Conceptual Physics Fundamentals

Transcription

1 Conceptual Physics Fundamentals Chapter 3: EQUILIBRIUM AND LINEAR MOTION

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 Equilibrium of Moving Things The Force of Friction Speed and Velocity Acceleration

3 Aristotle (384 BC 322 BC)

4 Aristotle Added 5 th element Ether Aristotle added 5 th Element - Ether

5 Aristotle on Motion Aristotle s classification of motion natural motion every object in the universe has a proper place determined by a combination of four elements: earth, water, air, and fire any object not in its proper place will strive to get there examples: stones fall puffs of smoke rise

6 Aristotle on Motion natural motion (continued) straight up or straight down for all things on beyond Earth, motion is circular example: Sun and Moon continually circle Earth violent motion produced by external pushes or pulls on objects example: wind imposes motion on ships

7 Aristotle on Motion

8 Aristotle on Motion Aristotle s laws of motion The speed of falling is proportional to the weight of the object. Heavier objects fall faster. The speed by which an object falls depends inversely on the density of the medium it falls through (thus there can be no void)

9 Galileo ( ) Phases of the Venus

10 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

11 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)

12 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.

13 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.

14 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)

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

16 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! :-)

17 Mass A Measure of Inertia CHECK YOUR NEIGHBOR If the mass of an object is halved, the weight of the object is A. halved. B. twice. C. depends on location. D. none of the above.

18 Mass A Measure of Inertia CHECK YOUR ANSWER If the mass of an object is halved, the weight of the object is A. halved. B. twice. C. depends on location. D. none of the above.

19 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 weight of an object on Moon is less than on Earth (1/6 th ) mass of an object is the same in both locations

20 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

21 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.

22 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.

23 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.

24 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.

25 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.

26 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.

27 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.

28 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 length of arrow represents magnitude and arrowhead shows direction examples: force, velocity, acceleration

29 The Equilibrium Rule The equilibrium rule the vector sum of forces acting on a nonaccelerating object equals zero in equation form: F = 0

30 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 equal in magnitude and opposite in direction when added, cancel to zero bag of flour remains at rest

31 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.

32 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.

33 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 table a compresses atoms in the table, and the compressed atoms produce the support force

34 The Support Force CHECK YOUR NEIGHBOR 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.

35 The Support Force CHECK YOUR ANSWER 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.

36 Equilibrium of Moving Things Equilibrium a state of no change with no net force acting static equilibrium example: hockey puck at rest on slippery ice dynamic equilibrium example: hockey puck sliding at constant speed on slippery ice

37 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.

38 Equilibrium of Moving Things 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

39 Equilibrium of Moving Things 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

40 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.

41 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

42 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

43 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.

44 The Force of Friction CHECK YOUR NEIGHBOR 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.

45 The Force of Friction CHECK YOUR ANSWER 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.

46 The Force of Friction CHECK YOUR NEIGHBOR 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.

47 The Force of Friction CHECK YOUR ANSWER 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.

48 Speed and Velocity Speed defined as the distance covered per amount of travel time units are meters per second in equation form speed = distance covered time example: A girl runs 6 meters in 1 sec. Her speed is 6 m/s.

49 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: average speed = total distance covered travel time example: drive a distance of 80 km in 1 hour and your average speed is 80 km/h

50 Speed and Velocity Instantaneous speed is the speed at any instant. Velocity a description of how fast and in what direction a vector quantity

51 Speed and Velocity CHECK YOUR NEIGHBOR 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.

52 Speed and Velocity CHECK YOUR ANSWER 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.

53 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.

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

55 Acceleration Acceleration rate at which velocity changes over time involves a change in speed, direction, or both speed and direction example: car making a turn

56 Acceleration in equation form: acceleration = change of velocity time interval 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.

57 Acceleration CHECK YOUR NEIGHBOR An automobile cannot maintain a constant speed when A. accelerating. B. rounding a curve. C. both of the above D. neither of the above

58 Acceleration CHECK YOUR ANSWER 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.

59 Acceleration CHECK YOUR NEIGHBOR 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.

60 Acceleration CHECK YOUR ANSWER 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.

61 Acceleration Free-fall falling under the influence of gravity only with no air resistance freely falling objects on Earth gain speed at the rate of 10 m/s each second (more precisely, 9.8 m/s 2 )

62 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

63 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.

64 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.

65 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.