Lect06.ppt

Physics 211 Lecture 6, Slide Physics 211 Lecture 6, Slide 11

Physics 211Physics 211Lecture 6Lecture 6

Today's Concept:Today's Concept:FrictionFriction

fmfm

http://www.youtube.com/watch?v=dfIwnDoWnrw


How could the Bears lose?How could the Bears lose?Stuff you asked about:Stuff you asked about:

go over boxes being pulled on a table by a string connected to go over boxes being pulled on a table by a string connected to a box hanging off the tablea box hanging off the table

The wheel rolling, not sliding.The wheel rolling, not sliding.

If you tape a piece of toast butter side up on the back of the cat If you tape a piece of toast butter side up on the back of the cat and drop the entire system, which law of the universe is and drop the entire system, which law of the universe is violated? But more to the point, can you go over in more detail violated? But more to the point, can you go over in more detail the derivations of the equations in the prelecture?the derivations of the equations in the prelecture?

In the case of the car going around the track, how are we able In the case of the car going around the track, how are we able to assume that the friction is counteracting the acceleration? It to assume that the friction is counteracting the acceleration? It makes sense but I don't fully understand it from a physics point makes sense but I don't fully understand it from a physics point of view. I thought centripetal acceleration pointed towards the of view. I thought centripetal acceleration pointed towards the center of the circle, thus to counter act it why wouldn't the static center of the circle, thus to counter act it why wouldn't the static friction force point outwards?friction force point outwards?

PuddingPudding


A box sits on the horizontal bed of a moving truck. Static A box sits on the horizontal bed of a moving truck. Static friction between the box and the truck keeps the box from friction between the box and the truck keeps the box from sliding around as the truck drives. sliding around as the truck drives.

PreflightPreflight

S

aa

If the truck moves with constant accelerating to the left as If the truck moves with constant accelerating to the left as shown, which of the following diagrams best describes the shown, which of the following diagrams best describes the static frictional force acting on the box:static frictional force acting on the box:

AA BB CC


S

aa


AA BB CC

A)A) The friction is causing the box to accelerate in the same The friction is causing the box to accelerate in the same direction as the truck. direction as the truck.

B)B) Frictional force is always in the opposite direction of the motion Frictional force is always in the opposite direction of the motion


S

aa


AA BB CC


A box of mass A box of mass MM sits on a horizontal table. A horizontal sits on a horizontal table. A horizontal string having tension string having tension TT applies a force on the box, but static applies a force on the box, but static friction between the box and the table keeps the box from friction between the box and the table keeps the box from moving. moving.

ActAct

What is the magnitude of the total force acting on the box?What is the magnitude of the total force acting on the box?

A) A) MgMgB) B) MgMgC) C) TTD) D) 00

f


A box of mass A box of mass MM sits on a horizontal table. A horizontal sits on a horizontal table. A horizontal string having tension string having tension TT applies a force on the box, but static applies a force on the box, but static friction between the box and the table keeps the box from friction between the box and the table keeps the box from moving. moving.

PreflightPreflight

What is the magnitude of the static frictional force acting on What is the magnitude of the static frictional force acting on the box?the box?

A) A) MgMgB) B) MgMgC) C) TTD) D) 00

f

Since there is no acceleration, Since there is no acceleration, the forces must be equal. the forces must be equal.


A block slides on a table pulled by a string attached to a A block slides on a table pulled by a string attached to a hanging weight. In Case 1 the block slides without hanging weight. In Case 1 the block slides without friction and in Case 2 there is kinetic friction between the friction and in Case 2 there is kinetic friction between the sliding block and the table. sliding block and the table.

In which case is the tension in the string biggest?In which case is the tension in the string biggest?

A) Case 1 B) Case 2 C) SameA) Case 1 B) Case 2 C) Same

PreflightPreflight

m2

m1

gCase 1Case 1(no friction)(no friction)

m2

m1

gCase 2Case 2(with friction)(with friction)


A 3kg block (mA 3kg block (m22) slides on a table pulled by a string attached ) slides on a table pulled by a string attached

to a 5kg mass (mto a 5kg mass (m11) hanging over the side. The coefficient of ) hanging over the side. The coefficient of

kinetic friction between the sliding block and the table is 0.3. kinetic friction between the sliding block and the table is 0.3.

What is the tension TWhat is the tension T22 in the string? in the string?

ActAct

m2

m1

gWhat is the relationship between the magnitude of What is the relationship between the magnitude of the tension of the string at block 2 and the the tension of the string at block 2 and the magnitude of the tension in the string at block 1?magnitude of the tension in the string at block 1?

A)A) T T11 > T > T22 B)B) T T11 = T = T2 2 C)C) T T11 < T < T22

TT22

TT11


In which case is the tension in the string biggest?In which case is the tension in the string biggest?

A) Case 1 B) Case 2 C) SameA) Case 1 B) Case 2 C) Same

m2

m1

gCase 1Case 1(no friction)(no friction)

m2

m1

gCase 2Case 2(with friction)(with friction)

B)B) In case 2, the acceleration of m1 is less than in case 1, In case 2, the acceleration of m1 is less than in case 1, which means there is a larger tension force holding it back in which means there is a larger tension force holding it back in case 2. case 2.

C)C) They both have the same pull from m1 They both have the same pull from m1





What is the tension in the string?What is the tension in the string?

Lets work it outLets work it out

m2

gm1





What is the tension in the string?What is the tension in the string?

ActAct

m2

m1

g

What is the relationship between the magnitudes of the What is the relationship between the magnitudes of the acceleration of the two blocks?acceleration of the two blocks?

A)A) a a11 = a = a22 B)B) a a11 < a < a2 2 C)C) a a11 > a > a22


m1

g

m2

m2

1)1) FBDFBD

2)2) F=maF=ma

TTNN

mm22gg

ffm1

TT

mm11gg


m1

g

m2

m2

1)1) FBDFBD

2)2) F=maF=ma

TTNN

mm22gg

ffm1

TT

mm11gg

2N m g

2 2T m g m a 1 1m g T m a

addadd

1 2 1 2m g m g m a m a

1 2

1 2

m g m ga

m m


m1

g

m2

m2

1)1) FBDFBD

2)2) F=maF=ma

TTNN

mm22gg

ffm1

TT

mm11gg

1 1m g T m a 1 2

1 2

m g m ga

m m

T is smaller when a is biggerT is smaller when a is bigger

1 1T m g m a

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