# Chapter 4 - Forces and Newton s Laws of Motion w./ QuickCheck Questions

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1 Chapter 4 - Forces and Newton s Laws of Motion w./ QuickCheck Questions 2015 Pearson Education, Inc. Anastasia Ierides Department of Physics and Astronomy University of New Mexico September 8, 2015

2 Review of Last Time Looking at 1-D (x or y) and 2-D (x and y) motion diagrams, position, velocity, & acceleration graphs Equations of motion in 1-D and 2-D, Projectile Motion Vectors, Free Fall, Coordinate System, & Motion on a ramp Relative Velocity, Circular Motion

3 QuickCheck Question 3.15 A factory conveyor belt rolls at 3 m/s. A mouse sees a piece of cheese directly across the belt and heads straight for the cheese at 4 m/ s. What is the mouse s speed relative to the factory floor? A. 1 m/s B. 2 m/s C. 3 m/s D. 4 m/s E. 5 m/s

4 QuickCheck Question 3.15 A factory conveyor belt rolls at 3 m/s. A mouse sees a piece of cheese directly across the belt and heads straight for the cheese at 4 m/ s. What is the mouse s speed relative to the factory floor? Break the problem up into the relative parts! A. 1 m/s B. 2 m/s C. 3 m/s D. 4 m/s E. 5 m/s

5 QuickCheck Question 3.16 A heavy red ball is released from rest 2.0 m above a flat, horizontal surface. At exactly the same instant, a yellow ball with the same mass is fired horizontally at 3.0 m/s. Which ball hits the ground first? 2.0 m v = 3.0 m/s A. The red ball hits first. B. The yellow ball hits first. C. They hit at the same time.

6 QuickCheck Question 3.16 A heavy red ball is released from rest 2.0 m above a flat, horizontal surface. At exactly the same instant, a yellow ball with the same mass is fired horizontally at 3.0 m/s. Which ball hits the ground first? 2.0 m v = 3.0 m/s A. The red ball hits first. B. The yellow ball hits first. C. They hit at the same time.

7 Projectile Acceleration Ignoring air resistance, vertical acceleration, ay, is the same at all points Why? - Free Fall ay = afree fall = -g = -9.8 m/s 2 Horizontal (x-) component, ax = 0 m/s 2

8 QuickCheck Question 3.17 A 100-g ball rolls off a table and lands 2.0 m from the base of the table. A 200-g ball rolls off the same table with the same speed. It lands at distance A. 1.0 m B. Between 1 m and 2 m C. 2.0 m D. Between 2 m and 4 m E. 4.0 m

9 QuickCheck Question 3.17 A 100-g ball rolls off a table and lands 2.0 m from the base of the table. A 200-g ball rolls off the same table with the same speed. It lands at distance A. 1.0 m B. Between 1 m and 2 m C. 2.0 m D. Between 2 m and 4 m E. 4.0 m Remember no acceleration in the x direction only in the y.

10 Analyzing Projectile Motion The ball finishes its motion moving downward at the same speed as it started moving upward Two independent motions: uniform motion at constant velocity in the x- direction and free-fall motion in the y-direction

11 QuickCheck Question 3.18 Projectiles 1 and 2 are launched over level ground with the same speed but at different angles. Which hits the ground first? Ignore air resistance. A. Projectile 1 hits first. B. Projectile 2 hits first. C. They hit at the same time. D. There s not enough information to tell.

12 QuickCheck Question 3.18 Projectiles 1 and 2 are launched over level ground with the same speed but at different angles. Which hits the ground first? Ignore air resistance. Remember: The time in the air is determined by the vertical component of the velocity A. Projectile 1 hits first. B. Projectile 2 hits first. C. They hit at the same time. D. There s not enough information to tell.

13 What have we learned? Acceleration is a vector that points in the direction of change in velocity; if there is a change in velocity, there is an acceleration

14 What is the Cause of Motion? Acceleration is a vector that points in the direction of change in velocity; if there is a change in velocity, there is an acceleration What causes this change in velocity or acceleration?

15 What Causes Motion?

16 What Causes Motion? When you (agent) push on an object to cause it to move from rest (accelerate) what are you doing?

17 What Causes Motion? When you (agent) push on an object to cause it to move from rest (accelerate) what are you doing? You are applying a FORCE!

18 What Causes Motion? When you (agent) push on an object to cause it to move from rest (accelerate) what are you doing? You are applying a FORCE! A push or pull, an action on an object, is a FORCE

19 Newton s First Law A push or pull, an action on an object, is a FORCE

20 Newton s First Law A push or pull, an action on an object, is a FORCE A force acts on an object

21 Newton s First Law A push or pull, an action on an object, is a FORCE A force acts on an object An agent imparts that force on the object

22 What is FORCE? Forces cause acceleration

23 What is FORCE? Forces cause acceleration A force is a vector, denoted by the symbol F, whose magnitude (size) is F.

24 What is FORCE? Forces cause acceleration A force is a vector, denoted by the symbol F, whose magnitude (size) is F. Contact forces are forces that act on an object by touching it at a point of contact

