Chapter 5 Force and Motion I 5 Chapter

Chapter 5 Force and Motion - I 第 5章 力與運動(一)

Chapter 5 Force and Motion - I 5 -1 What is Physics? 5 -2 Newtonian Mechanics 牛頓力學 5 -3 Newton’s First Law 5 -4 Force 力 5 -5 Mass 質量 5 -6 Newton’s Second Law 5 -7 Some Particular Forces 5 -8 Newton’s Third Law 5 -9 Applying Newton’s Law

5 -1 What is Physics? • • • Acceleration 加速度 Force 力 Dragster Quarterback 四分衛 Knock down 撞倒

5 -2 Newtonian Mechanics • Isaac Newton (1642 – 1727)艾薩克．牛頓 • Very high speed – Newtonian mechanics Einstein’s special theory of relativity • Atomic scale – Newtonian mechanics Quantum mechanics

5 -3 Newton’s First Law • Frictionless surface 無摩擦面 • Newton’s First Law of Motion – If no force acts on a body, the body’s velocity cannot change; that is, the body cannot accelerate. – In other words, if the body is at rest, it stays at rest. If it is moving, it continues to move with the same velocity.

A force on the standard kilogram gives that body an acceleration

5 -4 Force • net force (淨力) or resultant force (合力 ) • Principle of superposition forces • Newton’s First Law of Motion – If no net force acts on a body, the body’s velocity cannot change; that is, the body cannot accelerate. • Inertial reference frame or Inertial frame 慣性參考坐標系

Noninertial frame

5 -5 Mass • The mass of a body is the characteristic that relates a force on the body to the resulting acceleration.

5 -6 Newton’s Second Law • Newton’s Second Law of Motion – The net force on a body is equal to the product of the body’s mass and its acceleration. • pound 磅 (lb) (1 lb = 0. 454 kg, ? ? ? ) • slug 斯拉格（斯勒格、斯） – 1 slug = 14. 59 kg

• free-body diagram 自由體受力圖 • external force 外力 • internal force 內力

Sample Problem

Sample Problem

5 -7 Some Particular Force • Gravitational Force, Fg 重力 • Weight, W 重量 • Normal Force, FN 正向力 • Friction (Frictional Force), f 摩擦力 • Tension, T 張力

An equal-arm balance 等臂天平

A spring scale 彈簧秤 • Apparent weight 視重、視重量

Normal Force, FN 正向力

Friction, f 摩擦力

Tension, T 張力

5 -8 Newton’s Third Law • When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. • Third-law force pair 第三定律力偶

Third-law force pair 第三定律力偶

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Sample Problem