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Physics · 术语表

什么是 The common contact forces？

又称： normal reaction · tension · friction · thrust

Definition 6.4 High School Physics · 第 6 章 — Forces and the Principle of Inertia

Four contact forces cover most everyday situations:

the normal reaction $\vect R$ of a support, perpendicular to its surface, pushing away from it;
the tension $\vect T$ of a wire, rope or chain, directed along it, pulling toward it;
friction $\vect f$ from a surface, the air or water, opposing the sliding or the motion;
the thrust of an engine, jet or hand, pushing along the effort.

The book on the table: weight P and normal reaction R — same line, equal lengths, opposite ways. — The book on the table: weight $\vect P$ and normal reaction $\vect R$ — same line, equal lengths, opposite ways.

示例

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.

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