Rook and pawn beat a knight almost always; the knight is too slow to both blockade the pawn and avoid the rook. The king advances, the rook pins or chases the knight, and the pawn promotes.
The knight is a worse defender than the bishop against rook and pawn, because it has no long-range control and can be dominated by the rook. This position shows a typical win: Black's king on g8 and knight on h6 try to stop the g-pawn, but White's king on f4 and rook do the rest.