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

The weightless feeling from a rollercoaster is caused by gravitational force, also known as g-force. Different rollercoaster elements lead to different types of g-force: there is positive g-force, which drives blood downwards towards the feet, and negative g-force, which drives blood upwards towards the head. Negative g-force acts opposite to gravity, and it occurs when you accelerate downwards faster than free-fall. The acceleration causes you to pull out of your seat, evoking a sense of weightlessness.