Why can’t people open their car doors when they drive into a lake?
In: 20
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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