1. Stuff you asked about:
Relationship between friction and tension and how one effects other really confused me. can you show how to derive the equations from the force diagrams. I generally make mistakes on the formulas I just really want my comment to be shown in lecture some day... So here's my chance. Can you please explain the blocks and pulley problem? I'm having a lot of trouble with it. In fact, please explain everything. How much wood could a woodchuck chuck if a woodchuck could chuck wood This is madness! NO, THIS IS SPAAAR- Nope, actually, this is what it has to be to do what it does. Blocks and Pulley problems I thought this lecture was really easy. A neutron walked into a bar and asked, "How much for a drink?" The bartender replied, "For you, no charge." Wading thought the grammar of the saying "It is what it has to be to do what it does." I mean, how does that string of words even function as a sentence? I think I need to start remembering to do these before Wednesday at midnight. I think I'm delirious! It's a little scary that an entire semester of Physics 100 is the equivalent of only two weeks of Physics 211.

2. Today’s Concept: Friction
Physics 221 Lecture 6
Today’s Concept:
Friction

3. Stuff you asked about: 90 q mgcos(q) 90 q y mgsin(q) mg x
i would like to go over the x and y trig simplifications of newtons 2nd law on the ramp 90 q
90-q
mgcos(q) 90 q x y
mgsin(q) mg

4. Friction Direction: Always opposes the relative motion of two surfaces
Magnitude: Kinetic fk=mkN
Static: Does what needs to do up to a point:

5. 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 accelerating to the left as shown, which of the following diagrams best describes theC static frictional force acting on the box: S a A B C
“I was confused about the first checkpoint problem. The box should move towards the back of the truck if it is accelerating, but which way would that make the friction go?”

6. Clicker Question
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.
Friction is the only horizontal force that acts on the box:
A) True B) False S a

7. 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: A B C
A) The box is accelerating to the left so the force must be to the left.
B) Friction always acts in the direction to oppose motion so the force is to the right.

8. 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: A B C
Friction is the only horizontal force acting on the box. Since the box accelerates to the left, the force must be to the left.

9. 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 net force acting on the box? M f T
A) Mg B) mMg C) T
D) 0
Since acceleration is zero.

10. Checkpoint
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?
A) Mg B) mMg C) T
D) 0 M f T
B) F=uN. N = mg.
C) The force of friction must be equal to the tension force if it doesn't move

11. Could you please go over the friction having to do with a car going around a corner?

12. 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
60% got this right 12

13. Clicker Question m2 m2 Case 1 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) T
m1g – T = m1a
Case 2
(With Friction) g a m1 m1
T = m1g - m1a
m1g
What is the tension in the string in Case 1?
A) T = B) T = m1g C) T is between 0 and m1g 13 13

14. m2 m2
Case 1
(No Friction)
Case 2
(With Friction) m1 m1
In which case is the acceleration of the blocks biggest? A) Case 1 B) Case 2 C) Same 14

15. m2 m2
Case 1
(No Friction)
Case 2
(With Friction) m1 m1
In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same
B) Friction would slow down the acceleration in case two. Therefore, the tension must be greater. 15

16. 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 m1
“Can you go over the sliding box over a pulley in class? I feel like it's a trick question... And it probably is. I'm just thinking with common sense... which may be wrong also...” 16

17. Clicker Question
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? T2 m2 T1
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 17

18. Clicker Question
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? a2 m2 a1 m1
What is the relationship between the magnitudes of the acceleration of the two blocks?
A) a1 = a2 B) a1 < a2 C) a1 > a2 18

19. m2
1) FBD m2 m1 m1 19

20. m2
1) FBD N T m2 f T m1 m1
m2g
m1g 20

21. m2 1) FBD N 2) SF=ma T m2 f T m1 m1 m2g Block 2 m1g Block 1 N = m2g
T – m m2g = m2a
m1g – T = m1a
add
m1g – m m2g = m1a + m2a
m1g – m m2g
a =
m1 + m2 21

22. T is smaller when a is bigger
1) FBD
2) SF=ma N T m2 f T m1 m1
m2g
m1g
m1g – m m2g
T = m1g – m1a
a =
m1 + m2
T is smaller when a is bigger 22