Why can’t people open their car doors when they drive into a lake?
In: 20
Because water is heavy as fuck and a lot stronger than someone pushing against a door.
Headrests, however, are typically designed to be removable and their lil pointy ends are made for breaking windows from the inside.
They can. You just need to wait until the water fills up the inside of the car.
When the car first falls in, you have water outside the car and air inside. The pressure from the water on the door isn’t that high but the area of the door is large, so that small pressure results in a very large force…so large that you may not be able to push outwards hard enough to overcome it.
Once the car is full of water there’s no pressure difference and you can open the door.
Because the pressure of the water pushing onwards on the car is FAR greater than the pressure of the air in the car pushing out.
In day-to-day lives, the pressure is essentially equal, so there is little to no imbalance in force, so it is easy.
If you want to feel these forces, grab a plastic bag, put it over your hand, and try to hold your hand open (kind of like a claw) as you start putting your hand into a container of water (just make sure the water doesn’t spill into the bag). You will feel the pressure of the water pushing in.
As an extra extension, if/when you let the water rush over the top of the bag, filling it, the pressure will become balanced again and it will be easy to love the bag.
Water is heavy. Air is not.
A car’s passenger cabin contains air. It’s doors present an effectively water-tight seal which prevents lake water from immediately filling the passenger cabin. Instead, lake water presses firmly against the car’s partially submerged doors with force that exceeds what most people can overcome. Most water enters the cabin through ventilation ducts. Not until the cabin floods to the point that the water level inside the passenger cabin matches the water level outside the car will forces pressing the doors shut diminish to the point that the doors will easily open.
Unfortunately, unless the car already rests firmly on the bottom with its cabin still above the surface, equalization will not occur until well after the vehicle submerges — indeed a frightening outcome.
The only realistic means of escape is through an open window. However, after entering the water, a vehicle’s power windows often fail to function, leaving occupants trapped. Therefore, most first-responders recommend drivers keep a spring operated [glass punch](https://www.amazon.ca/SANZHENG-Center-Automatic-Spring-Replacement/dp/B09V56ZM39/ref=sr_1_5?crid=1NJH4IBJ5ILF2&keywords=glass+punch&qid=1669160150&qu=eyJxc2MiOiIzLjA3IiwicXNhIjoiMi43MCIsInFzcCI6IjIuNjYifQ%3D%3D&sprefix=glass+punch%2Caps%2C154&sr=8-5) within easy reach, and remember the mnemonic “POGO”:
* P: **P**op your seat-belt;
* O: **O**pen the window; and
* GO: **GO!!!**
If you’re deep underwater, all of the water outside and on top of ypur car is weighing down against the door, effectively crushing it inwards. You’re trying to open the door with a LOT of extra weight behind it. So why does cracking the glass and/or letting the water rush into the car help you open the door? Well, when the car is filled with water, then an (almost) equal amount of water is pushing outwards from the car as there is pushing inwards, so they (almost) cancel out!
A lot of misinformation in this thread :/
Don’t wait for the car to submerge, get out as quickly as possible.
Everybody here has already answered the question, so I’ll add this if you wanted to do an experiment to feel the force of water in a similar situation.
If you stand or sit upright underwater while using a snorkel to breathe, you’ll be able to notice it’s harder to inhale while the water is pushing in on your chest even at a depth of about 2 feet.
Pressure.
Water is heavy. Water pushes on things that fall into it.
Including doors. Importantly, most car doors open outwards. When opening a door, you are fighting against the water outside the door.
And water pushes *hard*. Every meter of depth, for a door of 1 square meter, is the weight of 1 metric ton pushing into the door. You aren’t strong enough to overcome that. So if you want to get out, you’re gonna need to use smarts instead.
Fortunately, you have a few options. Notably, if the car is filling with water you can just wait. If there is pressure from both sides, the force cancels itself out, and you are free to open the door just like you do in the air. (Literally. Air at sea level has the same pressure as about 10 m of water.) And the human body can go a good couple minutes holding its breath, giving you plenty of time. (For a couple more minutes, you can keep breathing – calmly – for as long as the air in your car lasts, though beware of CO2 buildup.)
If you have doors that slide (rather than swinging), you can slide them. Even just windows will work, if you have a way to move them (e.x. a hand crank).
Theoretically, you can also try to break the glass. By using a pointy object, you can concentrate an impact on a small part of glass, and hopefully do enough damage to break it. But this will be hard, since car glass (especially the windshield) is designed not to fall apart very easily. It will also be dangerous, since you now have entire tons worth of force applying to loose glass shards. But, if you can manage it, you can make an opening to exit, very quickly. (This is probably best for if the interior is, like, on fire or something.)
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