Welcome to Physics 100: Physics for Society

1. Welcome to Physics 100: Physics for Society Course website: http://faculty.wiu.edu/p-wang

2. Introduction to the syllabus • Word puzzle:Physics 100 is a GPA accelerator if and only if you P_ _ _ _ _ _ _ _ _ _. Answer: P A R T I C I P A T E.

3. Chapter 1 The Laws of Motion, Part 1 August 25: Skating−Newton’s first law of motion

4. Observations about the facts on skating: Movie: Ice skating −“Push … hold.” • When you’re at rest on a level surface, • without a push, you remain stationary. • with a push, you start moving that direction. • When you’re moving on a level surface, • without a push, you coast steady and straight. • with a push, you change direction or speed.

5. Question 1: Why does a stationary skater remain stationary? Demo: Tablecloth

6. Question 2: Why does a moving skater continue moving? Thought experiment: Galileo’s inclined planes

7. Physics concept: Inertia • A body at rest tends to remain at rest • A body in motion tends to remain in motion Newton’s first law of motion: (Start version) An object that is free of external influences moves in a straight line and covers equal distances in equal times. A motionless object also obeys this law.

8. Question: • Why is Newton’s first law of motion not so apparent to us? • Real-world complications mask simple physics • Solution: minimize or overwhelm complications • To demonstrate inertia: • Work on level ground (minimize gravity influence) • Use wheels, ice, or air support (minimize friction) • Work fast (overwhelm friction) • Use vacuum (remove air resistance)

9. Read: Ch1: 1 Homework: Ch1: E3,6. Due: September 5

10. August 27: Skating − Newton’s second law of motion

11. Physical quantities: • Position– an object’s location A vector quantity has magnitudeand direction. The magnitude of position is called distance. • Velocity – the change in position with time The magnitude of velocity is speed. • Force – a push or a pull

12. Newton’s first law of motion: (Start version review) An object that is free of external influences moves in a straight line and covers equal distances in equal times. Now let us use physical terms: Newton’s first law of motion An object that is not subject to any outside forces moves at a constant velocity.

13. Questions: • How does a skater start or stop moving? • He needs a push or pull to start or stop. • How does a skater respond to a push? • He changes his velocity. • Do all skaters respond equally to equal pushes? • Kids respond more quickly than adults do.

14. More physical quantities: • Mass – measure of an object’s inertiaEverything has a mass. • Acceleration – change in velocity with time.Acceleration is a vector which has the same direction as the force causing it.Deceleration is actually a type of acceleration.

15. Examples of acceleration: • A car is getting into the highway. • A car is going to a stop sign. • A car is shifting to the left to pass another car. • An elevator is leaving the first floor. • A ball is dropping from a window. • A cart is running down an incline. • The moon is circling the earth. • However, an object that is stationary or has a constant velocity is not accelerating.

16. Net force – sum of all the individual forces exerted on an object. Relation between net force, mass and acceleration: Cause Effect Resistance or in algebra

17. Newton’s second law of motion An object’s acceleration is equal to the net force exerted on it divided by its mass. That acceleration is in the same direction as the net force. Demo: Hammer, blocks, and hand

18. Question: You are given two black bottles, one empty and the other with water in it. How do you distinguish them without lifting them?

19. Read: Ch1: 1 Homework: Ch1: E8;P1 Due: September 5

20. August 29: Skating − Measurement and units

21. Newton’s second law of motion An object’s acceleration is equal to the net force exerted on it divided by its mass. That acceleration is in the same direction as the net force. Question: How do we calculate the net force if an object which receives more than one forces? Answer: Because forces are vectors, we must find the net force using the rule of addition of vectors.

22. Vector addition: Vector subtraction:

23. Measurement and units: • The SI (Systéme Internationale d’Unités) unit has been adopted as the standard unit system in physics. The three basic SI units in mechanics:

24. Merits of the SI units: • Different units for the same quantity are related by factors of 10, 100, 1000, … • There are only a few basic units (meter, kilogram, second for mechanics). • Examples: SI units 1 kilometer = 1000 meter 1 meter = 100 centimeter = 1000 millimeter 1 kilogram = 1000 gram English units 1 mile = 5280 feet = 1760 yard 1 foot = 12 inch 1 pound = 16 ounce

25. Derived units:

26. The SI units and the English units:

27. How to change a unit: • Example: 65 mile/hour = ? meter/second Answer:

28. Question: You have a mass of 80 kilogram and you are standing on ice. Your friend starts to push you with a force of 10 Newton. How much is your acceleration? Answer:

