When pressure is no longer applied to open an automatic valve, which unbalanced force closes it?

The correct answer is based on the function of an automatic valve in terms of its mechanics when pressure is removed. When pressure is applied to open the valve, it typically holds the valve in an open position against any counteracting forces. Upon the removal of this pressure, certain forces come into play to close the valve.

Gravity and spring tension are pivotal in this scenario. The valve's mechanical design incorporates a spring that applies tension to the valve components. This tension works to close the valve once the external pressure is removed. Simultaneously, gravity exerts a downward force on the valve components, contributing further to the closing action.

Understanding this interaction of forces is critical. The spring tension effectively pushes back against the movement of the valve when the pressure is no longer present, while gravity ensures that any components rely on their weight to assist in the closure. Thus, without the applied pressure, the combination of these two forces is what causes the automatic valve to close safely and reliably.