# Lecture Presentation Chapter 4 Forces and Newton s Laws of Motion

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1 Lecture Presentation Chapter 4 Forces and Newton s Laws of Motion

2 Suggested Videos for Chapter 4 Prelecture Videos Newton s Laws Forces Video Tutor Solutions Force and Newton s Laws of Motion Class Videos Identifying Forces Newton s Second Law Applications Newton s Second Law Video Tutor Demos Suspended Balls: Which String Breaks? Cart with Fan and Sail Tension in String between Hanging Weights Cart with Fan and Sail Weighing a Hovering Magnet Slide 4-2

3 Suggested Simulations for Chapter 4 PhETs Forces in 1D The Ramp Slide 4-3

4 Chapter 4 Forces and Newton s Laws of Motion Chapter Goal: To establish a connection between force and motion. Slide 4-4

5 Chapter 4 Preview Looking Ahead: Forces A force is a push or a pull. It is an interaction between two objects, the agent (the woman) and the object (the car). In this chapter, you ll learn how to identify different forces, and you ll learn their properties. Slide 4-5

6 Chapter 4 Preview Looking Ahead: Forces and Motion Acceleration is caused by forces. A forward acceleration of the sled requires a forward force. A larger acceleration requires a larger force. You ll learn this connection between force and motion, part of Newton s second law. Slide 4-6

7 Chapter 4 Preview Looking Ahead: Reaction Forces The hammer exerts a downward force on the nail. Surprisingly, the nail exerts an equal force on the hammer, directed upward. You ll learn how to identify and reason with action/reaction pairs of forces. Slide 4-7

8 Chapter 4 Preview Looking Ahead Text: p. 97 Slide 4-8

9 Chapter 4 Preview Looking Back: Acceleration You learned in Chapters 2 and 3 that acceleration is a vector pointing in the direction of the change in velocity. If the velocity is changing, there is an acceleration. And so, as you ll learn in this chapter, there must be a net force. Slide 4-9

10 Chapter 4 Preview Stop to Think A swan is landing on an icy lake, sliding across the ice and gradually coming to a stop. As the swan slides, the direction of the acceleration is A. To the left. B. To the right. C. Upward. D. Downward. Slide 4-10

11 Reading Question 4.1 If you are not wearing a seat belt and the car you are driving hits a fixed barrier, you will hit the steering wheel with some force. This is because A. The force of the collision has thrown you forward. B. The steering wheel has been pushed back toward you. C. You continue moving even after the car has stopped. Slide 4-11

12 Reading Question 4.1 If you are not wearing a seat belt and the car you are driving hits a fixed barrier, you will hit the steering wheel with some force. This is because A. The force of the collision has thrown you forward. B. The steering wheel has been pushed back toward you. C. You continue moving even after the car has stopped. Slide 4-12

13 Reading Question 4.2 If you stand on a trampoline, it depresses under your weight. When you stand on a hard stone floor, A. The floor does not deform under your weight; it is too stiff. B. The floor deforms very slightly under your weight. C. The floor deforms a slight amount if you are heavy enough. Slide 4-13

14 Reading Question 4.2 If you stand on a trampoline, it depresses under your weight. When you stand on a hard stone floor, A. The floor does not deform under your weight; it is too stiff. B. The floor deforms very slightly under your weight. C. The floor deforms a slight amount if you are heavy enough. Slide 4-14

15 Reading Question 4.3 Which of these is not a force discussed in this chapter? A. The tension force B. The normal force C. The orthogonal force D. The thrust force Slide 4-15

16 Reading Question 4.3 Which of these is not a force discussed in this chapter? A. The tension force B. The normal force C. The orthogonal force D. The thrust force Slide 4-16

17 Reading Question 4.4 If you are standing on the floor, motionless, what are the forces that act on you? A. Weight force B. Weight force and normal force C. Normal force and friction force D. Weight force and tension force Slide 4-17

18 Reading Question 4.4 If you are standing on the floor, motionless, what are the forces that act on you? A. Weight force B. Weight force and normal force C. Normal force and friction force D. Weight force and tension force Slide 4-18

19 Reading Question 4.5 A skydiver has reached terminal velocity she now falls at a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction? A. There is a net force directed upward. B. There is no net force. C. There is a net force directed downward. Slide 4-19

20 Reading Question 4.5 A skydiver has reached terminal velocity she now falls at a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction? A. There is a net force directed upward. B. There is no net force. C. There is a net force directed downward. Slide 4-20

