Hi and welcome,
Cranium that is hard covering which make up head can withstand incredible amount of force. Head knocking does nt result in any physical or cognitive function and usually whenever there is a physical injury associated with head injury there are signs like nausea, vomiting, loss of consciousness . And cognitive function only gets affected by repeated knocks enough to cause minor physical injury in the long run. So nothing much to worry about those light knocks as every one suffers them multiple times without any damage.
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Thanks for reply,
Any severe head bang which cause you to feel fogy, dizzy, or difficulty in remembering can cause concussion. The exact force is difficult to consider as it varies from person to person so it is the symptoms which are leading in this case.