Physics 218 Lecture 9

1. Physics 218Lecture 9 Dr. David Toback Physics 218, Lecture IX

2. Checklist for Today • Things that were due last Thursday: • Reading for Chapter 6 • Things that were due Monday: • Chaps. 3 and 4 HW on WebCT • Things due for Wednesday’s Recitation: • Problems from Chap 3&4 • Exam review • Things for Thursday: • Study for Exam 1 • Mini-practice exam for Extra Credit Physics 218, Lecture IX

3. Exam 1 Exam 1 is Thursday during Lecture • Formula sheet provided (copy on web) • Topics: through Chapter 4 on Syllabus • Mostly full length problems with variables • Dominated by Chapter 3 & 4 type-problems Today’s lecture not on the exam Physics 218, Lecture IX

4. Other Notes • Lab 3 will not be formed during the normal lab time • It will not count as part of your physics grade • Since you need it for engineering class that class will worth with you to get what you need Physics 218, Lecture IX

5. Overview • Today’s lecture is about problem solving for Chapters 5 & 6 • Learn how to use everything we’ve learned so far to solve problems with: • Rope • Friction Physics 218, Lecture IX

6. Physics 218, Lecture IX

7. Rope Problems A 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 Physics 218, Lecture IX

8. Two Boxes and a Pulley You hold two boxes, m1 and m2, connected by a rope running over a pulley at rest. The coefficient of kinetic friction between the table and box I is m. You then let go and the mass m2 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 Physics 218, Lecture IX

9. Skiing Q 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 Q and needs an acceleration a in order to win the race What coefficient of friction, m, do her skis need to have? Physics 218, Lecture IX

10. 2 boxes connected with a string Two boxes with masses m1and m2 are placed on a frictionless horizontal surface and pulled with a Force FP. Assume the string between doesn’t stretch and is massless. • What is the acceleration of the boxes? • What is the tension of the strings between the boxes? M2 M1 Physics 218, Lecture IX

11. An Incline, a Pulley and two Boxes In the diagram given, m1 and m2 remain at rest and the angle Q is known. The coefficient of static friction is m and m1is known. What is the mass m2? Ignore the mass of the pulley and cord and any friction associated with the pulley m1 m2 Q Physics 218, Lecture IX

12. 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 Q, as shown in the figures. Assuming the sled doesn’t leave the ground and has a constant coefficient of friction, m, which is better? FP Physics 218, Lecture IX

13. Coming Up • Recitation on Wednesday: • Exam 1 Preparations • Exam 1 on Thursday • For Monday: • Get going on HW5 & 6 • For Lecture next Tuesday • Read Chapter 7 • Later next week: • Recitation on Chaps 5 & 6 • No Lab Physics 218, Lecture IX

14. End of Lecture Notes Physics 218, Lecture IX

15. 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, Q, should the road be banked so that no friction is required? Physics 218, Lecture IX

16. 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

17. 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 Physics 218, Lecture IX

18. 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

19. 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 • Today’s lecture not on the exam Physics 218, Lecture IX

20. 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

21. 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 Physics 218, Lecture IX

22. 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

23. Exam 1 Students • 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. Physics 218, Lecture IX

24. The Skier in slush A skier is going down a mountain which is at an angle Q. She has mass m but the snow is so slushy that she has constant velocity. What is the coefficient of friction m? Q Physics 218, Lecture IX

25. 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 = LsinQ. • What is the velocity of the ball? • Calculate the period of the ball. Physics 218, Lecture IX

26. 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. • What is the centripetal acceleration? • What is the centripetal force? Physics 218, Lecture IX