21 Reading Question 4.6 An action/reaction pair of forces A. Points in the same direction. B. Acts on the same object. C. Are always long-range forces. D. Acts on two different objects. Slide 4-21

22 Reading Question 4.6 An action/reaction pair of forces A. Points in the same direction. B. Acts on the same object. C. Are always long-range forces. D. Acts on two different objects. Slide 4-22

23 Section 4.1 Motion and Forces

24 What Causes Motion? In the absence of friction, if the sled is moving, it will stay in motion. Slide 4-24

25 What Causes Motion? Slide 4-25

26 What Is a Force? A force is a push or a pull. A force acts on an object. Every force has an agent, something that acts or pushes or pulls. Slide 4-26

27 What Is a Force? A force is a vector. The general symbol for a force is the vector symbol F. The size or strength of such a force is its magnitude F. Contact forces are forces that act on an object by touching it at a point of contact. Long-range forces are forces that act on an object without physical contact. Slide 4-27

28 Force Vectors Text: p. 100 Slide 4-28

29 Force Vectors Slide 4-29

30 Combining Forces Experiments show that when several forces F1, F2, F3, are exerted on an object, the combine to form a net force that is the vector sum of all the forces: COMP: Symbols with overarrows are MathType The net force is sometimes called the resultant force. It is not a new force. Instead, we should think of the original forces being replaced by F net. Slide 4-30

31 QuickCheck 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? A. B. C. D. Slide 4-31

32 QuickCheck 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? A. B. C. D. Vertical components cancel Slide 4-32

33 Section 4.2 A Short Catalog of Forces

34 Weight The gravitational pull of the earth on an object on or near the surface of the earth is called weight. The agent for the weight forces is the entire earth pulling on an object. An object s weight vector always points vertically downward, no matter how the object is moving. Slide 4-34

35 Spring Force Springs come in in many forms. When deflected, they push or pull with a spring force. Slide 4-35

36 Tension Force When a string or rope or wire pulls on an object, it exerts a contact force that we call the tension force. The direction of the tension force is always in the direction of the string or rope. Slide 4-36

37 Normal Force The force exerted on an object that is pressing against a surface is in a direction perpendicular to the surface. The normal force is the force exerted by a surface (the agent) against an object that is pressing against the surface. Slide 4-37

38 Normal Force The normal force is responsible for the solidness of solids. The symbol for the normal force is n. COMP: n[overarrow] is MathType. Slide 4-38

39 Friction Friction, like the normal force, is exerted by a surface. The frictional force is always parallel to the surface. Kinetic friction, denoted by f, acts as an object slides k across a surface. Kinetic friction is a force that always opposes the motion. Static friction, denoted by f s, is the force that keeps an object stuck on a surface and prevents its motion relative to the surface. Static friction points in the direction necessary to prevent motion. Slide 4-39

40 Friction Slide 4-40

41 Drag The force of a fluid (like air or water) on a moving object is called drag. Like kinetic friction, drag points opposite the direction of motion. You can neglect air resistance in all problems unless a problem explicitly asks you to include it. Slide 4-41

42 Thrust Thrust is a force that occurs when a jet or rocket engine expels gas molecules at high speed. Thrust is a force opposite the direction in which the exhaust gas is expelled. Slide 4-42

43 Electric and Magnetic Forces Electricity and magnetism, like gravity, exert long-range forces. The forces of electricity and magnetism act on charged particles. These forces and the forces inside the nucleus won t be important for the dynamics problems we consider in the next several chapters. Slide 4-43

44 QuickCheck 4.2 A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp? A. The weight of the ball acting vertically down. B. A horizontal force that maintains the motion. C. A force whose direction changes as the direction of motion changes. D. The weight of the ball and a horizontal force. E. The weight of the ball and a force in the direction of motion. Slide 4-44

45 QuickCheck 4.2 A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp? A. The weight of the ball acting vertically down. B. A horizontal force that maintains the motion. C. A force whose direction changes as the direction of motion changes. D. The weight of the ball and a horizontal force. E. The weight of the ball and a force in the direction of motion. Slide 4-45

46 QuickCheck 4.3 A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam? A. Gravity B. Gravity and tension in the cable C. Gravity and a force of motion D. Gravity and tension and a force of motion Slide 4-46

47 QuickCheck 4.3 A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam? A. Gravity B. Gravity and tension in the cable C. Gravity and a force of motion D. Gravity and tension and a force of motion Slide 4-47

