Answer:
The answer to this question is that people´s ability to maintain balance, a process called propioreception, and posture, as well as perception of the position of the body, and motion, depends majorly on the vestibular system in the ear, a series of hairs, fluid, and structures within the ear that are responsible for the sense of propioreception.
When the head moves in certain ways, or gravity plays its part, the sensors in the vestibular system sense the change thanks to the motion of the fluid in them, which stimulates the hairs. This will tell the brain that the head is moving, and it explains also the reason why some people, when in a car, boat, or plain, may feel dizzy, or nauseous. This is a system the body uses to ensure correct posture, and also the correct use of reflexes responsible for the movement of the body.