29. Read: Ch1: 1 • Homework: • Dr. Wang’s height is 1.74 meters. His mass is 76.5 kilograms. Please express his height in foot+inch, and his mass in pound mass. • Change the units: • i) 60 mile/hour = ? foot/second = ? kilometer/day • ii) 170 pound force = ? Newton • Due: September 5

30. September 3: Falling Balls − Gravity

31. Observations about the facts on falling balls: • When you drop a ball, it • begins at rest, but acquires downward speed. • covers more and more distance each second. • When you toss a ball straight up, it • rises to a certain height and comes briefly to a stop. • begins to descend, like a dropped ball. • A thrown ball travels in an arc.

32. Question 1: Why does a dropped ball fall downward? Gravity and weight • Gravity is a physical phenomenon that exerts a force on the ball. This force is the ball’s weight. • The earth’s gravity produces the ball’s weight. The weight points toward the earth’s center. • The ball’s weight causes it to accelerate downward.

33. Question 2: Do different balls fall at different acceleration? Weight and mass • An object’s weight is always proportional to its mass. • Near the surface of the earth,whichis called the “acceleration due to gravity”. weight = mass · acceleration due to gravity in algebra (Demo using weights and a spring scale)

34. Acceleration due to gravity • Why is the name? • weight/mass = force/mass = acceleration • Acceleration due to gravity is indeed an acceleration. • 9.8 Newton/kilogram = 9.8 meter/second2 • On the surface of the earth, all falling balls accelerate downward at • Therefore different balls fall at the same acceleration.

35. Question: Dr. Wang’s mass is 76.5 kilogram. How much is his weight on the earth? How much is his weight in the far space? If he falls down from a cliff, how much is his acceleration?

36. More about gravity: the law of universal gravitation gravitational Near the surface of the earth,

37. Acceleration due to gravity (g) varies: • At mountains • Shape of the earth: oblate spheroidAt the equator (r = 6378 km), at the poles (r = 6357 km) • At the moon • Dig a hole?

38. Read: Ch1: 2 Homework: Ch1: P9,10. Due: September 12

39. September 5: Falling Balls − Projectile motion

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41. The velocity of a falling ball Observation:A falling ball accelerates downward. Its acceleration is a constant. Its velocity increases in the downward direction. Question:How do we calculate the velocity of an object which has a constant acceleration? in algebra,

42. The position of a falling ball Observation:A falling ball accelerates downward steadily. Its altitude decreases ever faster. Question:How do we calculate the position of an object which is accelerating constantly? If the acceleration is constant, we can use “average velocity  time” to find the change of position. in algebra,

43. The position and velocity of a ball falling from rest Velocity: “-” means downward. Position:

44. The position and velocity of a ball thrown upward Velocity: Position:

45. Throwing a ball at an angle Simplification of physics: A vector can be separated into its horizontal and vertical components. The two components follow Newton’s laws of motion independently. • In the vertical direction, the ball is falling. It goes up initially at viy. • In the horizontal direction, the ball coasts at vix. Gravity only affects the ball’s vertical motion: Movie: Projectile motion

46. Question: A hunter is shooting at a monkey far away on a tree with a gun. If the monkey knows a little physics, when he sees the flash from the gun, should he 1) stay still on the tree, or 2) jump down from the tree?

47. Read: Ch1: 2 Homework: Ch1: P11,12,13. Due: September 12

48. September 8: Ramps − Newton’s third law of motion

49. How to Preapprove an Absence to Physics 100 • Email p-wang@wiu.edu before the planned absence. Your email should describe the event that prevents you from coming to the class. Usually you will get a quick reply from the instructor if the absence is approved. If it is not approved, a reason will be given in the reply. • In case of emergency you can leave at any time, but a document for the emergency is required thereafter. • Filing a WIU OARS (Online Absence Reporting System) should not be automatically treated as an approved absence because in this method there is no place for the instructors to reply back. • For prearranged university business, such as travel of athlete teams, appropriate document should be submitted in the beginning of the semester.

50. Question: A ball is resting on a table. The ball has weight, why doesn’t it fall into the table? Answer: The ball is pushing downward on the table, but the table is also pushing upward on the ball. The upward force exerted by the table on the ball is a support force, which equals the ball’s downward weight in magnitude. The ball is stationary because there is no net force on it.