48 QuickCheck 4.4 A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she s jumped in? A. Gravity and kinetic friction B. Gravity and a normal force C. Gravity and the force of the push D. Gravity, a normal force, and kinetic friction E. Gravity, a normal force, kinetic friction, and the force of the push Slide 4-48

49 QuickCheck 4.4 A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she s jumped in? A. Gravity and kinetic friction B. Gravity and a normal force C. Gravity and the force of the push D. Gravity, a normal force, and kinetic friction E. Gravity, a normal force, kinetic friction, and the force of the push Slide 4-49

50 Section 4.3 Identifying Forces

51 Identifying Forces Text: p. 105 Slide 4-51

52 Identifying Forces Slide 4-52

53 Conceptual Example 4.1: Identifying forces on a bungee jumper A bungee jumper has leapt off a bridge and is nearing the bottom of her fall. What forces are being exerted on the bungee jumper? Slide 4-53

54 Conceptual Example 4.2: Identifying forces on a skier A skier is being towed up a snow-covered hill by a tow rope. What forces are being exerted on the skier? Slide 4-54

55 Section 4.4 What Do Forces Do?

56 What Do Forces Do? How does an object move when a force is exerted on it? Slide 4-56

57 What Do Forces Do? As the block starts to move, in order to keep the pulling force constant you must move your hand in just the right way to keep the length of the rubber band and thus the force constant. Slide 4-57

58 What Do Forces Do? The experimental findings of the motion of objects acted on by constant forces are: An object pulled with a constant force moves with a constant acceleration. Acceleration is directly proportional to force. Acceleration is inversely proportional to an object s mass. Slide 4-58

59 QuickCheck 4.5 A cart is pulled to the right with a constant, steady force. How will its acceleration graph look? A. B. C. Slide 4-59

60 QuickCheck 4.5 A cart is pulled to the right with a constant, steady force. How will its acceleration graph look? A. B. C. A constant force produces a constant acceleration. Slide 4-60

61 Example 4.4 Finding the mass of an unknown block When a rubber band is stretched to pull on a 1.0 kg block with a constant force, the acceleration of the block is measured to be 3.0 m/s 2. When a block with an unknown mass is pulled with the same rubber band, using the same force, its acceleration is 5.0 m/s 2. What is the mass of the unknown block? PREPARE Each block s acceleration is inversely proportional to its mass. Slide 4-61

62 Example 4.4 Finding the mass of an unknown block (cont.) SOLVE We can use the result of the Inversely Proportional Relationships box to write ASSESS With the same force applied, the unknown block had a larger acceleration than the 1.0 kg block. It makes sense, then, that its mass its resistance to acceleration is less than 1.0 kg. Slide 4-62

63 Section 4.5 Newton s Second Law

64 Newton s Second Law A force causes an object to accelerate. The acceleration a is directly proportional to the force F and inversely proportional to the mass m: The direction of the acceleration is the same as the direction of the force: Slide 4-64

65 Newton s Second Law Slide 4-65

66 QuickCheck 4.6 A constant force causes an object to accelerate at 4 m/s 2. What is the acceleration of an object with twice the mass that experiences the same force? A. 1 m/s 2 B. 2 m/s 2 C. 4 m/s 2 D. 8 m/s 2 E. 16 m/s 2 Slide 4-66

67 QuickCheck 4.6 A constant force causes an object to accelerate at 4 m/s 2. What is the acceleration of an object with twice the mass that experiences the same force? A. 1 m/s 2 B. 2 m/s 2 C. 4 m/s 2 D. 8 m/s 2 E. 16 m/s 2 Slide 4-67

68 QuickCheck 4.7 An object, when pushed with a net force F, has an acceleration of 2 m/s 2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be A. ½ m/s 2 B. 1 m/s 2 C. 2 m/s 2 D. 4 m/s 2 Slide 4-68

69 QuickCheck 4.7 An object, when pushed with a net force F, has an acceleration of 2 m/s 2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be A. ½ m/s 2 B. 1 m/s 2 C. 2 m/s 2 D. 4 m/s 2 Slide 4-69

70 Conceptual Example 4.5 Acceleration of a wind-blown basketball A basketball is released from rest in a stiff breeze directed to the right. In what direction does the ball accelerate? Slide 4-70

