The common contact forces কী?

Example 6.5 (Inventory on a towed sled)

A sled towed across the snow: weight $\vect P$ (distance), normal reaction $\vect R$ of the snow, tension $\vect T$ of the rope, friction $\vect f$ opposing the sliding. Nothing else touches the sled: the inventory is complete — weight always, then one contact force per touching object.

Example 6.11 (The hockey puck)

Released by the stick, a puck crosses $30\,\mathrm{m}$ of smooth ice in $2.5\,\mathrm{s}$, straight at the constant $v = 30/2.5 = 12\,\mathrm{m}/\mathrm{s}$. Inventory: weight and normal reaction (friction negligible). They compensate, and the principle predicts exactly the observed motion: nothing pushes the puck forward — and nothing needs to.

Example 6.16 (The book on the table)

A book of mass $0.50\,\mathrm{kg}$ rests on a table: weight $P = 0.50 \times 9.81 \approx 4.9\,\mathrm{N}$ and normal reaction $\vect R$. At rest, $\vect R$ compensates $\vect P$: vertical, upward, $R = 4.9\,\mathrm{N}$. Add a second book and $R$ adjusts — a support pushes exactly as hard as needed.