What is Compensating forces? — Physics

Definition 6.7 High School Physics · Chapter 6 — Forces and the Principle of Inertia

Forces acting on a body compensate when their combined effect is nil: tip to tail, their arrows return to the start. Two forces compensate exactly when they share the same line of action with equal magnitudes and opposite ways.

A curling stone at equal time intervals. Left: P and R compensate — equal steps. Right: an uncompensated friction f remains — the steps shrink, the velocity changes. — A curling stone at equal time intervals. Left: $\vect P$ and $\vect R$ compensate — equal steps. Right: an uncompensated friction $\vect f$ remains — the steps shrink, the velocity changes.

Examples

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.

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