Actually, they are wrong. Einstein showed it to be so. All scientific theories are mathematical models of phenomena. Most are just first-order approximations. The whole notions of tension, normal force and friction are methods of dealing with extraordinarily complex behavior.
> Actually, they are wrong. Einstein showed it to be so.
WRONG. Your assignment for tonight is to write a 1,000 word essay on the topic: "Why Relativity only tweaked Newtonian physics, and didn't actually prove it 'wrong'."