# Conceptual Physics Fundamentals

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1 Conceptual Physics Fundamentals Chapter 4: NEWTON S LAWS OF MOTION

2 Newton s Laws of Motion I was only a scalar until you came along and gave me direction. Barbara Wolfe

3 This lecture will help you understand: Newton s First Law of Motion Newton s Second Law of Motion Forces and Interactions Newton s Third Law of Motion Vectors Summary of Newton s Laws of Motion

4 Sir Isaac Newton ( ) Newton invented calculus described the laws of universal gravitation (gravitas-weight) and the three laws of motion built the first reflecting telescope.

5 Newton s First Law of Motion Newton s First Law (the law of inertia) a restatement of Galileo s concept of inertia states that every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero force

6 Newton s First Law of Motion example: dishes remain in their initial state of rest when a tablecloth is whipped from beneath them Inertia is the property of objects to resist changes in motion.

7 Newton s First Law of Motion CHECK YOUR NEIGHBOR A sheet of paper can be quickly withdrawn from under a soft-drink can without the can toppling, because A. gravity pulls harder on the can than on the paper. B. the can has weight. C. the can has inertia. D. none of the above

8 Newton s First Law of Motion CHECK YOUR ANSWER A sheet of paper can be quickly withdrawn from under a soft-drink can without the can toppling, because A. gravity pulls harder on the can than on the paper. B. the can has weight. C. the can has inertia. D. none of the above

9 Newton s First Law of Motion CHECK YOUR NEIGHBOR If you swing a stone overhead in a horizontal circle and the string breaks, the tendency of the stone is to follow a A. curved path. B. straight-line path. C. spiral path. D. vertical path.

10 Newton s First Law of Motion CHECK YOUR ANSWER If you swing a stone overhead in a horizontal circle and the string breaks, the tendency of the stone is to follow a A. curved path. B. straight-line path. C. spiral path. D. vertical path.

11 Newton s First Law of Motion The Moving Earth Theory of Earth revolving around the Sun was proposed by Copernicus, a Polish astronomer in 1543 Newton

12 Newton s Second Law of Motion Isaac Newton was the first to connect the concepts of force and mass to produce acceleration.

13 Newton s Second Law of Motion Newton s Second Law (the law of acceleration) relates acceleration to force states that the acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object

14 Newton s Second Law of Motion in equation form: acceleration = net force mass example: net force acting on object is doubled object s acceleration will be doubled mass doubled acceleration halved

15 Newton s Second Law of Motion

16 Newton s Second Law of Motion CHECK YOUR NEIGHBOR Consider a cart pushed along a track with a certain force. If the force remains the same while the mass of the cart decreases to half, the acceleration of the cart A. remains the same. B. halves. C. doubles. D. changes unpredictably.

17 Newton s Second Law of Motion CHECK YOUR ANSWER Consider a cart pushed along a track with a certain force. If the force remains the same while the mass of the cart decreases to half, the acceleration of the cart A. remains the same. B. halves. C. doubles. D. changes unpredictably.

18 Newton s Second Law of Motion CHECK YOUR NEIGHBOR Push a cart along a track so twice as much net force acts on it. If the acceleration remains the same, what is a reasonable explanation? A. The mass of the cart doubled when the force doubled. B. The cart experiences a force that it didn t before. C. The track is not level. D. Friction reversed direction.

19 Newton s Second Law of Motion CHECK YOUR ANSWER Push a cart along a track so twice as much net force acts on it. If the acceleration remains the same, what is a reasonable explanation? A. The mass of the cart doubled when the force doubled. B. The cart experiences a force that it didn t before. C. The track is not level. D. Friction reversed direction.

