Conceptual Physical Science

Transcription

1 Hewitt/Suchocki/Hewitt Conceptual Physical Science Fourth Edition Chapter 1: PATTERNS OF MOTION AND EQUILIBRIUM

2 This lecture will help you understand: Aristotle on Motion Galileo s Concept of Inertia Mass A Measure of Inertia Net Force The Equilibrium Rule Support Force Dynamic Equilibrium The Force of Friction Speed and Velocity Acceleration

3 1. ARISTOTLE ON MOTION Aristotle attempted to understand motion by classification. Two Classes: Natural and Violent

4 Natural Natural motion depended on nature of the object. Examples: Rocks fall Smoke rises

5 Natural Motion The falling speed of an object was supposed to be proportional to its weight. Natural motion could be circular (perfect objects in perfect motion with no end).

6 Violent Pushing or pulling forces imposed motion. Some motions were difficult to understand. Example: the flight of an arrow There was a normal state of rest except for celestial bodies.

7 Aristotle was unquestioned for 2000 years. Most thought that the Earth was the center of everything. No one could imagine a force that could move it.

8 2. COPERNICUS AND THE MOVING EARTH Sun was center, not earth. He was hesitant to publish because he didn't really believe it either. De Revolutionibus reached him on his day of death, May 24, 1543.

9 3. GALILEO AND THE LEANING TOWER 16th Century scientist who supported Copernicus. He refuted many of Aristotle's ideas. Worked on falling body problem - used experiment.

10 GALILEO'S INCLINED PLANES Knocked down Aristotle's push or pull ideas. Rest was not a natural state. The concept of inertia was introduced. Galileo is sometimes referred to as the father of experimentation.

11 Galileo s Concept of Inertia Italian scientist Galileo demolished Aristotle s assertions in early 1500s. In the absence of a force, objects once set in motion tend to continue moving indefinitely.

12 Galileo s Concept of Inertia Legend of the Leaning Tower of Pisa: Galileo showed that dropped objects fall to the ground at the same time when air resistance is negligible.

13 Galileo s Concept of Inertia Discovery: In the absence of friction, no force is necessary to keep a horizontally moving object moving.

14 He tested with planes. Experiment - Ball and incline plane The change in speed depended on the slope of the incline.

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16 Galileo s Concept of Inertia Experiment: Balls rolling down inclined planes and then up others tend to roll back up to their original heights.

17 Galileo s Concept of Inertia Conclusion: The tendency of a moving body to keep moving is natural every material object resists change in its state of motion. This property of things to resist change is called inertia.

18 Galileo s Concept of Inertia CHECK YOUR NEIGHBOR 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.

19 Galileo s Concept of Inertia CHECK YOUR ANSWER 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.

20 Mass A Measure of Inertia The amount of inertia possessed by an object depends on the amount of matter the amount of material that composes it its mass: greater mass greater inertia smaller mass smaller inertia

21 Units of Measurement System Length Mass Time SI CGS BE Meter (m) Kilogram (kg) Seconds (s) Centimeter (cm) Gram (gm) Seconds (s) Foot (ft) Slug (sl) Seconds (s)

22 Mass A Measure of Inertia Mass Quantity of matter in an object Measure of inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way

23 Mass A Measure of Inertia Weight Amount of gravitational pull on an object Proportional to mass Twice the mass twice the weight Half the mass half the weight

24 Mass A Measure of Inertia Mass versus volume: Mass involves how much matter an object contains Volume involves how much space an object occupies

25 Mass A Measure of Inertia CHECK YOUR NEIGHBOR The concept of inertia mostly involves A. mass. B. weight. C. volume. D. density.

26 Mass A Measure of Inertia CHECK YOUR ANSWER 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! :-)

27 Mass A Measure of Inertia Kilogram standard unit of measurement for mass on Earth s surface, 1 kg of material weighs 9.8 Newton (N) 10 Newton (N) away from Earth (on the Moon), 1 kg of material weighs less than 9.8 Newton (N)

28 Mass A Measure of Inertia CHECK YOUR NEIGHBOR 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.

29 Mass A Measure of Inertia CHECK YOUR ANSWER 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.

30 Mass A Measure of Inertia CHECK YOUR NEIGHBOR 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.

31 Mass A Measure of Inertia CHECK YOUR ANSWER 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.

32 Mass A Measure of Inertia Measure of compactness Density is the measure of how much mass occupies a given space Equation for density: Density = mass volume in grams per cubic centimeter or kilograms per cubic meter

33 Mass A Measure of Inertia CHECK YOUR NEIGHBOR The density of 1 kilogram of iron is A. less on the Moon. B. the same on the Moon. C. greater on the Moon. D. All of the above.

34 Mass A Measure of Inertia CHECK YOUR ANSWER The density of 1 kilogram of iron is A. less on the Moon. B. the same on the Moon. C. greater on the Moon. D. All of the above. Explanation: Both mass and volume of 1 kilogram of iron is the same everywhere, so density is the same everywhere.

