1. Chapter 2: Motion and Forces

2. Gravity and Motion Gravity and motion
Aristotle believed that the rate an object fell depended on its mass
400 B.C. Greece

3. All Objects Fall with the same Velocity
1500s Galileo questioned Aristotle and said that all things fall at a constant rate
Acceleration Due to Gravity
Acceleration due to gravity is the same for all objects
Acceleration at a constant rate
9.8 m/s/s
Every second an object falls—its downward velocity increases by 9.8 m/s
Calculate the velocity of a falling object
Change in Velocity= acceleration due to gravity x time

4. Air Resistance Slows Down Acceleration
Air resistance is fluid friction
Example: Air resistance slows down flat paper more than it does a ball of paper

6. Air Resistance Slows Down Acceleration
Air Resistance Affects Some Objects More than Others
Amount of effect depends on the size and shape of the object
Forces of Air resistance acts against forces of gravity to produce a net force

7. Air Resistance Slows Down Acceleration
Acceleration Stops at the Terminal Velocity
Amount of air resistance increases with the speed of an object
Increases until air resistance is equal to forces of gravity
Net force equals at terminal velocity
Benefits: hail storms—hail is slowed by terminal velocity

9. Air Resistance Slows Down Acceleration
Free Fall Occurs When There is No Air Resistance
Free Falling: gravity is pulling down and no other forces are acting on it
Can only occur in a vacuum (without air)

10. Orbiting Objects are In Free Fall
Orbiting objects are not weightless
Always some gravitational forces acting upon you—always have some weight
Astronauts ‘float’ because of free fall

11. Objects are In Free Fall
Two Motions Combine to Cause Orbiting
Orbiting: traveling in a circular pattern around another object
Spaceships—moving forward and in freefall—causes orbiting
Astronauts fall with the space shuttle
Free fall appears as floating

12. Orbiting Objects are In Free Fall
The Role of Gravity in Orbiting
Centripetal force—circular paths caused by this unbalanced force
Centripetal force—”towards the center”

14. Projectile Motion and Gravity
Projectile Motion—curved path an object follows when thrown or propelled
Made up of horizontal and vertical motion
Motions are independent of each other/do not effect each other
Horizontal Motion
Motion parallel to the ground
Ex. Hands throwing ball
Velocity of horizontal motion is constant

15. Projectile Motion and Gravity
Vertical Motion
Motion perpendicular to the ground
Must aim above a target to hit the target
Gravity pulls down at acceleration of 9.8 m/s/s

16. Newton’s Laws of Motion
1668 Newton published Principia
Principia include Newton’s three laws of motion
1. Object in motion remains at rest/motion at a constant speed straight path—unless acted on by unbalanced force

17. Newton’s First Law of Motion
Part 1: Objects at Rest
Object Not Moving
Will not start moving without a force (push/pull)

18. Newton’s First Law of Motion
Part 2: Objects in Motion
Object in motion will continue to travel in the same direction forever at the same speed and direction

19. Newton’s First Law of Motion
Friction and Newton’s First Law
Friction stops motion
Because of friction objects don’t travel forever

20. Newton’s First Law of Motion
Inertia is Related to Mass
Inertia is related to mass of an object
1st Law: Objects have a tendency to resist change in motion (inertia)

21. Newton’s First Law of Motion
Mass is the Measure of Inertia
Large mass means large inertia
Easier to change motion of smaller objects

22. Newton’s Second Law of Motion
The acceleration of an object depends on the mass of the object and the amount of force applied.
Part 1: Acceleration Depends on Mass
Takes more force to increase the acceleration of a more massive object then a less massive object
Ex. It is easier to push a empty shopping cart than a full shopping cart.

23. Newton’s Second Law of Motion
Part 2: Acceleration Depends on Force
Acceleration increases as force increases and decreases as force decreases
Acceleration is in the direction of the applied force

24. Newton’s Second Law of Motion
Expressing Newton’s Second Law Mathematically
Acceleration = Force/mass
Force = mass X acceleration
Equation demonstrates acceleration due to gravity (9.8 m/s/s)
Example: Apple and watermelon

25. Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first
All forces act in pairs (equal size/strength and opposite direction)
Action/Reaction pairs occur even when there is no motion
Ex. Sitting in a chair; your body pushes down on the chair, the chair pushes up on your body

26. Newton’s Third Law of Motion
Force Pairs Do Not Act on the Same Object
If action/reaction occurred on the same object the net force would be zero and there would be no motion
Action/reaction occurs on different objects
Ex. Action—swimmers hands on the water; Reaction—water on the hands