20 Newton s Second Law of Motion When acceleration is g free-fall Newton s second law provides an explanation for the equal accelerations of freely-falling objects of various masses acceleration is equal when air resistance is negligible acceleration depends on force (weight) and inertia example: a double brick and a single brick fall at the same rate accelerate equally

21 Newton s Second Law of Motion CHECK YOUR NEIGHBOR At one instant, an object in free-fall has a speed of 40 m/s. Its speed one second later is A. also 40 m/s. B. 45 m/s. C. 50 m/s. D. none of the above

22 Newton s Second Law of Motion CHECK YOUR ANSWER At one instant, an object in free-fall has a speed of 40 m/s. Its speed one second later is A. also 40 m/s. B. 45 m/s. C. 50 m/s. D. none of the above Comment: We assume the object is falling downward.

23 Newton s Second Law of Motion CHECK YOUR NEIGHBOR A 5-kg iron ball and a 10-kg iron ball are dropped from rest. For negligible air resistance, the acceleration of the heavier ball will be A. less. B. the same. C. more. D. undetermined.

24 Newton s Second Law of Motion CHECK YOUR ANSWER A 5-kg iron ball and a 10-kg iron ball are dropped from rest. For negligible air resistance, the acceleration of the heavier ball will be A. less. B. the same. C. more. D. undetermined.

25 Newton s Second Law of Motion CHECK YOUR NEIGHBOR A 5-kg iron ball and a 10-kg iron ball are dropped from rest. When the free-falling 5-kg ball reaches a speed of 10 m/s, the speed of the free-falling 10-kg ball is A. less than 10 m/s. B. 10 m/s. C. more than 10 m/s. D. undetermined.

26 Newton s Second Law of Motion CHECK YOUR ANSWER A 5-kg iron ball and a 10-kg iron ball are dropped from rest. When the free-falling 5-kg ball reaches a speed of 10 m/s, the speed of the free-falling 10-kg ball is A. less than 10 m/s. B. 10 m/s. C. more than 10 m/s. D. undetermined.

27 Newton s Second Law of Motion When acceleration of fall is less than g non-free fall occurs when air resistance is non-negligible depends on two things: speed and frontal surface area

28 Newton s Second Law of Motion example: A skydiver jumps from plane. Weight is the only force until air resistance acts. As falling speed increases, air resistance on diver builds up, net force is reduced, and acceleration becomes less. When air resistance equals the diver s weight, net force is zero and acceleration terminates. Diver reaches terminal velocity, then continues the fall at constant speed.

29 Newton s Second Law of Motion Terminal speed occurs when acceleration terminates (when air resistance equals weight and net force is zero) Terminal velocity same as terminal speed, with direction implied or specified

30 Newton s Second Law of Motion CHECK YOUR NEIGHBOR When a 20-N falling object encounters 5 N of air resistance, its acceleration of fall is A. less than g. B. more than g. C. g. D. terminated.

31 Newton s Second Law of Motion CHECK YOUR ANSWER When a 20-N falling object encounters 5 N of air resistance, its acceleration of fall is A. less than g. B. more than g. C. g. D. terminated. Comment: Acceleration of a nonfree-fall is always less than g. Acceleration will actually be (20 N 5 N)/2 kg = 7.5 m/s 2.

32 Newton s Second Law of Motion CHECK YOUR NEIGHBOR If a 50-N person is to fall at terminal speed, the air resistance needed is A. less than 50 N. B. 50 N. C. more than 50 N. D. none of the above

33 Newton s Second Law of Motion CHECK YOUR ANSWER If a 50-N person is to fall at terminal speed, the air resistance needed is A. less than 50 N. B. 50 N. C. more than 50 N. D. none of the above Explanation: Then, F = 0 and acceleration = 0.

