講者： 許永昌 老師 1

Contents Equilibrium Dynamics Mass, weight, gravity, and weightlessness Friction & drag 2

Equilibrium ( 請預讀 P152~P154) 3

Exercises (Step by step) Ex6.2: The 3 ropes in Figure Ex6.2 are tied to a small, very light ring. Two of these ropes are anchored to wall at right angles with the tensions shown in the figure. What are the magnitude and direction of the tension T 3 in the third rope? Ex6.4: A football coach sits on a sled while two of his player build their strength by dragging the sled across the field with ropes. The kinetic friction force on the sled is 1000 N and the angle between the two ropes is 20 o. How hard must each player pull to drag the coach at a steady 2.0 m/s? 4 T 1 =50 N T 2 =80 N 0.80 m 0.60 m

Simple second-law problem ( 請預讀 P155~P157) 5

Exercises Ex6.11: A 50 kg box hangs from a rope. What is the tension in the rope if: The box is at rest? The box moves up at a steady 5.0 m/s? The box has v y =5.0 m/s and is speeding up at 5.0 m/s 2 ? The box has v y =5.0 m/s and is slowing down at 5.0 m/s 2 ? Ex6.8: Figure Ex6.8 shows the velocity graph of a 2. kg object as it moves along the x-axis. What is the net force acting on this object at t=1 s? At 4 s? At 7s? 6 t (s) v x (m/s)

Mass, Weight, Gravity and Weightlessness ( 請預讀 P158~P162) 7

Exercises The components (F G ) x and (F G ) y in each of the three coordinate systems shown below. Ex6.13: An astronaut’s weight while standing on earth is 800 N. What is his weight on Mars, where g=3.76 m/s 2 ? Ex6.16: Figure Ex6.16 shows the velocity graph of a 75 kg passenger in an elevator. What is the passenger’s weight at t= 1 s? At 5 s? At 9 s? 8 x y x y x y   t (s) v y (m/s)

Friction ( 請預讀 P162~P167) 9 * 切記，相對速度為零時，就是用靜摩擦。 Materials Static  s Kinetic  k Rolling  r Rubber on Concrete Steel on Steel (dry) Steel on Steel (lubricat ed) Wood on Wood Ice on ice

Exercise Based on the model of the friction forces, are the kinetic friction forces of these two objects the same? The mass and the material of these two objects are the same. 10

Homework Student Workbook: ( 請小心作答，免得被騙 )

Drag ( 請預讀 P168~P170) The force exerted by air on a moving object. Basic Concept: Object: air Newton’s 3 rd Law: 12 A  x Q: A=?

Terminal Speed An object will reach a terminal speed when the gravity and drag forces exerted on the object are balance. i.e. F net =0. 13

Summary It provides us the strategies for equilibrium problems and dynamics problems. Mass is an intrinsic property of an object, gravity is a force and weight is a measure of a force. They are different. Friction & Drag are complicated forces, the model we used in this chapter are 14

Homework Student Workbook Textbook: 照樣，請將 terms and notation 製作成卡片，以方便記憶。 15