"Physics also applies to her."
Translation:Physik gilt auch für sie.
Also, I think it should be ‘irgendwas mit den Fällen/Kasus zu tun?’ (assuming you meant to say ‘something to do with the cases?’), but I don't know whether you can use the expression this way.
I have a problem understanding the meaning of gelten. If it means 'to be valid' what is the sentence saying? Does anyone have a better example for the verb?
Yes, it means “to be valid, to hold true, to count” or “to be of value”. The first meaning is being used here: “physics holds true for her too”. Another example could be “diese Regel gilt immer noch” (“this rule is still valid/still in force”) or “das gilt als vollständig” (“that counts as/is deemed complete”).