34 Newton s Second Law of Motion CHECK YOUR NEIGHBOR As the skydiver falls faster and faster through the air, air resistance A. increases. B. decreases. C. remains the same. D. not enough information

35 Newton s Second Law of Motion CHECK YOUR ANSWER As the skydiver falls faster and faster through the air, air resistance A. increases. B. decreases. C. remains the same. D. not enough information

36 Newton s Second Law of Motion CHECK YOUR NEIGHBOR As the skydiver continues to fall faster and faster through the air, net force A. increases. B. decreases. C. remains the same. D. not enough information

37 Newton s Second Law of Motion CHECK YOUR ANSWER As the skydiver continues to fall faster and faster through the air, net force A. increases. B. decreases. C. remains the same. D. not enough information

38 Newton s Second Law of Motion CHECK YOUR NEIGHBOR As the skydiver continues to fall faster and faster through the air, her acceleration A. increases. B. decreases. C. remains the same. D. not enough information

39 Newton s Second Law of Motion CHECK YOUR ANSWER As the skydiver continues to fall faster and faster through the air, her acceleration A. increases. B. decreases. C. remains the same. D. not enough information

40 Newton s Second Law of Motion A situation to ponder Consider a heavy and light person jumping together with the same-size parachutes from the same altitude.

41 A situation to ponder CHECK YOUR NEIGHBOR Who will reach the ground first? A. the light person B. the heavy person C. both will reach at the same time D. not enough information

42 A situation to ponder CHECK YOUR ANSWER Who will reach the ground first? A. the light person B. the heavy person C. both will reach at the same time D. not enough information Explanation: The heavier person has a greater terminal velocity. Do you know why?

43 Newton s Second Law of Motion Coin and feather fall feather reaches terminal velocity very quickly and falls slowly at constant speed, reaching the bottom after the coin does coin falls very quickly and air resistance doesn t build up to its weight over short-falling distances, which is why the coin hits the bottom much sooner than the falling feather

44 Newton s Second Law of Motion CHECK YOUR NEIGHBOR When the air is removed by a vacuum pump and the activity is repeated, A. the feather hits the bottom first, before the coin hits. B. the coin hits the bottom first, before the feather hits. C. both the coin and feather drop together side-by-side. D. not enough information

45 Newton s Second Law of Motion CHECK YOUR ANSWER When the air is removed by a vacuum pump and the activity is repeated, A. the feather hits the bottom first, before the coin hits. B. the coin hits the bottom first, before the feather hits. C. both the coin and feather drop together side-by-side. D. not enough information

46 Air Resistance How to be a flying squirrel: Jeb Corliss "Grinding the Crack"

47 Forces and Interactions Interaction is between one thing and another requires a pair of forces acting on two objects example: interaction of fingers and wall pushing on each other force pair you push on wall; wall pushes on you

48 Newton s Third Law of Motion Newton s Third Law (law of action and reaction) states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

49 Newton s Third Law of Motion CHECK YOUR NEIGHBOR A soccer player kicks a ball with 1500 N of force. The ball exerts a reaction force against the player s foot of A. somewhat less than 1500 N. B N. C. somewhat more than 1500 N. D. none of the above

50 Newton s Third Law of Motion CHECK YOUR ANSWER A soccer player kicks a ball with 1500 N of force. The ball exerts a reaction force against the player s foot of A. somewhat less than 1500 N. B N. C. somewhat more than 1500 N. D. none of the above

51 Newton s Third Law of Motion Action and reaction forces one force is called the action force; the other force is called the reaction force are copairs of a single interaction neither force exists without the other are equal in strength and opposite in direction always act on different objects

52 Newton s Third Law of Motion reexpression of Newton s third law: to every action there is always an opposed equal reaction example: tires of car push back against the road while the road pushes the tires forward

53 Newton s Third Law of Motion Simple rule to identify action and reaction identify the interaction one thing interacts with another action: object A exerts a force on object B reaction: object B exerts a force on object A example: action rocket (object A) exerts force on gas (object B) reaction gas (object B) exerts force on rocket (object A)

54 Newton s Third Law of Motion CHECK YOUR NEIGHBOR When you step off a curb, Earth pulls you downward. The reaction to this force is A. a slight air resistance. B. nonexistent in this case. C. you pulling Earth upward. D. none of the above

55 Newton s Third Law of Motion CHECK YOUR ANSWER When you step off a curb, Earth pulls you downward. The reaction to this force is A. a slight air resistance. B. nonexistent in this case. C. you pulling Earth upward. D. none of the above

56 Newton s Third Law of Motion Action and Reaction on Different Masses cannonball: F/m = a cannon: F/m = a the same force exerted on a small mass produces a large acceleration the same force exerted on a large mass produces a small acceleration

57 Newton s Third Law of Motion CHECK YOUR NEIGHBOR When a cannon is fired, the accelerations of the cannon and cannonball are different because the A. forces don t occur at the same time. B. forces, although theoretically the same, in practice are not. C. masses are different. D. ratios of force to mass are the same.