35 Net Force Force simply a push or a pull Net force combination of all forces that act on an object changes an object s motion

36 Net Force CHECK YOUR NEIGHBOR 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.

37 Net Force CHECK YOUR ANSWER 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.

38 The Equilibrium Rule The equilibrium rule: The vector sum of forces acting on a nonaccelerating object or system of objects equals zero. Mathematical notation: F = 0.

39 The Equilibrium Rule CHECK YOUR NEIGHBOR The equilibrium rule, F = 0, applies to A. vector quantities. B. scalar quantities. C. Both of the above. D. Neither of the above.

40 The Equilibrium Rule CHECK YOUR ANSWER 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.

41 Support Force Support force is force that supports an object on the surface against gravity is also normal force

42 Support Force CHECK YOUR NEIGHBOR When you stand on two bathroom scales, with one foot on each scale and weight evenly distributed, each scale will read A. your weight. B. half your weight. C. zero. D. actually more than your weight.

43 Support Force CHECK YOUR ANSWER When you stand on two bathroom scales, with one foot on each scale and weight evenly distributed, each scale will read A. your weight. B. half your weight. C. zero. D. actually 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.

44 Dynamic Equilibrium An object that moves at constant velocity is in equilibrium. When two or more forces cancel to zero on a moving object, then the object is in equilibrium.

45 Dynamic Equilibrium CHECK YOUR NEIGHBOR 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.

46 Dynamic Equilibrium CHECK YOUR ANSWER 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.

47 The Force of Friction Friction the resistive force that opposes the motion or attempted motion of an object through a fluid or past another object with which it is in contact always acts in a direction to oppose motion

48 The Force of Friction Friction (continued) between two surfaces, the amount depends on the kinds of material and how much they are pressed together due to surface bumps and also to the stickiness of atoms on the surfaces of the two materials

49 The Force of Friction CHECK YOUR NEIGHBOR The force of friction can occur A. with sliding objects. B. in water. C. in air. D. All of the above.

50 The Force of Friction CHECK YOUR ANSWER 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.

51 The Force of Friction CHECK YOUR NEIGHBOR When Nellie pushes a crate across a factory floor at constant speed, the force of friction between the crate and the floor is A. less than Nellie s push. B. equal to Nellie s push. C. equal and opposite to Nellie s push. D. more than Nellie s push.

52 The Force of Friction CHECK YOUR ANSWER When Nellie pushes a crate across a factory floor at constant speed, the force of friction between the crate and the floor is A. less than Nellie s push. B. equal to Nellie s push. C. equal and opposite to Nellie s push. D. more than Nellie s push.

53 The Force of Friction CHECK YOUR NEIGHBOR When Nellie pushes a crate across a factory floor at an increasing speed, the amount of friction between the crate and the floor is A. less than Nellie s push. B. equal to Nellie s push. C. equal and opposite to Nellie s push. D. more than Nellie s push.

54 The Force of Friction CHECK YOUR ANSWER When Nellie pushes a crate across a factory floor at an increasing speed, the amount of friction between the crate and the floor is A. less than Nellie s push. B. equal to Nellie s push. C. equal and opposite to Nellie s push. D. more than Nellie s push. Explanation: The increasing speed indicates a net force greater than zero. Her push is greater than the friction force. The crate is not in equilibrium.

55 DESCRIPTION OF MOTION Speed Velocity Acceleration

56 Speed and Velocity Speed is described as the distance covered per amount of travel time Equation for speed: Speed = distance covered travel time

57 Speed Average Speed = total distance traveled / time Units - m/s, ft/s, etc. Instantaneous Speed is the speed you would read from a speedometer. Is speed at any instant of time

58 Example of Average Speed A 30 mph 2 miles B You take a trip from A to B and back to A. You want to average 60 mph for the round trip A to B to A. From A to B you average 30 mph. What is your average speed on the return trip from B to A?

59 Velocity Average Velocity = Displacement/time Units - m/s, ft/s, etc. Instantaneous Velocity of an object is its instantaneous speed plus the direction it is traveling. Velocity is a vector.

60 Displacement and Average Velocity Distance traveled is the length of the path taken. D Displacement Average velocity = v D t D

61 Speed and Velocity CHECK YOUR NEIGHBOR The average speed in 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.

62 Speed and Velocity CHECK YOUR ANSWER The average speed in 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.

63 Motion is Relative Everything is always moving. At this moment, your speed relative to the Sun is about 100,000 kilometers per hour. When we say a space shuttle travels at 30,000 kilometers per hour, we mean relative to the Earth.

