No one knows for sure, but when you experience that ‘stomach drop’ feeling on a rollercoaster, or abruptly descending in a lift, or even going over a big bump in the car, it’s likely to do with how your body and internal systems respond to rapid changes in motion and gravity.
Normally, your organs are held in place by a network of tissues and muscles, and gravity is pulling everything down. But when you go over a bump or a dip in a car, you experience a sudden change in speed and direction – especially when you quickly accelerate downward.