58 Newton s Third Law of Motion CHECK YOUR ANSWER When a cannon is fired, the accelerations of the cannon and cannonball are different because the A. forces don t occur at the same time. B. forces, although theoretically the same, in practice are not. C. masses are different. D. ratios of force to mass are the same.

59 Newton s Third Law of Motion A situation to ponder Consider a high-speed bus colliding head-on with an innocent bug. The force of impact splatters the unfortunate bug over the windshield.

60 A situation to ponder CHECK YOUR NEIGHBOR Which is greater, the force on the bug or the force on the bus? A. bug B. bus C. both are the same D. cannot say

61 A situation to ponder CHECK YOUR ANSWER Which is greater, the force on the bug or the force on the bus? A. bug B. bus C. both are the same D. cannot say Comment: Although the forces are equal in magnitude, the effects are very different. Do you know why?

62 Newton s Third Law of Motion CHECK YOUR NEIGHBOR Two people of equal mass on slippery ice push off from each other. Will both move at the same speed in opposite directions? A. yes B. yes, but only if both push equally C. no D. no, unless acceleration occurs

63 Newton s Third Law of Motion CHECK YOUR ANSWER Two people of equal mass on slippery ice push off from each other. Will both move at the same speed in opposite directions? A. yes B. yes, but only if both push equally C. no D. no, unless acceleration occurs Explanation: However they push, the result is equal-magnitude forces on equal masses, which produces equal accelerations; therefore, there are equal changes in speed.

64 Newton s Third Law of Motion Myth busters video two cars head on crash test

65 Newton s Third Law of Motion Defining Your System consider a single enclosed orange applied external force causes the orange to accelerate in accord with Newton s second law action and reaction pair of forces is not shown

66 Newton s Third Law of Motion consider the orange and the apple pulling on it action and reaction do not cancel (because they act on different things) external force by apple accelerates the orange

67 Newton s Third Law of Motion consider a system comprised of both the orange and the apple the apple is no longer external to the system force pair is internal to system, which doesn t cause acceleration action and reaction within the system cancel with no external forces, there is no acceleration of system

68 Newton s Third Law of Motion consider the same system, but with external force of friction on it same internal action and reaction forces (between the orange and apple) cancel a second pair of action-reaction forces (between the apple s feet and the floor) exists

69 Newton s Third Law of Motion one of these acts by the system (apple on the floor) and the other acts on the system (floor on the apple) external frictional force of floor pushes on the system, which accelerates second pair of action and reaction forces do not cancel

70 Newton s Third Law CHECK YOUR NEIGHBOR When lift equals the weight of a helicopter, the helicopter A. climbs down. B. climbs up. C. hovers in midair. D. none of the above

71 Newton s Third Law CHECK YOUR ANSWER When lift equals the weight of a helicopter, the helicopter A. climbs down. B. climbs up. C. hovers in midair. D. none of the above

72 Newton s Third Law CHECK YOUR NEIGHBOR When lift is greater, the helicopter A. climbs down. B. climbs up. C. hovers in midair. D. none of the above

73 Newton s Third Law CHECK YOUR ANSWER When lift is greater, the helicopter A. climbs down. B. climbs up. C. hovers in midair. D. none of the above

74 Newton s Third Law CHECK YOUR NEIGHBOR A bird flies by A. flapping its wings. B. pushing air down so that the air pushes it upward. C. hovering in midair. D. inhaling and exhaling air.

75 Newton s Third Law CHECK YOUR ANSWER A birds flies by A. flapping its wings. B. pushing air down so that the air pushes it upward. C. hovering in midair. D. inhaling and exhaling air.