25 What is FORCE? Forces cause acceleration A force is a vector, denoted by the symbol F, whose magnitude (size) is 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

26 Contact Forces

27 Contact Forces

28 Contact Forces

29 Contact Forces

30 Long Range Forces

31 Long Range Forces

32 Force Vectors

33 More than One Force? Many objects in motion experience more than one force acting on them

34 More than One Force? Many objects in motion experience more than one force acting on them The sum of all the forces acting on an object is the resultant or net force

35 More than One Force? Many objects in motion experience more than one force acting on them The sum of all the forces acting on an object is the resultant or net force F NET = F = F 1 + F 2 + F 3 +

36 QuickCheck Question 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force?

37 QuickCheck Question 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? We know that F NET points to the left

38 QuickCheck Question 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? We also know that the sum of F 1 and F 2 points to the top right F r F NET

39 QuickCheck Question 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? Subtracting Fr from F NET will give us the third force F r F NET

40 QuickCheck Question 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? Adding -Fr and F NET together we see that F 3 has to be F r F NET -F r

41 QuickCheck Question 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? Adding -Fr and F NET together we see that F 3 has to be F r F NET -F r

42 Where Are Forces Found? There is a plethora of forces out there

43 Where Are Forces Found? There is a plethora of forces out there You just have to know where to look!

44 Where Are Forces Found? There is a plethora of forces out there You just have to know where to look! Think of your weight or falling down

45 Where Are Forces Found? There is a plethora of forces out there You just have to know where to look! Think of your weight or falling down Rolling or skiing down a hill

46 Where Are Forces Found? There is a plethora of forces out there You just have to know where to look! Think of your weight or falling down Rolling or skiing down a hill Jumping on a trampoline or your car s shocks

47 Types of Forces

48 Types of Forces - Weight The gravitational pull of the earth on an object on or near the surface of the earth

49 Types of Forces - Weight The gravitational pull of the earth on an object on or near the surface of the earth What is the agent?

50 Types of Forces - Weight The gravitational pull of the earth on an object on or near the surface of the earth What is the agent? - Earth (pulling down)

51 Types of Forces - Weight The gravitational pull of the earth on an object on or near the surface of the earth What is the agent? - Earth (pulling down) An object s weight vector always points vertically downward

52 Types of Forces - Spring

53 Types of Forces - Spring These come in many forms (springs, trampoline, etc.)

54 Types of Forces - Spring These come in many forms (springs, trampoline, etc.) They push or pull when pushed or pulled

55 Types of Forces - Tension

56 Types of Forces - Tension Contact force exerted when pulling a string, rope, or wire

57 Types of Forces - Tension Contact force exerted when pulling a string, rope, or wire The direction of this force is in the direction the string, rope, or wire is pulled

58 Types of Forces - Normal Force

59 Types of Forces - Normal Force The force exerted by a surface (the agent) against an object that is pressing against it

60 Types of Forces - Normal Force The force exerted by a surface (the agent) against an object that is pressing against it This forces is always perpendicular to the surface

61 Types of Forces - Normal Force The force exerted by a surface (the agent) against an object that is pressing against it This forces is always perpendicular to the surface Even on inclined plane

62 Types of Forces - Normal Force Responsible for solidness of solids

63 Types of Forces - Normal Force Responsible for solidness of solids Denoted by the symbol n n

64 Types of Forces - Friction

65 Types of Forces - Friction Like Normal Force, exerted by a surface

66 Types of Forces - Friction Like Normal Force, exerted by a surface Unlike Normal Force, parallel to the surface

67 Types of Forces - Friction Static Friction, f s, keeps an object from moving

68 Types of Forces - Friction Static Friction, f s, keeps an object from moving Kinetic Friction, f k, acts as an object moves along the surface

69 Types of Forces - Drag The force of a fluid (like air or water) on a moving object

70 Types of Forces - Drag The force of a fluid (like air or water) on a moving object Points opposite the direction of motion

71 Types of Forces - Drag The force of a fluid (like air or water) on a moving object Points opposite the direction of motion Neglect air resistance in all problems except when told to include it

72 Types of Forces - Thrust

73 Types of Forces - Thrust Occurs when a jet or rocket engine expels gas molecules at high speed

74 Types of Forces - Thrust Occurs when a jet or rocket engine expels gas molecules at high speed Directed opposite the direction of the exhaust gas

75 QuickCheck Question 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.

76 QuickCheck Question 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.

77 QuickCheck Question 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

78 QuickCheck Question 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

79 QuickCheck Question 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

80 QuickCheck Question 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

81 Identifying Forces

82 What Forces Are There?

83 Identifying Forces

84 Forces and Motion What do forces do?

85 Forces and Motion What do forces do? - They cause an object to move, accelerate

86 Forces and Motion What do forces do? - They cause an object to move, accelerate An object pulled with a constant force experiences a constant acceleration (both pointing in the same direction)

87 Forces and Motion What do forces do? - They cause an object to move, accelerate An object pulled with a constant force experiences a constant acceleration (both pointing in the same direction) Acceleration is directly proportional to the force (F a)

88 Forces and Motion What do forces do? - They cause an object to move, accelerate An object pulled with a constant force experiences a constant acceleration (both pointing in the same direction) Acceleration is directly proportional to the force (F a) Acceleration is inversely proportional to an object s mass (a 1/m)

89 Forces and Motion

90 Forces and Motion 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.

91 QuickCheck Question 4.5 A cart is pulled to the right with a constant, steady force. How will its acceleration graph look?

