What mechanism prevents escalators from moving upward without a passenger's weight?

The correct answer focuses on the role of counterweights in escalator systems. Counterweights are used to balance the weight of the escalator's steps and facilitate smooth operation. They are designed so that when there is no weight on the escalator, the counterweights counterbalance the force of gravity on the escalator steps, preventing them from moving upward. This mechanism ensures the escalator remains in a stable position when not in use, effectively avoiding unintended movement.

Counterweights play a crucial part in the overall design and safety of escalators, allowing for controlled and steady operation while minimizing the risk of mechanical failure or accidents. They are essential for providing the necessary balance and stability required for safe escalator functionality.