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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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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