71 Conceptual Example 4.5 Acceleration of a wind-blown basketball (cont.) REASON Wind is just air in motion. If the air is moving to the right with respect to the ball, then the ball is moving to the left with respect to the air. There will be a drag force opposite the velocity of the ball relative to the air, to the right. So, as FIGURE 4.22a shows, two forces are acting on the ball: its weight ww directed downward and the drag force directed to the right. Newton s second law tells us that the direction of the acceleration is the same as the direction of the net force. In FIGURE 4.22b we find by graphical vector addition of and. We see that and therefore point downward and to the right. ASSESS This makes sense on the basis of your experience. Weight pulls the ball down, and the wind pushes the ball to the right. The net result is an acceleration down and to the right. Slide 4-71

72 QuickCheck 4.8 A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the net force greater? A. The train B. The skier C. The net force is the same on both. D. There s not enough information to tell. Slide 4-72

73 QuickCheck 4.8 A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the net force greater? A. The train B. The skier C. The net force is the same on both. D. There s not enough information to tell. Slide 4-73

74 QuickCheck 4.9 An object on a rope is lowered at constant speed. Which is true? A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. Slide 4-74

75 QuickCheck 4.9 An object on a rope is lowered at constant speed. Which is true? Constant velocity Zero acceleration A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. Slide 4-75

76 QuickCheck 4.10 An object on a rope is lowered at a steadily decreasing speed. Which is true? A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. Slide 4-76

77 QuickCheck 4.10 An object on a rope is lowered at a steadily decreasing speed. Which is true? Decreasing downward velocity Acceleration vector points up points up A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. Slide 4-77

78 Units of Force The basic unit of force is called a newton. One newton is the force that causes a 1 kg mass to accelerate at 1 m/s 2. 1 pound = 1 lb = 4.45 N Slide 4-78

79 Example 4.6 Racing down the runway A Boeing 737 a small, short-range jet with a mass of 51,000 kg sits at rest. The pilot turns the pair of jet engines to full throttle, and the thrust accelerates the plane down the runway. After traveling 940 m, the plane reaches its takeoff speed of 70 m/s and leaves the ground. What is the thrust of each engine? Slide 4-79

80 Example 4.6 Racing down the runway (cont.) PREPARE If we assume that the plane undergoes a constant acceleration (a reasonable assumption), we can use kinematics to find the magnitude of that acceleration. Then we can use Newton s second law to find the force the thrust that produced this acceleration. FIGURE 4.23 is a visual overview of the airplane s motion. Slide 4-80

81 Example 4.6 Racing down the runway (cont.) SOLVE We don t know how much time it took the plane to reach its takeoff speed, but we do know that it traveled a distance of 940 m. We can solve for the acceleration by using the third constant-acceleration equation in Synthesis 2.1: (v x ) f2 = (v x ) i2 + 2a x Δx The displacement is Δx = x f x i = 940 m, and the initial velocity is 0. We can rearrange the equation to solve for the acceleration: Slide 4-81

82 Example 4.6 Racing down the runway (cont.) We ve kept an extra significant figure because this isn t our final result we are asked to solve for the thrust. We complete the solution by using Newton s second law: F = ma x = (51,000 kg)(2.61 m/s 2 ) = 133,000 N The thrust of each engine is half of this total force: Thrust of one engine = 67,000 N = 67 kn Slide 4-82

83 Example 4.6 Racing down the runway (cont.) ASSESS An acceleration of about ¼g seems reasonable for an airplane: It s zippy, but it s not a thrill ride. And the final value we find for the thrust of each engine is close to the value given in Table 4.2. This gives us confidence that our final result makes good physical sense. Slide 4-83

84 Section 4.6 Free-Body Diagrams

85 Free-Body Diagrams Text: p. 112 Slide 4-85

86 QuickCheck 4.11 An elevator, lifted by a cable, is moving upward and slowing. Which is the correct free-body diagram? A. B. C. D. E. Slide 4-86

87 QuickCheck 4.11 An elevator, lifted by a cable, is moving upward and slowing. Which is the correct free-body diagram? A. B. C. D. E. Slide 4-87

88 QuickCheck 4.12 A ball has been tossed straight up. Which is the correct freebody diagram just after the ball has left the hand? Ignore air resistance. A. B. C. D. Slide 4-88

89 QuickCheck 4.12 A ball has been tossed straight up. Which is the correct freebody diagram just after the ball has left the hand? Ignore air resistance. A. B. C. D. No points of contact. Gravity is the only force. Slide 4-89

90 QuickCheck 4.13 A ball, hanging from the ceiling by a string, is pulled back and released. Which is the correct free-body diagram just after its release? A. B. C. D. E. Slide 4-90

91 QuickCheck 4.13 A ball, hanging from the ceiling by a string, is pulled back and released. Which is the correct free-body diagram just after its release? A. B. C. D. E. Slide 4-91

