Physics 218, Lecture VIII 1

Physics 218Lecture 8

Dr. David Toback

Physics 218, Lecture VIII 2

Before we begin…

Exam is Tuesday at 8:00 AM

• Formula sheet provided (copy on web)

• Topics: through Chapter 3 on Syllabus

• Mostly full length problems with variables

• Dominated by Chapter 3 type-problems

Today’s lecture not on the exam

Physics 218, Lecture VIII 3

Yet more before we begin

• Note: Need to a 100% HW1 and 2 in WebCT to open HW3 on WebCT

• Homework 3 due on Monday• Mini-practice exam available for

perfect scores on Math Quizzes and HW1-3

• 5 Extra Credit exam points for a perfect score on the Mini-Practice BEFORE the exam on Tuesday

Physics 218, Lecture VIII 4

Chapter 4 & Chapter 5

Further applications of Newton’s Laws

• This time:–Inclined Plane–Friction

• Next time: –Uniform Circular Motion

Physics 218, Lecture VIII 5

Physics 218, Lecture VIII 6

Box on an inclined planeA box with mass m is placed on a frictionless incline

with angle and is allowed to slide down.

a) What is the normal force?

b) What is the acceleration of the box?

Physics 218, Lecture VIII 7

Friction

Two types of friction:1. Kinetic: The friction force

that slows things down2. Static: The force that makes

it hard to even get things moving

Physics 218, Lecture VIII 8

Refrigerator

• If you push a refrigerator when there is no friction what happens?

• In the real world what happens? Especially when it’s fully loaded and on a sticky kitchen floor?–When does static friction kick in?

–When does kinetic friction kick in?

Physics 218, Lecture VIII 9

Kinetic Friction• For kinetic friction, it turns out that the

larger the Normal Force the larger the friction. We can write

FFriction = KineticFNormal

Here is a constant• Warning:

– THIS IS NOT A VECTOR EQUATION!

Physics 218, Lecture VIII 10

Static Friction• This is more complicated• For static friction, the friction force can vary

FFriction StaticFNormal

Example of the refrigerator: – If I don’t push, what is the static friction

force?– What if I push a little?

Physics 218, Lecture VIII 11

Pulling Against FrictionA box of mass m is on a surface with coefficient

of kinetic friction . You pull with constant force FP at angle The box does not leave the surface and moves to the right.

1. What is the magnitude of the acceleration?

2. What angle maximizes the acceleration?

Physics 218, Lecture VIII 12

Exam 1 Results from Previous Semester

•Overall: Mean=52/75 or ~70%•Green: People who took the mini-practice exam: Mean = 56(+5)/75 or ~80%•Red: People who didn’t: Mean = 38/75 or ~50%

Advice: Test yourself BEFORE the exam. How? Get a 100 on the mini-practice exam by

yourself in as few tries as possible

Physics 218, Lecture VIII 13

Next Week• Tuesday at 8:00AM: Exam

– Formula sheet provided (copy on the web)– Topics through Chapter 3 on Syllabus– Bring your Section Number and UIN to

check in your exam• Thursday:

– Lecture: Problems from Chapters 4 & 5• HW3 Due Monday as exam preparation

– Don’t forget about extra credit!

Physics 218, Lecture VIII 14

Physics 218, Lecture VIII 15

Circular Motion Example

A ball of mass m is at the end of a string and is revolving uniformly in a horizontal circle (ignore gravity) of radius R. The ball makes N revolutions in a time t.

a)What is the centripetal acceleration?

b)What is the centripetal force?

Physics 218, Lecture VIII 16

Rope Problems

A PHYS218 rope is a perfect rope. It doesn’t stretch. This means:

• The acceleration of any part of a rope is the same as any other part

• The magnitude of the acceleration of the two things it attaches are the same

• The tension (Force) exerted by each end is the same

Physics 218, Lecture VIII 17

2 boxes connected with a stringTwo boxes with masses m1 and m2 are placed on a

frictionless horizontal surface and pulled with a Force FP. Assume the string between doesn’t stretch and is massless.

a)What is the acceleration of the boxes?

b)What is the tension of the strings between the boxes?

M2 M1

Physics 218, Lecture VIII 18

Is it better to push or pull a sled?You can pull or push a sled with the same force magnitude, FP, but different angles , as shown in the figures.Assuming the sled doesn’t leave the ground and has a constant coefficient of friction, , which is better?

FP

Physics 218, Lecture VIII 19

Further Applications

Now put it all together!

• Friction: Static and Kinetic

• Dynamics of Uniform Circular Motion

• Highway Curves, Banked and Unbanked

Physics 218, Lecture VIII 20

More…

• It has come to my attention that students are not going to Recitation having attempted the HW, or having brought questions. This is UNACCEPTABLE.

• I’m guessing this is because of my mistake on the syllabus. I’ll give it one week to correct itself. If not, I’ll take action.

Physics 218, Lecture VIII 21

Before we begin

• Exam Tuesday– I will give you a formula sheet

• A copy is already on web

– Here at 8:00AM– Only on topics through Chapter 3 on

Syllabus• Not circular motion!

• Today’s lecture not on the exam

Physics 218, Lecture VIII 22

Example 3-12

The Moon’s nearly circular orbit around the Earth has a radius of about 384,000km and a period T of 27.3 days.

1. Determine the acceleration of the Moon towards the Earth.

2. Determine the force

Physics 218, Lecture VIII 23

Example 5-1

A box of mass m = 10kg sits on a horizontal floor with coefficient of static friction s = 0.40 and coefficient of kinetic friction k=0.30. Determine the Friction Force acting on the box if a horizontal external force is exerted on it with a magnitude:

a) 0b) 20 Nc) 40 N

Physics 218, Lecture VIII 24

Next Time

• Reading: Section 3-9 and Chapter 5

• Lecture: Lots of problems

• HW4 Due, Labs/Rec/Web quiz as usual