## Grandpa’s Flying Hammer

Rotation Around the Center of Mass

By Joel Hernandez, Canan Karaalioglu

### 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.

01/15/2021

### 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;

Engineering (General)
Physics

EDUCATIONAL LEVEL

FORMAT

PDF

TOPICAL AREAS

N/A

LANGUAGE

English

TYPE/METHODS

Analysis (Issues), Directed, Interrupted