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Announcements. CAPA Set #6 due Friday at 10 pm This week in Section  Lab 2: Acceleration due to gravity Make sure to do the pre-lab and print materials before lab * Slight mismatch of pre-lab file and web input. Just one simple question added on web input. Follow web input.

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Announcements

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Announcements

• CAPA Set #6 due Friday at 10 pm

• This week in Section 

• Lab 2: Acceleration due to gravity

• Make sure to do the pre-lab and print materials before lab

• * Slight mismatch of pre-lab file and web input. Just one

• Read Chapter 5 Sections 5.1-5.5 (Circular Motion)

• Advanced reminder  Exam #2 on Tuesday, October 11

Friction

Why have you been ignoring me all this time?

Force resisting the movement of two objects in physical contact past each other.

Force of Friction depends on:

1. Characteristics of materials

2. Vertical force pushing the materials together

Coefficient of Friction (m)

When we try to push this

green block sideways…

The Force of Friction acts in the opposite direction to resist this pushing force.

Maximum Force of Friction =

coefficient of friction (m) x

Force pushing surfaces together (Normal Force)

Ff(max) = m N

Static versus Kinetic Friction

When an object is not moving (static), molecules can form more bonds and materials can deform into each other.

Thus, the coefficient of friction can be larger

(call it coefficient of static friction ms).

Once the object is moving, friction is lower

(call it the coefficient of kinetic friction mk).

Also, the force of friction never creates an acceleration,

it always acts to oppose motion

caused by some other effect.

Demonstration with Newton Meter and Blocks

Idealization – Not an Exact Law of Physics

Room Frequency BA

Clicker Question

You are pushing horizontally with a force of 5000 Newtons on a car that has a weight of 10,000 Newtons.

The car is not moving.

What can you say for certain about the coefficient of friction?

ms = 0

ms = 0.1

ms = 0.5

D) mk= 0.5

E) None of the above

Fnet= 0 = Fpush – Ffriction

0=Fpush – ms x Normal

ms x Mg = Fpush

ms = Fpush/Mg = 5000/10k

ms= 0.5

But actually, ms could be even larger since we do not know if we have reached the maximum.

A block of mass m is being pushed across a rough horizontal table. A constant velocity v is maintained with an external force Fext. What is μK?

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Room Frequency BA

Clicker Question

An object with mass M is resting on a rough table whose coefficients of static and kinetic friction are μS and μK, respectively.

Which of the following is a necessary condition to start the object in motion?

A) M > m

B) M < m

C) mg > μK Mg

D) Mg > μS mg

E) mg > μs Mg

fmax= -μSN = -μSMg

N=Mg

T

M

T

m

Mg

mg

T=mg (little block)

T>msN (large block)

For motion to begin:

mg > μSMg

Room Frequency BA

Clicker Question

A block of mass m is pulled across a rough (μK) flat table with a constant force Fext at an angle θ.

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What is the correct expression for the magnitude of the Normal force to use for calculating the friction force?

N=mg

N=mg sinq

N=mg sinq-Fextsinq

N=mg–Fextcosq

None of the above

Fnet,y=may=0

0=N-mg+Fextsinq

N=mg-Fextsinq

A block of mass m slides down a rough (μK) incline tilted at an angle θ from the horizontal.

f = μKN

N

mg cosθ

θ

+y

Fg = mg

+x

mg sinθ

θ

Circular Motion