64 Motion is Relative CHECK YOUR NEIGHBOR A hungry bee sees a flower in a 5-m/s breeze. How fast and in what direction should the bee fly in order to hover above the flower? A. It should fly toward the flower, then at 5 m/s into the breeze. B. It should fly with the breeze at 5 m/s away from the flower. C. The bee will not be able to fly in a 5-m/s breeze. D. The bee will not be able to reach the flower. Explanation: When just above the flower, it should fly at 5 m/s in order to hover at rest. This is why bees grip onto a flower to prevent from being blown off.

65 Motion is Relative CHECK YOUR ANSWER A hungry bee sees a flower in a 5-m/s breeze. How fast and in what direction should the bee fly in order to hover above the flower? A. It should fly toward the flower, then at 5 m/s into the breeze. B. It should fly with the breeze at 5 m/s away from the flower. C. The bee will not be able to fly in a 5-m/s breeze. D. The bee will not be able to reach the flower. Explanation: When just above the flower, it should fly at 5-m/s in order to hover at rest. This is why bees grip onto a flower to prevent from being blown off.

66 Acceleration Galileo first formulated the concept of acceleration in his experiments with inclined planes.

67 Acceleration on Galileo's Inclined Planes

68 Acceleration Acceleration is the rate at which velocity changes with time. The change in velocity may be in magnitude, in direction, or both. Equation for acceleration: Acceleration = change of velocity time interval

69 Acceleration Acceleration = "change" in velocity/time Units - m/s 2, ft/s 2, etc. Acceleration is also a vector.

70 Motion at constant velocity Accelerated motion Here Here, too

71 Velocity and Acceleration Galileo used inclined planes to study accelerations. He found constant accelerations for inclines. (It was too hard to measure time for free-falls.) He also found that the size of the objects didn't matter.

72 Relationships Between v and a for Linear Motion. v v t 0 a v v 0 at v v 0 at If initial velocity is zero, then v at

73 Example A jogger starts at zero velocity with an acceleration of 3 m/s 2. How fast is she moving after 4 seconds? v0 0 a 3 m/ s t 4s 2 v at v 3 m/ s v 12 m / 2 (4s) s

74 Acceleration CHECK YOUR NEIGHBOR An automobile cannot maintain a constant speed when A. accelerating. B. rounding a curve. C. Both of the above. D. None of the above.

75 Acceleration CHECK YOUR ANSWER An automobile cannot maintain a constant speed when A. accelerating. B. rounding a curve. C. Both of the above. D. None of the above. Comment: When rounding a curve, the automobile is accelerating, for it is changing direction.

76 Acceleration CHECK YOUR NEIGHBOR Acceleration and velocity are actually A. much the same as each other. B. rates, but for different quantities. C. the same when direction is not a factor. D. the same for free-fall situations.

77 Acceleration CHECK YOUR ANSWER Acceleration and velocity are actually A. much the same as each other. B. rates, but for different quantities. C. the same when direction is not a factor. D. the same for free-fall situations. Explanation: Velocity is the rate at which distance changes with time; acceleration is the rate at which velocity changes with time.

78 Acceleration Free fall When the only force acting on a falling object is gravity, (with negligible air resistance), the object is in a state of free fall.

79 FREE FALL Motion near the surface of the earth in the absence of air resistance. The acceleration of an object is g = 9.81 m/s 2 10 m/s 2

80 How Fast Velocity in gravitational field: v = gt = 10t

81 How Far d d vt v d t 2 gt d t 2 1 gt 2 16t 2 (If initial velocity is zero) 2

82 What is the acceleration of an object at top of its flight? g, you should know this one.

83 Acceleration CHECK YOUR NEIGHBOR 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.

84 Acceleration CHECK YOUR ANSWER 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.

85 Acceleration CHECK YOUR NEIGHBOR 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.

86 Acceleration CHECK YOUR ANSWER 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.

87 Acceleration CHECK YOUR NEIGHBOR The distance fallen by a free-falling body A. remains constant each second of fall. B. increases each second when falling. C. decreases each second when falling. D. None of the above.

88 Acceleration CHECK YOUR ANSWER The distance fallen by a free-falling body A. remains constant each second of fall. B. increases each second when falling. C. decreases each second when falling. D. None of the above. Explanation: See Table 1.2 for verification of this. Falling distance time squared.

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90 Chapter 1 Review Questions

91 What is the average speed of a horse that gallops a round-trip distance of 15 km in a time of 30 min? (a) 0 (b) 0.5 km/h (c) 30 km/h (d) 500 m/s (e) None of the above

92 What is the average velocity for the round-trip of the horse in the previous question? (a) 0 (b) 0.5 km/h (c) 30 km/h (d) 500 m/s (e) None of the above

93 You throw a stone downward. It leaves your hand with a speed of 10 m/s. What is its speed two seconds after leaving your hand? (Neglect air resistance.) (a) 10 m/s (b) 30 m/s (c) 40 m/s (d) 60 m/s (e) 70 m/s