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## Exploring the Effective Potential: Instructor's Guide

### Main Ideas

• The effect of angular momentum, the force constant, and the reduced mass on the shape of the effective potential function;
• Practicing exploring parameter space for a function;
• Developing intuition about how the orbit shape depends on these parameters.

Estimated Time: 15 minutes

The students use Maple to explore how the shape of the effective potential function changes as the various parameters (angular momentum, force constant, reduced mass) are varied.

### Prerequisite Knowledge

• Students should know that the effective potential is used to reduce the 2-D central force problem to a 1-D problem.
• Students should know that the shape of the orbit need not be elliptical - all conic sections are solutions to the equations of motion (depending on the values of the various parameters).

### Activity: Introduction

We usually start this activity with a general discussion about sets of solutions to an equation and the professional sense-making activity of exploring parameter space to build an understanding of the types of solutions that exist in the set.

### Activity: Student Conversations

• In exploring the parameter space of solutions, students should be encouraged to identify limiting and special cases (e.g. when is the orbit is circular, what is the shape of the function for large and small separations, etc).
• Some students point out that the force constant $k$ ought to depend on the reduced mass $\mu$. This is a result of the gravitational force being the typical example of a central force; reminding students that other central forces exist (e.g, the Coulomb force) clears up this confusion.

### Extensions

This activity works particularly well when sequenced with other activities.

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