Two united pawns in the sixth line

Hello everyone, this is my first post in chess.com and I want to explain an important idea in the endgame.
Two pawns in the sixth line are more strong to a rook , only a rook can't prevent the promotion of one of these two pawn.

Diagram 1

The diagram is a simple example of this, in fact , the rook can takes only a pawn and the other pawn goes to promotion.

It's very important know simple position to solve hard problem.

The past year I played in a rapid tournament against a young and strong chess player, it was a very hard match , I had this position(diagram 2) with only 1 or 2 minutes on my clock , I managed to find  the right move just because I knew the simple position.

I think is better that you try to solve the problem first to see the solution.

Diagram 2

We must remember that the value of the pieces is a relative concept

  Under certain circumstances , a knight is better than a queen or two paws are better then a rook.