76 Newton s Third Law CHECK YOUR NEIGHBOR Slightly tilted wings of airplanes deflect A. oncoming air downward to produce lift. B. oncoming air upward to produce lift. C. both A and B D. neither A nor B

77 Newton s Third Law CHECK YOUR ANSWER Slightly tilted wings of airplanes deflect A. oncoming air downward to produce lift. B. oncoming air upward to produce lift. C. both A and B D. neither A nor B Explanation: When a wing diverts air downward, it exerts a downward force on the air. The air simultaneously exerts an upward force on the wing. The vertical component of this upward force is lift. (The horizontal component is drag.)

78 Newton s Third Law CHECK YOUR NEIGHBOR Compared with a light-weight glider, a heavier glider would have to push air A. downward with greater force. B. downward with the same force. C. downward with less force. D. none of the above

79 Newton s Third Law CHECK YOUR ANSWER Compared with a light-weight glider, a heavier glider would have to push air A. downward with greater force. B. downward with the same force. C. downward with less force. D. none of the above Explanation: The force on the air deflected downward must equal the weight of the glider.

80 Vector quantity Vectors has magnitude and direction is represented by an arrow example: velocity, force, acceleration Scalar quantity has magnitude example: mass, volume, speed

81 Vectors Resultant the sum of two or more vectors for vectors in the same direction, add arithmetically for vectors in opposite directions, subtract arithmetically two vectors that don t act in the same or opposite direction use parallelogram rule two vectors at right angles to each other use Pythagorean theorem: R 2 = V 2 + H 2

82 Vectors CHECK YOUR NEIGHBOR Referring to the figure, which of the following are true statements? A. 50 N is the resultant of the 30 and 40-N vectors. B. The 30-N vector can be considered a component of the 50-N vector. C. The 40-N vector can be considered a component of the 50-N vector. D. All of the above are correct.

83 Vectors CHECK YOUR ANSWER Referring to the figure, which of the following are true statements? A. 50 N is the resultant of the 30 and the 40-N vectors. B. The 30-N vector can be considered a component of the 50-N vector. C. The 40-N vector can be considered a component of the 50-N vector. D. All of the above are correct.

84 Vectors CHECK YOUR NEIGHBOR Referring to the figure, which of the following are true statements? A. 100 km/h is the resultant of the 80 and 60-km/h vectors. B. The 80-km/h vector can be considered a component of the 100-km/h vector. C. The 60-km/h vector can be considered a component of the 100-km/h vector. D. All of the above are correct.

85 Vectors CHECK YOUR ANSWER Referring to the figure, which of the following are true statements? A. 100 km/h is the resultant of the 80 and 60-km/h vectors. B. The 80-km/h vector can be considered a component of the 100-km/h vector. C. The 60-km/h vector can be considered a component of the 100-km/h vector. D. All of the above are correct.

86 Vectors Vector Components vertical and horizontal components of a vector are perpendicular to each other determined by resolution

87 Vectors CHECK YOUR NEIGHBOR You run horizontally at 4 m/s in a vertically falling rain that falls at 4 m/s. Relative to you, the raindrops are falling at an angle of A. 0. B. 45. C. 53. D. 90.

88 Vectors CHECK YOUR ANSWER You run horizontally at 4 m/s in a vertically falling rain that falls at 4 m/s. Relative to you, the raindrops are falling at an angle of A. 0. B. 45. C. 53. D. 90. Explanation: The horizontal 4 m/s and vertical 4 m/s combine by the parallelogram rule to produce a resultant of 5.6 m/s at 45.

89 Summary of Newton s Three Laws of Motion Newton s first law of motion (the law of inertia) An object at rest tends to remain at rest; an object in motion tends to remain in motion at constant speed along a straight-line path. Newton s second law of motion (the law of acceleration) When a net force acts on an object, the object will accelerate. The acceleration is directly proportional to the net force and inversely proportional to the mass. Newton s third law of motion (the law of action and reaction) Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

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