1. The problem statement, all variables and given/known data A book is at rest on a table. What is the "reaction" force according to Newton's third law to the gravitational force by the earth on the book? (1)the normal force exerted by the table on the book (2)the normal force exerted by the table on the ground (3)the normal force exerted by the ground on the table (4)the gravitational force exerted on the earth by the book 2. Relevant equations 3. The attempt at a solution Analysing the FBD of book, the forces acting on it are its own weight and the normal reaction from the table. So the answer should be 1 but the answer key states 4. I understand that weight is the "gravitational force" exerted on the book by the earth. I don't see how 4 is the answer. Any help is appreciated. Thanks!