Two physics books, a and b, are placed on top of each other on a table. book a's mass is 2 kg and book b's mass is 1 kg. book a is placed on book b. what is the reaction book b exerts onto book a?

We don't care about the mass of book-B, the one on the bottom. Book-A is resting on something, and it's not accelerating up or down. So the vertical forces on it are balanced. The upward "reaction" force from whatever it's resting on is equal to it's own weight. Book-A's weight is (mass)x(gravity)=(2kg)x(9.8m/S2) = 19.8 newtons. That's also the reaction force acting upward on its underside.

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