For instance if someone is hit by a train or a car, why does it kill them? How does the impact kill them.
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Rupturing internal organs, rupturing blood vessels, crushing the head, even pulping the body if the impact is strong enough.
We think of our bodies as single objects that generally stay the same shape (not counting motion of our joints), but we’re really squishy bags of skin and bones with organs loosely anchored inside.
When you’re hit by something large, fast, and solid, the part of your body it touches first basically immediately has to accelerate up to the speed of the thing. That means a very large acceleration, which means a very large force. That can crush/break things because they get squashed before they can move out of the way.
As the motion continues, the now fast-moving parts of your body run into the parts that haven’t started moving yet…like your organs. And bendy things like bones are bending as one end moves but the other end hasn’t got up to speed yet. Organs get whacked and can burst, bones break, blood vessels stretch and break.
Once your whole body is up to speed the immediate acceleration damage stops but all the initial damage is still there, and it starts causing other problems. Broken bones stay broken, and those jagged ends can poke/stab adjacent things…more organ & blood vessel damage. All the holes poked in your circulatory system lead to blood loss and shock. If lungs are punctured by rib bits, you can’t breath. Leaking blood can put pressure on your brain. Things that should have stayed inside their respective tubes (food, blood, feces, urine) may leak out into places they shouldn’t be and are now poisoning the things around them.
Edit:typo
Have you ever stomped on a bug or watched a bug hit the car windshield?
Replace the bug with a human. Human organs puréed do not function.
Smack a human hard enough and every thing goes full stop upon a big enough squish.
Get a friend to punch you hard in the arm (or whatever body part you prefer). A bruise will form due to blood leaking from damaged capillaries into the surrounding tissue. A train or a car hits a lot harder than your friend can and will easily cause a lot more damage because of it. And, if that damage is done to your internal organs, that can interfere with their functioning and kill you.
By the law of conservation, energy can’t be created or destroyed, only transformed. Impacts involve a lot of energy, and that energy has to go somewhere.
When you dive into a swimming pool, the water takes away the energy of the impact. It does this by slowing you down. You sink a little bit into the water as this happens, while some of the water splashes around and other water resists your movement. But eventually the water siphons away all of that energy, you stop sinking, and other forces (like buoyancy) take over. Even if you do touch the bottom, as long as the pool is deep enough, you won’t be going very fast, so you don’t get hurt.
Let’s change up the scenario: you dive into the pool, but it’s empty. Now there’s nothing slowing you down, so you hit the concrete at full speed, and all that energy has to be dissipated *right now*. Some of it will go into the concrete, and you might leave some cracks or even a dent in it. But most of it will remain in your body, which just didn’t evolve to withstand those kinds of forces. Tissues tear, organs rupture, bones shatter, chunks of you splash around, and energy dissipates as this happens. It gets gruesome very quickly.
The impact of a train or car is like diving into a pool with no water. The energy of the impact has to dissipate very quickly, and your body just isn’t up to the task. It absorbs as much as it can, and whatever energy is left over will send you flying, which typically ends in *another* impact. If you’re wearing a lot of padding, it can absorb some of this impact energy just like water in a pool, but it would take an awful lot of padding to keep you safe from a car or train impact.
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