92 QuickCheck 4.14 A car is parked on a hill. Which is the correct free-body diagram? Slide 4-92

93 QuickCheck 4.14 A car is parked on a hill. Which is the correct free-body diagram? C. Slide 4-93

94 QuickCheck 4.15 A car is towed to the right at constant speed. Which is the correct free-body diagram? Slide 4-94

95 QuickCheck 4.15 A car is towed to the right at constant speed. Which is the correct free-body diagram? D. Slide 4-95

96 Example 4.7 Forces on an elevator An elevator, suspended by a cable, speeds up as it moves upward from the ground floor. Draw a free-body diagram of the elevator. Slide 4-96

97 Example 4.7 Forces on an elevator (cont.) PREPARE The elevator is moving upward, and its speed is increasing. This means that the acceleration is directed upward that s enough to say about acceleration for the purposes of this problem. Next, we continue with the forces. FIGURE 4.24 illustrates the steps listed in Tactics Box 4.3. We know that the acceleration is directed upward, so must be directed upward as well. Slide 4-97

98 Example 4.7 Forces on an elevator (cont.) ASSESS Let s take a look at our picture and see if it makes sense. The coordinate axes, with a vertical y-axis, are the ones we would use in a pictorial representation of the motion, so we ve chosen the correct axes. is directed upward. For this to be true, the magnitude of must be greater than the magnitude of, which is just what we ve drawn. Slide 4-98

99 Example 4.9 Forces on a towed skier A tow rope pulls a skier up a snow-covered hill at a constant speed. Draw a full visual overview of the skier. Slide 4-99

100 Example Problem Consider pushing a block across the table at a steady speed. Since you re exerting a force on it, why isn t it accelerating? Identify all the forces and draw a free-body diagram. Compare the size of the pushing force and the size of the friction force. Slide 4-100

101 Section 4.7 Newton s Third Law

102 Newton s Third Law Motion often involves two or more objects interacting with each other. As the hammer hits the nail, the nail pushes back on the hammer. A bat and a ball, your foot and a soccer ball, and the earth-moon system are other examples of interacting objects. Slide 4-102

103 Interacting Objects An interaction is the mutual influence of two objects on each other. The pair of forces shown in the figure is called an action/reaction pair. An action/reaction pair of forces exists as a pair, or not at all. Slide 4-103

104 Reasoning with Newton s Third Law Slide 4-104

105 Runners and Rockets In order for you to walk, the floor needs to have friction so that your foot sticks to the floor as you straighten your leg, moving your body forward. The friction that prevents slipping is static friction. The static friction has to point in the forward direction to prevent your foot from slipping. Slide 4-105

106 Runners and Rockets The rocket pushes hot gases out the back, and this results in a forward force (thrust) on the rocket. Slide 4-106

107 QuickCheck year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is A. Greater than the force of Sarah on Jack. B. Equal to the force of Sarah on Jack. C. Less than the force of Sarah on Jack. Slide 4-107

108 QuickCheck year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is A. Greater than the force of Sarah on Jack. B. Equal to the force of Sarah on Jack. C. Less than the force of Sarah on Jack. Slide 4-108

109 QuickCheck 4.17 A mosquito runs head-on into a truck. Splat! Which is true during the collision? A. The mosquito exerts more force on the truck than the truck exerts on the mosquito. B. The truck exerts more force on the mosquito than the mosquito exerts on the truck. C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito. D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck. E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito. Slide 4-109

110 QuickCheck 4.17 A mosquito runs head-on into a truck. Splat! Which is true during the collision? A. The mosquito exerts more force on the truck than the truck exerts on the mosquito. B. The truck exerts more force on the mosquito than the mosquito exerts on the truck. C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito. D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck. E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito. Slide 4-110

111 Summary: General Principles Text: p. 118 Slide 4-111

112 Summary: General Principles Text: p. 118 Slide 4-112

113 Summary: General Principles Text: p. 118 Slide 4-113

114 Summary: Important Concepts Text: p. 118 Slide 4-114

115 Summary: Important Concepts Text: p. 118 Slide 4-115

116 Summary: Important Concepts Text: p. 118 Slide 4-116

117 Summary: Applications Text: p. 118 Slide 4-117

118 Summary: Applications Text: p. 118 Slide 4-118

119 Summary Text: p. 118 Slide 4-119

120 Summary Text: p. 118 Slide 4-120

121 Summary Text: p. 118 Slide 4-121

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