What would happen if someone hit the back of the head.?

Don't know. It all depends upon the person, their medical/surgical history, medications & supplements they're taking, and the force of the blow, among other issues. I can imagine nothing happening to laceration (cut) to skull fracture to internal bleeding (epidural vs subdural hematoma) to death. Sorry but too many variables for specific prediction.

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What would happen if someone hit the back of the head?

It depends. the degree of injury would depend on how much force there was in the blow and the persons health. It could range from annoying pain to death. Read more...