92 QuickCheck Question 4.5 A cart is pulled to the right with a constant, steady force. How will its acceleration graph look? Remember: A constant force means a constant acceleration!

93 QuickCheck Question 4.5 A cart is pulled to the right with a constant, steady force. How will its acceleration graph look? Remember: A constant force means a constant acceleration!

94 Example 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?

95 Example 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. Meaning a 1/m

96 Example Finding the mass of an unknown block SOLVE We can use the result of the Inversely Proportional Relationships box to write

97 Example Finding the mass of an unknown block SOLVE We can use the result of the Inversely Proportional Relationships box to write

98 Example Finding the mass of an unknown block 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.

99 Newton s Second Law This is the same as writing Fnet = m a

100 QuickCheck Question 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

101 QuickCheck Question 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 Remember: Acceleration is inversely proportional to mass a 1/m

102 QuickCheck Question 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 Remember: Acceleration is inversely proportional to mass a 1/m If the new mass is twice as big as the old one, m new = 2 m old

103 QuickCheck Question 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 Remember: Acceleration is inversely proportional to mass a 1/m If the new mass is twice as big as the old one, m new = 2 m old a new = (m old /m new ) a old = 1/2 a old

104 QuickCheck Question 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 Remember: Acceleration is inversely proportional to mass a 1/m If the new mass is twice as big as the old one, m new = 2 m old a new = (m old /m new ) a old = 1/2 a old

105 QuickCheck Question 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

106 QuickCheck Question 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 Remember: Acceleration is directly proportional to force and inversely proportional to mass a = F/m

107 QuickCheck Question 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 Remember: Acceleration is directly proportional to force and inversely proportional to mass a = F/m If the new force is twice as big as the old one, F new = 2 F old and the new mass is 4 times as big as the old one, m new = 4 m old, then

108 QuickCheck Question 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 Remember: Acceleration is directly proportional to force and inversely proportional to mass a = F/m If the new force is twice as big as the old one, F new = 2 F old and the new mass is 4 times as big as the old one, m new = 4 m old, then a new = F new /m new = (2 F old )/(4 m old ) = 2/4 F old /m old = 2/4 a old = (1/2)(2 m/s 2 ) = 1 m/s 2

109 QuickCheck Question 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 Remember: Acceleration is directly proportional to force and inversely proportional to mass a = F/m If the new force is twice as big as the old one, F new = 2 F old and the new mass is 4 times as big as the old one, m new = 4 m old, then a new = F new /m new = (2 F old )/(4 m old ) = 2/4 F old /m old = 2/4 a old = (1/2)(2 m/s 2 ) = 1 m/s 2

110 QuickCheck Question 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.

111 QuickCheck Question 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.

112 QuickCheck Question 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.

113 QuickCheck Question 4.9 An object on a rope is lowered at constant speed. Which is true? Remember: Constant velocity means 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.

114 QuickCheck Question 4.9 An object on a rope is lowered at constant speed. Which is true? Remember: Constant velocity means 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.

115 QuickCheck Question 4.9 An object on a rope is lowered at constant speed. Which is true? Remember: Constant velocity means 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.

116 QuickCheck Question 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.

117 QuickCheck Question 4.10 An object on a rope is lowered at a steadily decreasing speed. Which is true? Remember: Decreasing downward velocity means acceleration vector 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.

118 QuickCheck Question 4.10 An object on a rope is lowered at a steadily decreasing speed. Which is true? Remember: Decreasing downward velocity means acceleration vector 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.

119 QuickCheck Question 4.10 An object on a rope is lowered at a steadily decreasing speed. Which is true? Remember: Decreasing downward velocity means acceleration vector points up so the net force is also pointing 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.

120 QuickCheck Question 4.10 An object on a rope is lowered at a steadily decreasing speed. Which is true? Remember: Decreasing downward velocity means acceleration vector points up so the net force is also pointing 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.

121 Units of Force The basic unit of force, in SI, is called a Newton (N) 1 N = 1 kg m/s 2 One newton causes 1 kg mass to accelerate at 1 m/ s 2 The unit for force in the English system is a pound (lb) 1 pound = 1 lb = 4.45 N

122 Summary

123 Summary

124 Summary

125 Summary

126 Things that are due Homework #3 Due September 14, 2015 by 11:59 pm Reading Quiz #4 Due September 15, 2015 by 4:59 pm

127 EXAM #1 Covers Chapters 1-3 & Lectures 2-6 Thursday, September 10, 2015 Bring a calculator and cheat sheet (turn in with exam, one side of 8.5 x11 piece of paper) Practice exam is available on website along with solutions

128 QUESTIONS?

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