About the Teaching of Plane Motion of Rigid Bodies

TL;DR

This paper discusses common problems in teaching plane motion of rigid bodies, clarifying misconceptions about energy and motion in typical physics problems involving pendulums and sliding ladders.

Contribution

It provides detailed analysis and explanations of two classic physics problems to improve understanding of rigid body motion concepts for students.

Findings

Angular frequency varies with the disk’s ability to spin.

Contact forces can do work even when total energy is conserved.

Abstract

The study of the motion of a rigid body on a plane (RBP motion) is usually one of the most challenging topics that students face in introductory physics courses. In this paper, we discuss a couple of problems which are typically used in basic physics courses, in order to highlight some aspects related to RBP motion which are not usually well understood by physics students. The first problem is a pendulum composed of a rod and disk. The angular frequency of the pendulum is calculated in two situations: disk fixed to the rod and disk free to spin. A detailed explanation of the change in the angular frequency from one case to another is given. The second problem is a ladder which slides touching a frictionless surface. We use this problem to highlight the fact that the contact forces applied by the surface perform translational and rotational work despite that the total mechanical energy of the ladder is conserved.