Q.Physics textbooks are stacked one upon another on a table, one volume above another.

Step 1. Consider several physics books stacked one upon another on a table, from bottom to top; label them (from the table upward) Book 1 (bottom) up to Book n (top).

Step 2. For the TOP book: only two forces act — its own weight $m_ng$ downward, and the normal (upward) reaction $N_{n-1}$ from the book directly below it; since it's at rest, $N_{n-1}=m_ng$.

Step 3. For any MIDDLE book (say book k): three forces act — its own weight $m_kg$ downward, the downward force exerted on it by the book immediately above (by Newton's third law, equal to the normal force that book received from below it), and the upward normal force from the book immediately below, which (by equilibrium) equals the total weight of book k and everything above it.

Step 4. For the BOTTOM book resting on the table: its own weight, the downward force from the book above, and the upward normal force from the TABLE, equal to the combined weight of the entire stack. …

Isolate each book one at a time as its own free body; the normal force it receives from below must support its o …

• Giving every book the same normal force, instead of increasing the normal force for books nearer the bottom of the stack. …