This chapter opens with Tris judging Lynn for her lunch choices. "Don't tell me you're going to eat a mashed potato sandwich" (20.3), she says, as if she's Jenny Craig herself.
Tris is not the only one being snippy in the cafeteria. Other boys are hissing "coward" (20.7) in Tobias's direction.
To prove he's not a coward (or maybe he's just hungry for a mashed potato sandwich), Tobias gets up and beats the crap out of his father. No, really, that's what he does. In front of everyone.
Tobias walks over to Marcus's table, throws Marcus to the ground, and beats him with a belt.
Then Tobias chucks a wedding ring at him and says, "My mother […] says hello" (20.26).
Tris runs after Tobias. His only explanation is "It was necessary" (20.30). Sometimes dads need a good beating, we guess.
That explanation isn't good enough for Tris, so she follows Marcus, who doesn't seem too damaged by the whole thing—physically, at least. We imagine his ego is more than a little bruised after getting beaten by his own son in a cafeteria.
Tris gets no information out of Tobias. She just snaps at him, tries to tell him that she's not useless, and then uselessly walks from the room. Very convincing.