Fisica Generale - Alan Giambattista, Betty McCarty Richardson Chapter 4: Motion with a Changing Velocity •Motion Along a Line •Graphical Representation of Motion •Free Fall •Projectile Motion •Apparent Weight •Air Resistance and Terminal Velocity Copyright © 2008 – The McGraw-Hill Companies s.r.l. 1 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §4.1 Motion Along a Line For constant acceleration the kinematic equations are: 1 x x f xi vix t a x t 2 2 v x v fx vix a x t v fx vix 2a x x 2 2 x vav, x t Also: vav, x vix v fx 2 Copyright © 2008 – The McGraw-Hill Companies s.r.l. 2 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example: In a previous example, a box sliding across a rough surface was found to have an acceleration of -2.94 m/s2. If the initial speed of the box is 10.0 m/s, how long does it take for the box to come to rest? Know: a= -2.94 m/s2, vix=10.0 m/s, vfx= 0.0 m/s Want: t. v x vix a x t 0 vix 10.0 m/s t 3.40 sec 2 ax -2.94 m/s Copyright © 2008 – The McGraw-Hill Companies s.r.l. 3 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 4.8): A train of mass 55,200 kg is traveling along a straight, level track at 26.8 m/s. Suddenly the engineer sees a truck stalled on the tracks 184 m ahead. If the maximum possible braking force has magnitude 84.0 kN, can the train be stopped in time? Know: vfx = 0 m/s, vix=26.8 m/s, x=184 m Determine ax and compare to the train’s maximum ax. v x vix 2a x x 0 2 2 vix 1.95 m/s 2 2x 2 ax Copyright © 2008 – The McGraw-Hill Companies s.r.l. 4 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: The train’s maximum acceleration is: a x ,max Fnet m Fbraking m 1.52 m/s 2 The maximum acceleration is not sufficient to stop the train before it hits the stalled truck. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 5 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §4.2 Visualizing Motion with Constant Acceleration Motion diagrams for three carts: Copyright © 2008 – The McGraw-Hill Companies s.r.l. 6 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Graphs of x, vx, ax for each of the three carts Copyright © 2008 – The McGraw-Hill Companies s.r.l. 7 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 4.13): A trolley car in New Orleans starts from rest at the St. Charles Street stop and has a constant acceleration of 1.20 m/s2 for 12.0 seconds. (a) Draw a graph of vx versus t. 16 14 12 v (m/sec) 10 8 6 4 2 0 0 2 4 8 6 10 12 14 t (sec) Copyright © 2008 – The McGraw-Hill Companies s.r.l. 8 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: (b) How far has the train traveled at the end of the 12.0 seconds? The area between the curve and the time axis represents the distance traveled. 1 x vt 12 sec t 2 1 14.4 m/s 12 s 86.4 m 2 (c) What is the speed of the train at the end of the 12.0 s? This can be read directly from the graph, vx=14.4 m/s. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 9 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §4.3 Free Fall A stone is dropped from the edge of a cliff; if air resistance can be ignored, the FBD for the stone is: y Apply Newton’s Second Law x w F y w mg ma a g 9.8 N/kg 9.8 m/s 2 The stone is in free fall; only the force of gravity acts on the stone. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 10 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example: You throw a ball into the air with speed 15.0 m/s; how high does the ball rise? y viy Given: viy=+15.0 m/s; ay=-9.8 m/s2 x ay To calculate the final height, we need to know the time of flight. Time of flight from: 1 y viy t a y t 2 2 v fy viy a y t Copyright © 2008 – The McGraw-Hill Companies s.r.l. 11 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: The ball rises until vfy= 0. The height: v fy viy a y t 0 viy 15.0 m/s t 1.53 sec 2 ay - 9.8 m/s 1 y viy t a y t 2 2 1 2 15.0 m/s 1.53 s 9.8 m/s 2 1.53 s 2 11.5 m Copyright © 2008 – The McGraw-Hill Companies s.r.l. 12 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 4.22): A penny is dropped from the observation deck of the Empire State Building 369 m above the ground. With what velocity does it strike the ground? Ignore air resistance. y Given: viy=0 m/s, ay=-9.8 m/s2, y=-369 m x ay Unknown: vyf 369 m Use: v fy viy 2a y y 2 2 2 a y y v yf 2a y y Copyright © 2008 – The McGraw-Hill Companies s.r.l. 13 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: v yf 2a y y 2 9.8 m/s 2 369 m 85.0 m/s (downward) How long does it take for the penny to strike the ground? Given: viy=0 m/s, ay=-9.8 m/s2, y=-369 m Unknown: t 1 1 2 y viy t a y t a y t 2 2 2 2y t 8.7 sec ay Copyright © 2008 – The McGraw-Hill Companies s.r.l. 14 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §4.4 Projectile Motion What is the motion of a struck baseball? Once it leaves the bat (if air resistance is negligible) only the force of gravity acts on the baseball. The baseball has ax = 0 and ay = -g, it moves with constant velocity along the x-axis and with nonzero, constant acceleration along the y-axis. