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 for 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

力圖

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Sample Problem