.


Objectives

• Engage with a real-world application of physics.
• Develop and reinforce their scientific thinking skills.
• Analyze real systems with simplified mathematical models that simulate the essence of the problem under study.
• Apply knowledge of Newton’s laws of motion, rotational kinematics, centripetal force, and the concept of center of mass to prove that a flying rotating object rotates around its center of mass.


Keywords

Center of mass; center of rotation; Newton’s laws of motion; centripetal force; rigid body; center of mass; rotational kinematics;

Topical Areas

N/A

Educational Level

Undergraduate lower division

Format

PDF

Type / Methods

Analysis (Issues), Directed, Interrupted

Language

English

Subject Headings

Engineering (General)  |   Physics  |  


Date Posted

01/15/2021

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Videos

The following video(s) are recommended for use in association with this case study.

  • MIT Physics Demo: Center of Mass Trajectory
    This brief video shows various objects in rotation after being tossed through the air, using black light to highlight the behavior of the center of mass. Running time: 1:03 min. Produced by Daniel Jibson, 2017.

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