1. Friction, air resistance1

2. How important is friction?
Think of the mousetrap powered vehicles
Think of your life
What would happen if there was no friction in the world? 2

3. What would change in your life if you lived in a world with no friction?3

4. The Evil Mr. Baker
And the Switch

5. FRICTION PARAGRAPH

6. Friction Paragraph
Identify a specific place and action you would be doing when all friction of the world is turned off.
Describe at least 4 things that would be altered related to what you are doing as a result of the loss of friction
Write a paragraph with opening and closing sentence, plus the above description of alterations
Due tomorrow 6

7. Next topic…
Write about what would be different in the way our species evolved over time if there was no friction on Earth.
Think of at least 4 things that would change about us if there was no friction

8. EFFECTS OF FRICTION ON YOUR LIFE

9. Dangers of friction Engines
Reduce or modify friction by introduction of lubricants
Smoothes out surfaces

10. Where does friction help in our lives?

11. Some Helpful examples Cleaning baked on food from pans/dishes
Placing a book on papers in front off a fan
Car tires gripping the road
Keeping mucus inside of the nose

12. Can you think of any more examples?

13. Hydroplaning
When a layer of water separates the tire from the rough road surface
Instance of no friction, results in no control over vehicle

14. DEFINITION OF FRICTION

15. Friction definition
A force that resists the motion of one material in sliding past another
Works with all states of matter

16. Friction Force applied opposite to the direction of motion
Amount of force depends on:
The roughness of the surfaces being put together
The force used to push surfaces together
Applied force
Friction force

17. HOW DOES FRICTION OCCUR?

18. What makes friction?
Force is a combination of physical interaction between the 2 surfaces
The meshing of peaks and valleys
The temporary bonds formed between molecules on the 2 surfaces 18

19. TYPES OF FRICTION

20. STATIC VS KINETIC VS ROLLING FRICTION20

21. Static friction
When an object is on a rough surface, an applied force is needed to overcome the frictional force keeping the object at rest
This frictional force is called static friction 21

22. Maximum static friction
Occurs at the point when the object is just about to move.
Depends on the normal friction requirements (surface conditions and amount of force pushing surfaces together)
Plus
The amount of force applied to object trying to make it move 22

23. Kinetic friction
The frictional force trying to slow down a sliding object in motion is called kinetic friction
Kinetic friction is always less than max. static friction (less bonding forces to overcome) 23

24. Rolling friction
Friction that occurs between surfaces as one or both move past each other
Rolling friction is much less than either of the other types
Tires and Road

25. DIRECTION OF THE FRICTIONAL FORCE25

26. Direction of friction Parallel to the surface contact
In a direction that opposes slipping between the two surfaces
Generally one surface does move, so friction opposes the motion of the other 26

27. DETERMINATION OF THE AMOUNT OF FRICTION

28. Calculation of friction
Ff = µ Fn
Ff = Force of friction
µ = Coefficient of friction
Fn = Normal force 28

29. How to calculate the frictional force
Calculate static and kinetic friction the same way using the same equation 29

30. Coefficient of friction
A constant that indicates the ability of the two surfaces to interact with each other
Usually a number between 0 and 1
The higher the #, the more interaction (rougher surfaces)
No units, a dimensionless value 30

31. Normal force
Reactive force of the “ground” pushing the surfaces together
Normal means perpendicular
Force’s direction is away from and perpendicular to surface 31

32. Determination of Normal Force
Ground pushes back with enough force to match both weight and foot
Fground (Normal force)
Weight
Fgravity
Foot 32

33. Calculating normal force
In this case the force of gravity = the reactive force of the ground pushing up (?)
The force of the ground pushing up = the normal force (it is = to the force pushing the surfaces together) 33

34. FORCES AND CONSTANT VELOCITY

35. Forces and constant velocity
What can be said about the forces on a block being pulled along rough ground at constant velocity?
Fapplied 35

36. The balance of forces
For something to move at constant velocity, the forces must be balanced.
Balanced means the net force on the object must = 0
Fapplied = Ffriction
Fgravity = ? ?
Ffriction
Fapplied
Fgravity 36

37. WORDS IN PROBLEMS AND FRICTION

38. Words that indicate amount of friction
Smooth surface indicates a frictionless surface
Any thing else indicates a surface that generates a friction force
Friction related values 38

39. The box problem
A 13.8 kg box is sliding horizontally at a constant velocity over a rough surface. It is being pushed by a hand with 59 N of force.
Draw a free-body diagram of the box
Determine the force of friction
Determine the normal force on the box
Determine the coefficient of friction between box and surface 39

40. Example question
A wooden block is placed on a smooth wooden tabletop. You find that you must exert a force of 14 N to keep the 40 N block moving at a constant velocity.
What is the coefficient of sliding friction for the block and table?
If a 20 N brick is placed on the block, what force will be required to keep block and brick moving at a constant velocity? 40

41. ADD FRICTION FORCE TO ALL PREVIOUS TYPES OF FORCE PROBLEMS
2nd Law, Third Law, Equilibrium…
ADD FRICTION FORCE TO ALL PREVIOUS TYPES OF FORCE PROBLEMS

42. Homework in textbook
Pg , 26 Pg

43. AIR RESISTENCE

44. Air creates a force
Each air molecule that hits the body creates an upward force
The combined effort of all these molecules pushing against the body creates a force opposing gravitational pull of Earth

45. Air Resistance Air resistance can play a significant role
at high speeds for all objects
at low speeds only for lightweight objects with a lot of surface area

46. On Earth, are objects that fall through the sky in freefall?

47. Air and fluids act the same way
Greater density of molecules in the fluid, therefore greater friction
Fluid friction differs from air resistance in whether the air molecules are impacting on the object or trying to slide past

48. TERMINAL VELOCITY

49. Is this man in free-fall?

50. Sky Diving 50

51. Terminal velocity The maximum velocity reached by a dropped object
Occurs when the downward gravitational force (weight) is matched by the upward force of air resistance 51

52. Factors that determine air resistance
Air resistance depends on mass, surface area and speed of object
Air resistance is more significant with:
Less mass
More surface area
At greater speeds

53. Terminal velocity At a high enough speeds,
the press of air pushing up on the falling object = the pull of gravity
The object will then no longer accelerate 53

54. If you dropped a spider, a penny and you off a tall building, which will land first?

55. Gliding ants, balloon spiders55