We frequently talk about how to make swords “safe”. Unfortunately unless you can come up with a definition which satisfies a few specific criteria, the word “safe” doesn’t have a lot of meaning.
It was the first two words that got you hooked, right?
The short answer to everything is always F = ma. For the longer answer, read the article.
From a physics point of view hitting ‘hard’ can be a somewhat difficult property to quantify. Learn a little bit about what can make a strike ‘hard’.
Sometimes people talk about how much ‘force’ something hits with. Unfortunately, they use the same term to describe average force, peak force, and impulse.