Opening a door underwater is a common question, often feeling like a difficult task. The challenges stem from fundamental scientific principles related to fluids and forces. Understanding these concepts clarifies why opening a door underwater can be demanding in some situations and manageable in others.
Understanding Water Pressure and Resistance
The primary factor making it difficult to open a door underwater is hydrostatic pressure. This pressure results from the weight of the water column pushing down on an object due to gravity. The deeper one goes, the more water is above, leading to higher pressure. For instance, pressure increases by approximately one atmosphere for every 10 meters (33 feet) of depth in water.
The total force exerted by this pressure on a door is a product of the pressure and the door’s surface area. For example, a typical car door with an area of about 1 square meter could experience thousands of Newtons of force at shallow depths. Water is also significantly denser than air, meaning any movement through it encounters considerable resistance or drag, further impeding the door’s opening.
Opening a Door in a Submerged Vehicle
When a vehicle becomes submerged, a significant pressure difference develops between the inside and outside. The air inside initially remains at atmospheric pressure, while the water outside exerts increasing pressure with depth. This differential creates a powerful force pushing the door inward, effectively sealing it shut. Even at shallow depths, this force can amount to hundreds or thousands of pounds, making it nearly impossible to overcome.
To open the door, this pressure differential must be reduced or eliminated. This occurs as water floods the vehicle, causing the internal pressure to rise and equalize with the external water pressure. Once the water level inside nearly matches the outside level, the significant force holding the door closed is no longer present, allowing the door to be opened with much less effort.
Accessing a Flooded Structure
When a door is already fully submerged on both sides, such as within a sunken ship or between flooded compartments, the dynamics of opening it differ. In these cases, water pressure on both sides of the door is already equalized. This means no substantial pressure differential pushes the door closed or open.
The primary resistance encountered when trying to open such a door stems from mechanical factors. These include the condition of the door’s hinges, debris obstructing its movement, or the inherent drag of water as the door moves. Opening a door in an already flooded structure is considerably easier than attempting to open a car door immediately after it submerges.