1. Classical Mechanics Lecture 6
Today’s Concept:
Friction

2. Midterm Redux Seems like time is a big issue!!! Practice makes faster…
Average=18.6/20

3. Friction

4. Friction

5. Friction

6. Always opposes the relative motion of two surfaces
Friction
Always opposes the relative motion of two surfaces

7. Friction

8. Static Friction

9. Static Friction

10. Static Friction

11. Without Friction

12. Friction

13. Friction

14. Friction

15. Friction

16. Friction

17. Friction

18. Friction

20. Main Points

21. Main Points

22. Pre-lecture thoughts

23. Microscopic explanation of Friction
What is Friction?
Why can’t we walk
through walls?
Basically the same
answer to both questions…
Electron clouds of atoms repel (or bond to) each other

32. Checkpoint
A box sits on the horizontal bed of a moving truck. Static friction between the box and the truck keeps the box from sliding around as the truck drives.
If the truck moves with constant acceleration to the left as shown, which of the following diagrams best describes the static frictional force acting on the box: S a A B C

33. CheckPoint S a
If the truck moves with constant accelerating to the left as shown, which of the following diagrams best describes the static frictional force acting on the box:
56% correct A B C
A) In order to keep the box from sliding to the back of the truck as it accelerates, the frictional force needs to pull/push the box forward.
B) Friction always opposes motion/acceleration.

34. Clicker Question
A box of mass M sits on a horizontal table. A horizontal string having tension T applies a force on the box, but static friction between the box and the table keeps the box from moving. What is the magnitude of the total force acting on the box? M f T
A) Mg B) mMg C) T
D) 0
Since acceleration is zero.

35. Clicker Question
A box of mass M sits on a horizontal table. A horizontal string having tension T applies a force on the box, but static friction between the box and the table keeps the box from moving.
What is the magnitude of the static frictional force acting on the box? M f T
A) Mg B) mMg C) T
D) 0
Since the box is not moving the forces must be equal, otherwise there would be an acceleration.
38% correct

36. CheckPoint
47% got this right on first try 36

37. Checkpoint

38. 38% got this right on first try

39. Checkpoint

40. CheckPoint m2 m2 Case 1 g Case 2 g m1 m1
A block slides on a table pulled by a string attached to a hanging weight. In Case 1 the block slides without friction and in Case 2 there is kinetic friction between the sliding block and the table. m2 m2
Case 1
(No Friction) g
Case 2
(With Friction) g m1 m1
In which case is the tension in the string biggest?
A) Case B) Case C) Same
65% got this right on first try 40

41. m2 m2
Case 1
(No Friction) g
Case 2
(With Friction) g m1 m1
In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same
B) M1 will not be accelerating as fast in case two than in case one because there is the extra force of friction acting against mass 1. Since the acceleration is smaller in case two, there has to be more of a force acting against gravity, the only other possible force is Tension. 41

42. Resume here

43. Lets work it out
A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk. What is the tension in the string? m2 g m1 43

44. Tension in String
A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk. What is the tension in the string? T1 m2 T2 g
What is the relationship between the magnitude of the tension of the string at block 2 and the magnitude of the tension in the string at block 1?
A) T1 > T2 B) T1 = T2 C) T1 < T2 m1 44

45. Acceleration of coupled blocks
A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk. What is the tension in the string? m2 g m1
What is the relationship between the magnitudes of the acceleration of the two blocks?
A) a1 = a2 B) a1 < a2 C) a1 > a2 45

46. m2
1) FBD N T m2 g f T m1 m1
m2g
m1g 46

47. m2 1) FBD 2) SF=ma N T m2 g f T m1 m1 m2g N = m2g m1g T – m m2g = m2a
m1g – T = m1a
add
m1g – m m2g = m1a + m2a
m1g – m m2g
a =
m1 + m2 47

48. T is smaller when a is bigger
1) FBD
2) SF=ma N T m2 g f T m1 m1
m2g
m1g
m1g – T = m1a
m1g – m m2g
T = m1g – m1a
a =
m1 + m2
T is smaller when a is bigger 48

49. Accelerating Blocks

50. Accelerating Blocks

51. Carnival Ride

52. Carnival Ride

53. Accelerating Truck

54. Accelerating Truck

55. Mass on Incline

56. Mass on Incline

57. Mass on Incline

58. Mass on Incline 2

59. Mass on Incline 2

60. Mass on Incline 2

61. Mass on Incline 2

62. Block

63. Pushing Blocks