Hi, As I was going through some SHM problems I came across a doubt regarding normal forces. Let's use as an example a book standing on a table: There are two forces acting on the book: It's weight acting downwards and a normal reaction force from the table acting upwards. My doubt: Is there a reaction force for the Normal Force acting on the box on the table? Or not even a reaction force, just a Normal force since the table is in contact with the book as much as the book is in contact in the table... I don't know if I am conveying my doubt clearly but, in case there was a Normal Force on the table as well, a free body diagram of it would show two forces pointing downwards, the normal force and the weight, no? Thanks, Peter G.