Reading & explanation
Isolate the system
A free-body diagram (FBD) includes forces acting ON the selected object. For a book resting on a table, Earth exerts gravitational weight downward and the table exerts a normal force upward. The equal and opposite force the book exerts on the table belongs on a different diagram. An FBD is not a picture of every force in the room.
Name the interaction agent
Labels such as 'Earth on book' and 'table on book' clarify which agent provides each force. The normal force is a contact force perpendicular to the supporting surface; weight is Earth's gravitational force. An arrow representing velocity does not belong in an FBD. Nor do we draw an extra force called 'acceleration'.
How to determine a normal force
For a block in contact with a horizontal surface and with no vertical acceleration, ΣFy=N+Fup−Fdown−mg=0. Therefore N=mg+Fdown−Fup. When no extra vertical forces act, N=mg, but this is a special situation, not a general identity. Normal force and weight act on the same object, so they cannot be a Newton's-third-law pair.
Our boundaries and limits
The Force Builder restricts upward applied forces to less than the minimum modeled weight. This guarantees contact in its idealized level-support model. Lifting off, tilting, rolling, friction transitions and contact loss are outside its scope. Draw the diagram and compare the displayed vertical force ledger to your calculation.