1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Conceptual Physics Fundamentals Chapter 3: EQUILIBRIUM AND LINEAR MOTION 1

2. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley This lecture will help you understand:  Aristotle on Motion  Galileo’s Concept of Inertia  Mass—A Measure of Inertia  Net Force  The Equilibrium Rule  Equilibrium of Moving Things  The Force of Friction  Speed and Velocity  Acceleration 2

3. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Equilibrium and Linear Motion “When you’re over the hill, that’s when you pick up speed.” —Quincy Jones 3

4. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Aristotle on Motion Aristotle’s classification of motion  natural motion o Every object in the universe has a proper place determined by a combination of four elements: earth, water, air, and fire o Any object not in its proper place will strive to get there. Example: stones fall; puffs of smoke rise 4

5. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Aristotle on Motion  natural motion (continued) o Straight up or straight down for all things on Earth o Beyond Earth, motion is circular. Example: Sun and moon continually circle the Earth.  violent motion o produced by external pushes or pulls on objects Example: Wind imposes motion on ships. 5

6. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Galileo’s Concept of Inertia Italian scientist Galileo demolished Aristotle’s assertions in early 1500s. Galileo’s discovery:  Objects of different weight fall to the ground at the same time in the absence of air resistance.  A moving object needs no force to keep it moving in the absence of friction. 6

7. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Galileo’s Concept of Inertia Force  is a push or a pull Inertia  is a property of matter to resist changes in motion  depends on the amount of matter in an object (its mass) 7

8. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The use of inclined planes for Galileo’s experiments helped him to _______. A.eliminate the acceleration of free fall B.discover the concept of energy C.discover the property called inertia D.discover the concept of momentum Galileo’s Concept of Inertia CHECK YOUR NEIGHBOR 8

9. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The use of inclined planes for Galileo’s experiments helped him to _______. A.eliminate the acceleration of free fall B.discover the concept of energy C.discover the property called inertia D.discover the concept of momentum Comment: Note that inertia is a property of matter, not a reason for the behavior of matter. Galileo’s Concept of Inertia CHECK YOUR ANSWER 9

10. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mass—A Measure of Inertia Mass  a measure of the inertia of a material object  independent of gravity  greater inertia  greater mass  unit of measurement is the kilogram (kg) Weight  the force on an object due to gravity  scientific unit of force is the Newton (N)  unit is also the pound (lb) 10

11. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The concept of inertia mostly involves _______. A.mass B.weight C.volume D.density Mass—A Measure of Inertia CHECK YOUR NEIGHBOR 11

12. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The concept of inertia mostly involves _______. A.mass B.weight C.volume D.density Comment: Anybody get this wrong? Check the title of this slide! :-) Mass—A Measure of Inertia CHECK YOUR ANSWER 12

13. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley If the mass of an object is halved, the weight of the object is _______. A.halved B.doubled C.depends on location D.none of the above Mass—A Measure of Inertia CHECK YOUR NEIGHBOR 13

14. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley If the mass of an object is halved, the weight of the object is _______. A.halved B.doubled C.depends on location D.none of the above Mass—A Measure of Inertia CHECK YOUR ANSWER 14

15. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mass—A Measure of Inertia Mass and weight in everyday conversation are interchangeable. Mass, however, is different and more fundamental than weight. Mass versus weight  On Moon and Earth o Weight of an object on the Moon is less than on the Earth. o Mass of an object is the same in both locations. 15

16. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mass—A Measure of Inertia One Kilogram Weighs 9.8 Newtons. Relationship between kilograms and pounds  1 kg = 2.2 lb = 9.8 N at Earth’s surface  1 lb = 4.45 N 16

17. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When the string is pulled down slowly, the top string breaks, which best illustrates the _______. A.weight of the ball B.mass of the ball C.volume of the ball D.density of the ball Mass—A Measure of Inertia CHECK YOUR NEIGHBOR 17

18. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When the string is pulled down slowly, the top string breaks, which best illustrates the _______. A.weight of the ball B.mass of the ball C.volume of the ball D.density of the ball Explanation: Tension in the top string is the pulling tension plus the weight of the ball; both of which break the top string. Mass—A Measure of Inertia CHECK YOUR ANSWER 18

19. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When the string is pulled down quickly, the bottom string breaks, which best illustrates the _______. A.weight of the ball B.mass of the ball C.volume of the ball D.density of the ball Mass—A Measure of Inertia CHECK YOUR NEIGHBOR 19

20. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When the string is pulled down quickly, the bottom string breaks, which best illustrates the _______. A.weight of the ball B.mass of the ball C.volume of the ball D.density of the ball Explanation: It is the “laziness” of the ball that keeps it at rest, resulting in the breaking of the bottom string. Mass—A Measure of Inertia CHECK YOUR ANSWER 20

21. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Net Force Net force is the combination of all forces that change an object’s state of motion. Example: If you pull on a box with 10 N and a friend pulls oppositely with 5 N, the net force is 5 N in the direction you are pulling. 21

22. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is _______. A.5 N to the left B.5 N to the right C.25 N to the left D.25 N to the right Net Force CHECK YOUR NEIGHBOR 22

23. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is _______. A.5 N to the left B.5 N to the right C.25 N to the left D.25 N to the right Net Force CHECK YOUR ANSWER 23

24. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Net Force Vector quantity  a quantity whose description requires both magnitude (how much) and direction (which way)  can be represented by arrows drawn to scale, called vectors o Length of arrow represents magnitude, and arrowhead shows direction Examples: force, velocity, acceleration 24

25. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Equilibrium Rule The equilibrium rule  the vector sum of forces acting on a non- accelerating object equals zero  in equation form:  F = 0 25

26. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Equilibrium Rule Example: A string holding up a bag of flour Two forces act on the bag of flour:  tension force acts upward  weight acts downward The forces are equal in magnitude and opposite in direction; when the forces are added they cancel to zero and the bag of flour remains at rest. 26

27. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The equilibrium rule,  F = 0, applies to _______. A.vector quantities B.scalar quantities C.both of the above D.neither of the above The Equilibrium Rule CHECK YOUR NEIGHBOR 27

28. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The equilibrium rule,  F = 0, applies to _______. A.vector quantities B.scalar quantities C.both of the above D.neither of the above Explanation: Vector addition takes into account + and - quantities that can cancel to zero. Two forces (vectors) can add to zero, but there is no way that two masses (scalars) can add to zero. The Equilibrium Rule CHECK YOUR ANSWER 28

29. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Support Force Support force (normal force) is an upward force on an object that is opposite to the force of gravity. Example: A book on a table compresses atoms in the table, and the compressed atoms produce the support force. 29

30. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you stand on two bathroom scales with one foot on each scale, and with your weight evenly distributed, each scale will read _______. A.your weight B.half your weight C.zero D.more than your weight The Support Force CHECK YOUR NEIGHBOR 30

31. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you stand on two bathroom scales, with one foot on each scale and with your weight evenly distributed, each scale will read _______. A.your weight B.half your weight C.zero D.more than your weight Explanation: You are at rest on the scales, so  F = 0. The sum of the two upward support forces is equal to your weight. The Support Force CHECK YOUR ANSWER 31

32. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Equilibrium of Moving Things Equilibrium  a state of no change with no net force acting o static equilibrium Example: Hockey puck at rest on slippery ice o dynamic equilibrium Example: Hockey puck sliding at constant speed on slippery ice 32

33. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Equilibrium of Moving Things Equilibrium test  whether something undergoes changes in motion Example: A refrigerator at rest is in static equilibrium. If it is moved at a steady speed across a floor, it is in dynamic equilibrium. 33

34. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A bowling ball is in equilibrium when it _______. A.is at rest B.moves steadily in a straight-line path C.Both of the above D.None of the above Equilibrium of Moving Things CHECK YOUR NEIGHBOR 34

35. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A bowling ball is in equilibrium when it _______. A.is at rest B.moves steadily in a straight-line path C.Both of the above D.None of the above Equilibrium of Moving Things CHECK YOUR ANSWER 35

36. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Force of Friction Friction  occurs when objects rub against one another  applies to solids, liquids, and gases  acts in a direction to oppose motion Example: When an object falls down through air, the force of friction (air resistance) acts upward. 36

37. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Force of Friction  depends on the kinds of material and how much they are pressed together  is due to tiny surface bumps and to “stickiness” of the atoms on a material’s surface Example: Friction between a crate on a smooth wooden floor is less than that on a rough floor. 37

38. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The force of friction can occur _______. A.with sliding objects B.in water C.in air D.All of the above The Force of Friction CHECK YOUR NEIGHBOR 38

39. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The force of friction can occur _______. A.with sliding objects B.in water C.in air D.All of the above Comment: Friction can also occur for objects at rest. If you push horizontally on your book and it doesn’t move, then friction between the book and the table is equal and opposite to your push. The Force of Friction CHECK YOUR ANSWER 39

40. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When Josh pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is _______. A.less than Josh’s push B.equal to Josh’s push C.equal and opposite to Josh’s push D.more than Josh’s push The Force of Friction CHECK YOUR NEIGHBOR 40

41. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When Josh pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is _______. A.less than Josh’s push B.equal to Josh’s push C.equal and opposite to Josh’s push D.more than Josh’s push The Force of Friction CHECK YOUR ANSWER 41

42. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When Josh pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is _______. A.less than Josh’s push B.equal to Josh’s push C.equal and opposite to Josh’s push D.more than Josh’s push The Force of Friction CHECK YOUR NEIGHBOR 42

43. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When Josh pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is _______. A.less than Josh’s push B.equal to Josh’s push C.equal and opposite to Josh’s push D.more than Josh’s push Explanation: The increasing speed indicates a net force greater than zero. The refrigerator is not in equilibrium. The Force of Friction CHECK YOUR ANSWER 43

44. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Speed and Velocity Speed  defined as the distance covered per amount of travel time  units are meters per second  in equation form Example: A girl runs 6 meters in 1 second. Her speed is 6 m/s. 44

45. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Speed and Velocity Average speed  the entire distance covered divided by the total travel time  doesn’t indicate various instantaneous speeds along the way  in equation form: Example: Drive a distance of 80 km in 1 hour and your average speed is 80 km/h. 45

46. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Speed and Velocity Instantaneous speed is the speed at any instant. Velocity  a description of how fast and in what direction  a vector quantity 46

47. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The average speed of driving 30 km in 1 hour is the same average speed as driving _______. A.30 km in one-half hour B.30 km in two hours C.60 km in one-half hour D.60 km in two hours Speed and Velocity CHECK YOUR NEIGHBOR 47

48. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The average speed of driving 30 km in 1 hour is the same average speed as driving _______. A.30 km in one-half hour B.30 km in two hours C.60 km in one-half hour D.60 km in two hours Speed and Velocity CHECK YOUR ANSWER 48

49. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Speed and Velocity  Constant speed is steady speed, neither speeding up nor slowing down.  Constant velocity is constant speed and constant direction (straight-line path with no acceleration).  Motion is relative to Earth, unless otherwise stated. 49

50. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Acceleration Galileo first formulated the concept of acceleration in his experiments with inclined planes. 50

51. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Acceleration  is the rate at which velocity changes over time  involves a change in speed, direction, or both speed and direction Example: Car making a turn 51

52. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Acceleration  in equation form: Example: If in 1 second you steadily increase your velocity from 30 km/h to 35 km/h, and in the next 1 second you steadily increase your velocity from 35 km/h to 40 km/h, you change your velocity by 5 km/h each second. Your acceleration is 5 km/h/s. 52

53. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley An automobile cannot maintain a constant speed when _______. A.accelerating B.rounding a curve C.Both of the above D.Neither of the above Acceleration CHECK YOUR NEIGHBOR 53

54. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley An automobile cannot maintain a constant speed when _______. A.accelerating B.rounding a curve C.Both of the above D.Neither of the above Comment: When rounding a curve, the automobile is accelerating because it is changing direction. Acceleration CHECK YOUR ANSWER 54

55. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Acceleration and velocity are actually _______. A.the same B.rates, but for different quantities C.the same when direction is not a factor D.the same in free-fall situations Acceleration CHECK YOUR NEIGHBOR 55

56. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Acceleration and velocity are actually _______. A.the same B.rates, but for different quantities C.the same, when direction is not a factor D.the same in free-fall situations Explanation: Velocity is the rate at which distance changes over time; acceleration is the rate at which velocity changes over time. Acceleration CHECK YOUR ANSWER 56

57. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Acceleration Free-fall falling under the influence of gravity only—with no air resistance o Freely falling objects on Earth gain speed at the rate of 10 m/s each second (more precisely, 9.8 m/s 2 ). 57

58. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley If a falling object gains 10 m/s each second it falls, its acceleration is _______. A.10 m/s B.10 m/s per second C.Both of the above D.Neither of the above Acceleration CHECK YOUR NEIGHBOR 58

59. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley If a falling object gains 10 m/s each second it falls, its acceleration is _______. A.10 m/s B.10 m/s per second C.Both of the above D.Neither of the above Explanation: It is common to express 10 m/s per second as 10 m/s/s, or 10 m/s 2. Acceleration CHECK YOUR ANSWER 59

60. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A free-falling object has a speed of 30 m/s at one instant. Exactly one second later, its speed will be _______. A.the same B.35 m/s C.more than 35 m/s D.60 m/s Acceleration CHECK YOUR NEIGHBOR 60

61. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A free-falling object has a speed of 30 m/s at one instant. Exactly one second later, its speed will be _______. A.the same B.35 m/s C.more than 35 m/s D.60 m/s Explanation: One second later, its speed will be 40 m/s, which is more than 35 m/s. Acceleration CHECK YOUR ANSWER 61