# PHYS 2211 Module 8.1

## Potential Energy of a System 8.1 Potential Energy of a System

### Learning Objectives

By the end of this section, you will be able to:

• Relate the difference of potential energy to work done on a particle for a system without friction or air drag
• Explain the meaning of the zero of the potential energy function for a system
• Calculate and apply the gravitational potential energy for an object near Earth’s surface and the elastic potential energy of a mass-spring system

### Gravitational Potential Energy

The work done on an object by the constant gravitational force, near the surface of Earth, over any displacement is a function only of the difference in the positions of the end-points of the displacement.

You can justify this statement by answering the following question:

Again, consider the work done by the force of gravity in this problem. Use the work-energy theorem: to answer the question. Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

A good, professional baseball pitcher throws a ball straight up in the air. Using the Work-Energy theorem, estimate how high the ball will go. (A good throw can reach 90 mph.)

The force of gravity is a special type of force called a conservative force. This is something we will discuss in the next section of the module. Without going into details about conservative forces just yet, I will give you a definition for potential energy:

The change in potential energy (U) of an object is equal to the negative of the work (Wcons) done by conservative forces.