27. Newton’s Third Law of Motion
The Effect of a Reaction Can Be Difficult to See
Gravity pulls falling objects to the Earth (action)
Earth is also pulled towards the ball (reaction)
Because the ball is small compared to the Earth it is difficult to see/feel the small acceleration force
Difficult to see Newton’s Third Law on falling objects

28. Newton’s Third Law of Motion
More Examples of Action and Reaction Force Pairs
Examples: Rabbit leg against Earth/Earth against leg
Example: Shuttle thruster gasses push down/gasses push the shuttle upwards

29. Momentum Is a Property of Moving Objects
Momentum: property of a moving object that depends on the object’s mass and velocity
Example: More difficult to stop a truck than a car traveling at the same speed
Momentum Is Conserved
Conservation of momentum: Any time two or more objects interact they exchange momentum—the total amount of momentum remains the same
Ex. Cue ball transfers all of its momentum to the #8 billiard ball

30. Momentum Is a Property of Moving Objects
Conservation of Momentum and Newton’s Third Law
Every action has an equal and opposite reaction
Because the forces are equal energy is conserved

33. Friction: A Force That Opposes Motion
Friction: force that opposes motion between two surfaces that are touching
The Source of Friction
The Hills and Valleys of two surfaces stick together
Amount of friction effected by roughness of surface and force pushing them together

34. The Force of Friction Greater Force Creates More Friction
Friction depends on forces pushing surfaces together
More force means hills and valleys have closer contact
Less massive objects exert less force than more massive objects
Changing the amount of surface does not change the amount of friction

36. Types of Friction Types: sliding, rolling, fluid, static
Sliding Friction
Pushing an object over a surface
Effective opposition to motion
Ex. Pushing a dresser across a room, sledding, brakes on a bike, writing with caulk

37. Rolling Friction Friction associated with wheels
Opposition to movement is less with rolling friction less than sliding
Easier to move something on wheels
Examples: transportation vehicles

38. The Force of Friction Rougher Surfaces Create More Friction
More hills and valley makes for more ‘stick’
Increased friction more effective at stopping sliding

39. Fluid Friction
Fluid frictions can involve the interaction of a liquid (milk) or gases (helium)
Fluid friction less than sliding friction when comparing dry to wet floor
Fluid friction opposes travel through the fluid
Ex. Air against a car; water against a swimmer

40. Static Friction
Force applied to an object but the object does not move
Static friction balances the force applied
Replaced by another type of friction as soon as movement occurs

41. Friction Can Be Harmful or Helpful
Harmful: parts wear out, hole in your socks, erosion of topsoil
Helpful: Motion of a car, walking, writing with a pencil

42. Friction Can Be Harmful or Helpful
Some Ways to Reduce Friction
Lubricants: substance applied to surfaces to reduce friction
Examples: motor oil, wax, grease
Switch from sliding to rolling friction
Examples: ball bearings in wheel/axles
Smoother surfaces 42

44. Some Ways to Increase Friction
Make surfaces rougher
Examples: sand on icy road, batting glove to improve grip
Increase forces pushing together
Example: rock on magazine, pushing on sandpaper

46. Gravity: A Force of Attraction
All Matter Is Affected by Gravity
Gravity: A force of attraction between objects due to their masses
All objects have attractions but most are too small to cause movement
Earth is large enough to cause movement

47. Gravity: A Force of Attraction
All Matter Is Affected by Gravity
Earth’s Gravitational Force Is Large
Earth’s gravitational force is towards the center of the Earth—that’s why things fall down 47

48. Gravity: A Force of Attraction
The Law of Universal Gravitational
Isaac Newton given credit for discovery of gravity
The Core of an Idea
Connect the force keeping planets in orbit was same force that caused apples to fall
A Law is Born
Describes relationship between gravitational force, mass, and distance throughout universe

49. Gravity: A Force of Attraction
The Law of Universal Gravitational
Part 1: Gravitational Force Increases as Mass Increases
Gravitational force is small between objects with small masses and large between objects of large masses
Example: cat lighter than elephant
Example: Moon less massive than Earth; less gravitational pull on the moon 49

50. Gravity: A Force of Attraction
The Law of Universal Gravitational
Part 2: Gravitational Force Decreased as Distance Increases
As distance between objects increases forces decrease 50

51. Gravity: A Force of Attraction
Weight Is a Measure of Gravitational Force
Weight and Mass Are Different
Weight—measure of the gravitational force exerted on an object
Measured in Newtons
Mass—amount of matter an object is made of; value does not change