講者： 許永昌 老師 1

Contents A Short Catalog of Forces Identifying Forces Action I Newton’s 2 nd Law Action II Newton’s 1 st Law Free-Body Diagrams 2

Preface 3

ForceSymbolCause(Agent) Spring ForceSpring Tension ForceRope Normal Forcesurface Frictionsurface DragAir or liquid ThrustExhaust gas GravityThe earth Electric & magnetic Forces A Short Catalog of Forces The graphical description: Catalog: particle (1) (2) (3) Long range 4

Action I: Identifying forces ( 請預讀 P126~P134) 5

Exercise (gravity) 6

Identifying forces (pictorial representation) 7 Example: Anna pull a block. Example: Anna pull a block.

Summary I 力是造成運動的 “ 因 ” 。 Identifying forces method 可以幫助我們找出作用在該 感興趣的物體的所有的力。 這些力的合力滿足向量疊加法則。 8

Homework Student Workbook

Newton’s 2 nd Law ( 請預讀 P135~P139) An object with mass m will undergo acceleration 10 Q: However, the object is not a PARTICLE!! How can we apply the Newton’s 2 nd Law on an object? Q: However, the object is not a PARTICLE!! How can we apply the Newton’s 2 nd Law on an object?

Newton’s 2 nd Law of motion 11 *Shipman, An introduction to Physical Science, 12 edition.

General Physics experiment II Reference: Newtons-Second-Law http:// Newtons-Second-Law Observed: When an object is isolated, its motion is a uniform motion, i.e. v=constant. (both the magnitude & direction are invariant) Use the paper clips to pull the cart. You will find that the graph of acceleration vs. # of paper clips is almost a straight line. (Not only F pull   a , but also F pull  a) Compare the acceleration of one cart & of two carts. Theoretically, a  1/# of cart. Note: The condition m cart >> m paper clips should be held for these experiments. 12

Newton’s 1 st Law of motion & inertial reference frame ( 請預讀 P139~P142) 13

Summary II Newton’s 1 st Law of motion : 一物體若在孤立狀態下，則該物體會表現出 uniform motion 。 藉此定義出 inertial reference frame 。 Newton’s Law is valid in inertial reference frame. Newton’s 2 nd Law of motion: 此方程將 Dynamics and Kinematics 關連在一起。 14

Homework Student Workbook

Free-body diagram 16 x y The direction of the forces will be discussed later.

Summary 17

Exercise(5.28) Toss a rock straight up into the air. The rock is “ the system” of interest. Please draw a picture and a complete free-body diagram for each case. The rock is rest on your palm. Your hand is moving up but before the rock leaves your hand. One-tenth of a second after the rock leaves your hand. After the rock has reached its highest point and is now falling straight down. 18

Homework Student Workbook Textbook: 一樣，將 terms and notation 製成卡片，以便自我練習。 19