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 15 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example: An object is projected from the origin. The initial velocity components are vix = 7.07 m/s, and viy = 7.07 m/s. Determine the x and y position of the object at 0.2 second intervals for 1.4 seconds. Also plot the results. 1 1 2 y viy t a y t a y t 2 2 2 x vix t Since the object starts from the origin, y and x will represent the location of the object at time t. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 16 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: t (sec) x (meters) y (meters) 0 0 0 0.2 1.41 1.22 0.4 2.83 2.04 0.6 4.24 2.48 0.8 5.66 2.52 1.0 7.07 2.17 1.2 8.48 1.43 1.4 9.89 0.29 Copyright © 2008 – The McGraw-Hill Companies s.r.l. 17 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: This is a plot of the x position (black points) and y position (red points) of the object as a function of time. 12 x,y (m) 10 8 6 4 2 0 0 0.5 1 1.5 t (sec) Copyright © 2008 – The McGraw-Hill Companies s.r.l. 18 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: This is a plot of the y position versus x position for the object (its trajectory). 3 2.5 y (m) 2 1.5 1 0.5 0 0 2 4 6 8 10 x (m) The object’s path is a parabola. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 19 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 4.36): An arrow is shot into the air with = 60° and vi = 20.0 m/s. (a) What are vx and vy of the arrow when t=3 sec? y The components of the initial velocity are: vix vi cos 10.0 m/s 60° x At t = 3 sec: viy vi sin 17.3 m/s v fx vix ax t vix 10.0 m/s v fy viy a y t viy gt 12.1 m/s Copyright © 2008 – The McGraw-Hill Companies s.r.l. 20 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: (b) What are the x and y components of the displacement of the arrow during the 3.0 sec interval? y r x 1 rx x vix t a x t 2 vix t 0 30.0 m 2 1 1 2 ry y viy t a y t viy t gt 2 7.80 m 2 2 Copyright © 2008 – The McGraw-Hill Companies s.r.l. 21 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example: How far does the arrow in the previous example land from where it is released? The arrow lands when y=0. Solving for t: t 2viy g 1 y viy t gt 2 0 2 3.53 sec 1 The distance traveled is: x vix t a x t 2 2 vix t 0 35.3 m Copyright © 2008 – The McGraw-Hill Companies s.r.l. 22 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §4.5 Apparent Weight Stand on a bathroom scale. FBD: y N Apply Newton’s 2nd Law: x w F y N w ma y N mg ma y Copyright © 2008 – The McGraw-Hill Companies s.r.l. 23 Fisica Generale - Alan Giambattista, Betty McCarty Richardson The normal force is the force the scale exerts on you. By Newton’s 3rd Law this is also the force (magnitude only) you exert on the scale. A scale will read the normal force. N mg a y is what the scale reads. When ay = 0, N = mg. The scale reads your true weight. When ay0, N>mg or N<mg. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 24 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example: A woman of mass 51 kg is standing in an elevator. If the elevator pushes up on her feet with 408 newtons of force, what is the acceleration of the elevator? FBD for woman: y N Apply Newton’s 2nd Law: x w F y N w ma y N mg ma y (1) Copyright © 2008 – The McGraw-Hill Companies s.r.l. 25 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: Given: N = 408 newtons, m = 51 kg, g = 9.8 m/s2 Unknown: ay Solving (1) for ay: N mg ay 1.8 m/s 2 m The elevator could be (1) traveling upward with decreasing speed, or (2) traveling downward with increasing speed. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 26 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §4.6 Air Resistance A stone is dropped from the edge of a cliff; if air resistance cannot be ignored, the FBD for the stone is: y Apply Newton’s Second Law Fd x w F y Fd w ma Where Fd is the magnitude of the drag force on the stone. This force is directed opposite the object’s velocity Copyright © 2008 – The McGraw-Hill Companies s.r.l. 27 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Assume that Fd bv 2 b is a parameter that depends on the size and shape of the object. Since Fdv2, can the object be in equilibrium? F y Fd w ma bv 2 mg 0 mg yes, when v vt b Copyright © 2008 – The McGraw-Hill Companies s.r.l. 28 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 4.61): A paratrooper with a fully loaded pack has a mass of 120 kg. The force due to air resistance has a magnitude of Fd = bv2 where b = 0.14 N s2/m2. (a) If he/she falls with a speed of 64 m/s, what is the force of air resistance? Fd bv 0.14 N s /m 64 m/s 570 N 2 2 2 2 Copyright © 2008 – The McGraw-Hill Companies s.r.l. 29 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: (b) What is the paratrooper’s acceleration? y FBD: Apply Newton’s Second Law and solve for a. Fd x F y w Fd w ma Fd mg 5.1 m/s 2 m (c) What is the paratrooper’s terminal speed? a F y Fd w ma 0 bvt2 mg 0 mg vt 92 m/s b Copyright © 2008 – The McGraw-Hill Companies s.r.l. 30 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Summary •The Kinematic Equations •Graphical Representations of Motion •Applications of Newton’s Second Law & Kinematics (free fall, projectiles, accelerated motion, air drag) •Terminal Velocity Copyright © 2008 – The McGraw-Hill Companies s.r.l. 31