Final answer:
Charges of the VDG and the pie pan must be added to the model to explain why the pie pan flies off a Van de Graaff generator (VDG). When the VDG generates a large positive charge on the metal sphere, it induces a large negative charge on the pie pan, causing repulsion and making the pie pan fly off.
Step-by-step explanation:
To make the model explain the phenomenon of why the pie pan flies off a Van de Graaff generator (VDG), charges of the VDG and the pan must be added to the model. When the VDG generates a large amount of positive charge on the metal sphere, it induces a large negative charge on the pie pan. The repulsion between the like charges causes the pie pan to be pushed away from the VDG and fly off.