Final answer:
We are able to sense movement in an elevator due to the semicircular canals in our inner ear. The fluid in these canals moves with our motion, stimulating hair cells that send signals to the cerebellum, helping us to maintain balance and perceive motion.
Step-by-step explanation:
We know when we are moving up and down in an elevator because of the movement of tiny crystals in the semicircular canals.
When an elevator suddenly begins to descend, we may feel lighter due to the acceleration. This is an effect of physics explained by Einstein. As the elevator accelerates, the forces acting on our body differ from the normal gravitational pull we are used to when standing still. Similarly, we would feel heavier when the elevator accelerates upwards. These changes in perceived weight can be measured if a person were standing on a scale in the elevator.
The biological reason we can feel this change is due to the liquid within our inner ear. Specifically, the semicircular canals contain fluids that move when our head changes position or when we experience acceleration. This movement triggers tiny hair cells in the canals to send impulses through the vestibular nerve to the cerebellum, which then helps us maintain our balance and perceive motion. Thus, the cerebellum plays a key role in how we perceive the acceleration and deceleration within an elevator.