Physics 218 Lecture 9

Physics 218, Lecture IX 1

Physics 218Lecture 9

Dr. David Toback


Overview• Today’s lecture is about problem

solving for Chapters 4 & 5• Learn how to use everything we’ve

learned so far to solve problems with:– Rope– Friction– Friction and Uniform Circular Motion



Rope ProblemsA PHYS218 rope is a perfect rope. It is

massless and 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 equal

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


Two Boxes and a PulleyYou hold two boxes, m and

m, connected by a rope running over a pulley at rest. The coefficient of kinetic friction between the table and box I is . You then let go and the mass m is so large that the system accelerates

Q: What is the magnitude of the acceleration of the system?

Ignore the mass of the pulley and rope and any friction associated with the pulley


Banking AngleYou are a driver on

the NASCAR circuit. Your car has m and is traveling with a speed V around a curve with Radius R

What angle, , should the road be banked so that no friction is required?


Skiing

You are the ski designer for the Olympic ski team. Your best skier has mass m. She plans to go down a mountain of angle and needs an acceleration a in order to win the race

What coefficient of friction, , do her skis need to have?


Skidding on a CurveA car of mass m rounds a curve on a flat road of radius R at a speed V. What coefficient of friction is required so there is no skidding?Kinetic or static friction?


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


An Incline, a Pulley and two BoxesIn the diagram given, m1 and m2 remain at rest and the angle is known. The coefficient of static friction is and m1 is known. What is the mass m2?

m2 m1

Ignore the mass of the pulley and cord and any friction associated with the pulley


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


Next Week• HW4 Due Monday• Labs & Recitation as usual: Chapter 5

problems• For Lecture next Tuesday

– Reading• Chapter 6• Chapter 7, Section 1 & 2

– Reading Questions Q7.1 and Q7.2



Before we begin…• Exam this Thursday

– 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– 3 multi-part problems: Some short answer,

dominated by chapter 3 topics and full length problems using only variables

• Today’s lecture not on the exam


Yet more before we begin• HW1, HW2, HW3 (+ associated quizzes)

and math quizzes are now past due– If you need an extension, I need an email

• Mini-practice exam becomes available when you have gotten perfect scores on all of the above

• 5 Bonus points on the exam if you get a perfect score on the Mini-Practice BEFORE the exam on Thursday


Next timeThursday

– 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– 3 multi-part problems: Some short answer,

dominated by chapter 3 topics and full length problems using only variables

• Today’s lecture not on the exam


Thursday and Next Week• Thursday: Exam

– I will give you a formula sheet (new version on the web)– Here at 8:00AM– Only on topics through Chapter 3 on Syllabus– Make sure you know your section number and your UIN

• HW4 Due Monday• Labs & Rec as usual, work on Chapter 5• For Lecture next Tuesday

– Chapter 6– Chapter 7, Section 1 & 2– Reading Questions Q7.1 and Q7.2


Exam 1 Results from Last Semester

• Overall:Mean=52/75 or ~70%

• Green: People who took the mini-practice exam: Mean = 56(+5)/75

• Red: People who didn’t: Mean = 38/75

My advice is to study for the exam by finishing all the HW and testing whether you really understand by turning them

all in and getting a 100 on the mini-practice exam by yourself in as few tries as possible

Extra problems at the end of my lecture notes


Need help?• Supplemental Instruction is free and meets

3 times a week.• Helpdesk has hours everyday and is free.

– Hours are posted on my website• Private tutoring available by appointmentFor more information about all of these see:

http://physics218.physics.tamu.edu/help/


Exam 1• Overall: Mean=52/75

or ~70%• Green: People who

took the mini-practice exam. Mean = 58(+5)/75 (not shown)

• Red: People who didn’t. Mean = 44/75

• Bonus points will be added later.

Stud

ents


The Skier in slushA skier is going down a mountain which is at an angle . She has mass m but the snow is so slushy that she has constant velocity. What is the coefficient of friction ?


Conical Pendulum

A small ball of mass m is suspended by a cord of length L and revolves in a circle with a radius given by r = Lsin.

1. What is the velocity of the ball?

2. Calculate the period of the ball.


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?

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Physics 218 Lecture 9