What causes that feeling of weightlessness when you're on a rollercoaster?

The feeling of weightlessness you may get when on a rollercoaster is caused by gravitational force (g-force), and more specifically, negative g-force. Negative g-force occurs when you accelerate downwards faster than freefall, and that downward acceleration produces a force in the upward direction, thus pulling you out of your seat and causing a feeling of weightlessness.