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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. mX a0 m0 a X a0 mX m0 aX 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. Fnet ma 1 N (1 kg)(1 m/s ) 1 kg m/s 2 • pound 磅 (lb) (1 lb = 0.454 kg, ???) • slug 斯拉格（斯勒格、斯） – 1 slug = 14.59 kg 2 • 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 Fg mg • Weight, W 重量 W mg • 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. FBC FCB FBC FCB • 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