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

• Advanced reminder Exam #2 on Tuesday, October 11

FrictionWhy 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

Clicker Question Room Frequency BAYou 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?

A) ms = 0B) ms = 0.1C) ms = 0.5 D) mk = 0.5E) None of the above

Fnet = 0 = Fpush – Ffriction

0=Fpush – ms x Normalms x Mg = Fpush

ms = Fpush/Mg = 5000/10kms= 0.5

But actually, ms could be even larger since we do not know if we have

reached the maximum.

Surfaces µ (static) µ (kinetic)

Steel on steel 0.74 0.57

Glass on glass 0.94 0.40

Metal on Metal (lubricated) 0.15 0.06

Ice on ice 0.10 0.03

Teflon on Teflon 0.04 0.04

Tire on concrete 1.00 0.80

Tire on wet road 0.60 0.40

Tire on snow 0.30 0.20

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?

-

-

mgNmaF yynet , 0 mgN

mgFNFmaF kextkextxxnet , 0

mg

Fextk

M

m

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

mg

fmax = -μSN = -μSMg

T=mg (little block)T>msN (large block)

For motion to begin:mg > μSMg

Clicker Question Room Frequency BA

T

T

Mg

N=Mg

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

-

-

Clicker Question Room Frequency BA

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

A) N=mgB) N=mg sinqC) N=mg sin -q FextsinqD) N=mg –FextcosqE) None of the above

Fnet,y=may=0

0=N-mg+Fext sinq

N=mg-Fext sinq

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

N

Fg = mg θ

θ

mg sinθ

mg cosθ

+x

+y

f = μKN

cos, mgNmaF yynet

NmgmaF kxxnet sin,

cos0 mgN

cossin mgmgma kx

Circular Motion