**Sample Problem 8-2a**
The potential energy at y1 is 98J if reference is assumed to be at y=0
The potential energy at y1 is 39J if reference is assumed to be at y=3
The potential energy at y1 is 0J if reference is assumed to be at y=5
The potential energy at y1 is -19J if reference is assumed to be at y=6

**Sample Problem 8-2b**
The change in potential energy is -98J same for all the references 