Force Energy is the ammunition used by the Force Gun.
In Dead Space 2, they can be stored in stacks of 15. Force Energy is inexpensive, and sold in stacks of three. Force energy acts like a shotgun at close range, but instead of expending shotgun shells, it fires gravitons instead. This will usually send an enemy flying away, and has a reasonably high chance of dismembering, or perhaps killing, enemies that were sufficiently close to the blast. Like Power Nodes and Ripper Blades, it emits a purple glow when not collected.