Physics 218 Lecture 9 Dr. David Toback Physics 218, Lecture IX 1 Overview Todays lecture is about problem solving for Chapters 4 & 5 Learn how to use everything weve learned so far to solve problems with: Rope Friction

Friction and Uniform Circular Motion Physics 218, Lecture IX 2 Physics 218, Lecture IX 3 Rope Problems A PHYS218 rope is a perfect rope. It is massless and it doesnt 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 Physics 218, Lecture IX 4 Two Boxes and a Pulley You hold two boxes, m and Ignore the mass of the pulley m, connected by a rope and rope and any friction running over a pulley at associated with the pulley

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? Physics 218, Lecture IX 5 Banking Angle You 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? Physics 218, Lecture IX 6 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? Physics 218, Lecture IX

7 Skidding on a Curve A 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? Physics 218, Lecture IX 8 2 boxes connected with a string Two boxes with masses m1 and m2 are placed on a

frictionless horizontal surface and pulled with a Force FP. Assume the string between doesnt 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 IX 9 An Incline, a Pulley and two Boxes

In the diagram given, m1 and m2 remain at rest and the angle is known. The coefficient of m2 static friction is and m1 is known. What is the mass m2? Ignore the mass of the pulley and cord and any friction associated with the pulley

Physics 218, Lecture IX m 1 10 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 doesnt leave the ground and has a

constant coefficient of friction, , which is better? FP Physics 218, Lecture IX 11 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 Physics 218, Lecture IX 12 Physics 218, Lecture IX 13 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 Todays lecture not on the exam Physics 218, Lecture IX 14 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 Physics 218, Lecture IX 15 Next time 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 Todays lecture not on the exam Physics 218, Lecture IX 16 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 Physics 218, Lecture IX 17 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 didnt: 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 atPhysics the end of my 218, Lecture

IX lecture notes 18 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 appointment For more information about all of these see: http://physics218.physics.tamu.edu/help/ Physics 218, Lecture IX

19 Overall: Mean=52/75 or ~70% Green: People who took the mini-practice exam. Mean = 58(+5)/ 75 (not shown) Red: People who didnt. Mean = 44/75 Bonus points will be added later. Students

Exam 1 Physics 218, Lecture IX 20 The Skier in slush A 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 Physics 218, Lecture IX friction ? 21 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. Physics 218, Lecture IX 22 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 IX 23