Lecture 06: Friction

Physics 2210Fall Semester 2014

Announcements

● Schedule next few weeks:● 9/08 – Unit 3● 9/10 – Unit 4● 9/15 – Unit 5 (guest lecturer)● 9/17 – Unit 6 (guest lecturer)● 9/22 – Unit 7, Exam Review

● work practice exam in discussion● 9/24 – Exam 1, Units 1-6● 9/25 – Unit 7 HW due

Prelecture Feedback

Mechanics Lecture 6, Slide 3

● Go over differences between static and kinetic friction.

● Need examples of “block and pulley” problems.

● Will friction be on the exam? (Yes.) What equations will be on our formula sheet?

Exam Details● Coverage: Units 1-6

● 1D Kinematics● 2D Kinematics● Relative and Circular Motion● Newton's Laws● Free Body Diagrams● Friction

● Types of problems: Multiple choice and workout.

● Study recommendations:● Review homework● See practice exam on course webpage● Solve problems, problems, problems...

Friction

Always opposes the relative motion of two surfaces

Mechanics Lecture 6, Slide 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 acceleration to the left as shown, which of the following diagrams best

describes the static frictional force acting on the box:

Mechanics Lecture 6, Slide 6

µS

a

A B C

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:

Mechanics Lecture 6, Slide 7

µS

a

A B C

Flashcard 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?

A) Mg B)

sMg

C) TD) 0

Mechanics Lecture 6, Slide 8

M

f T

Flashcard 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?

A) Mg B)

sMg

C) TD) 0

Mechanics Lecture 6, Slide 9

M

f T

Example: Two Masses and Pulley, w/ Friction

a) If the blocks are in motion, what is the acceleration? b) For a given mass m

2

what mass m1 is required

to start the system sliding?

Example: Mass on a Ramp, with Friction

The mass m = 13.0 kg is given an initial velocity of v

0 = 4.0 m/s down the

ramp, which now has friction on its surface. The mass slides 2.5 meters before coming to rest. The angle = 22°. What is the magnitude of the frictional force? What is the coefficient of kinetic friction?

Example: Car on a Curve

How fast can the car go (at what constant speed) without skidding off of the road?

r

Additional problems forexam practice...

FBD (with Pulley!) Problem

Friction Problem