Physics 121, Sections 9, 10, 11, and 12 Lecture 5 Today's Topics Homework 2: Due Friday Sept 16@6: 00PM Ch.3:#2.11,18,20,25,32,36,46,50,and56 Chapter 3: Forces and motion along a line Motion with constant acceleration Falling objects Apparent weight Chapter 4: Motion in 2-D Vectors Kinematics Projectile motion Physics 121: Lecture 5, Pg 1
Physics 121: Lecture 5, Pg 1 Physics 121, Sections 9, 10, 11, and 12 Lecture 5 Today’s Topics: Homework 2: Due Friday Sept. 16 @ 6:00PM Ch.3: # 2, 11, 18, 20, 25, 32, 36, 46, 50, and 56. Chapter 3: Forces and motion along a line Motion with constant acceleration Falling objects Apparent weight Chapter 4: Motion in 2-D Vectors Kinematics Projectile motion
Recap: constant acceleration in 1-D For constant acceleration Xo +vot+at V=Vo+at a= const From which we know v=2a(x-X0) (Vo +v) Physics 121: Lecture 5, Pg 2
Physics 121: Lecture 5, Pg 2 Recap: constant acceleration in 1-D For constant acceleration: v v at 0 = + 2 0 0 at 2 1 x = x + v t + a = const From which we know: (v v) 2 1 v v v 2a(x x ) av 0 0 2 0 2 = + − = −
Lecture 5 ACT 1 1D Freefall Alice and Bill are standing at the top of a cliff of height H. Both throw a ball with initial speed vo. Alice straight down and Bill straight up. The speed of the balls when they hit the ground are va and vB respectively. Which of the following is true: (a va< vB (b)VA=VB (c)VA> VB Alice Vol Bill 0 H B Physics 121: Lecture 5, Pg 3
Physics 121: Lecture 5, Pg 3 Lecture 5 ACT 1 1D Freefall Alice and Bill are standing at the top of a cliff of height H. Both throw a ball with initial speed v0 , Alice straight down and Bill straight up. The speed of the balls when they hit the ground are vA and vB respectively. v0 v0 Alice Bill H vA vB Which of the following is true: (a) vA < vB (b) vA = vB (c) vA > vB
About air resistance When a body moves through a fluid A drag force due to friction take place Like friction, its direction is opposite to the motion This force increases dramatically with speed It is proportional to v2 Fa=bv2 For an object falling through air As gravity accelerates it, v increases When the magnitude of Fd equals the weight mg 》 Not net force→ y becomes constant The object has reached its terminal speed vt mg Physics 121: Lecture 5, Pg 4
Physics 121: Lecture 5, Pg 4 About Air resistance ? When a body moves through a fluid A drag force due to friction take place Like friction, its direction is opposite to the motion This force increases dramatically with speed It is proportional to v2 For an object falling through air As gravity accelerates it, v increases When the magnitude of Fd equals the weight mg »Not net force v becomes constant The object has reached its terminal speed vt Fd mg v
Air resistance When a body reaches V, Both forces are equal in magnitude F d but mg So the coefficient b is 79 mg And d =m t. e 6 0 The terminal speed v, varies mass, shape, size Feather: 0.5 m/s Raindrop 7m/s Skydiver: 50-60 m/s(spread-eagle Skydiver: 100 m/s diving) Physics 121: Lecture 5, Pg 5
Physics 121: Lecture 5, Pg 5 Air resistance When a body reaches vt Both forces are equal in magnitude So the coefficient b is And The terminal speed vt varies mass, shape, size Feather: 0.5 m/s Raindrop: 7 m/s Skydiver: 50-60 m/s (spread-eagle) Skydiver: 100 m/s (diving) Fd mg vt