I've noticed, up til now, for every definite form of earth the word "jorda" has been used in this skill set, but in this sentence "jordens" is used. Is this because of the definite possessive? Would "jordas" also be an acceptable spelling choice?
You could use "tyngdekraft", "gravitasjon", and "gravitasjonskraft" interchangeably in this sentence. I'm not sure what's the preferred terminology among physicists, but for us mere mortals any of them will do.