Question

why does the boxer always move along the direction of the punch of the opponent

Answer 1

beacause
if they cannot dodge there in that situation
there is an effect of distance also here
distance is inversly proportional to momentum

Answer 2

As distance is indirectly proportional to momentum, then it would not hit the boxer much harder because if distance more the momentum will decrease.
This also helps the boxer to have a dodge.

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