Physics 218, Lecture VIII1 Physics 218 Lecture 8 Dr. David Toback

Physics 218, Lecture VIII2 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 VIII3 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 VIII4 Chapter 4 & Chapter 5 Further applications of Newton’s Laws This time: –Inclined Plane –Friction Next time: –Uniform Circular Motion

Physics 218, Lecture VIII5

6 Box on an inclined plane A 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 VIII7 Friction Two types of friction: 1.Kinetic: The friction force that slows things down 2.Static: The force that makes it hard to even get things moving

Physics 218, Lecture VIII8 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 VIII9 Kinetic Friction For kinetic friction, it turns out that the larger the Normal Force the larger the friction. We can write F Friction =  Kinetic F Normal Here  is a constant Warning: –THIS IS NOT A VECTOR EQUATION!

Physics 218, Lecture VIII10 Static Friction This is more complicated For static friction, the friction force can vary F Friction   Static F Normal Example of the refrigerator: –If I don’t push, what is the static friction force? –What if I push a little?

Physics 218, Lecture VIII11  Pulling Against Friction A box of mass m is on a surface with coefficient of kinetic friction . You pull with constant force F P 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 VIII12 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 VIII13 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 VIII14

Physics 218, Lecture VIII15 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 VIII16 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 VIII17 2 boxes connected with a string Two boxes with masses m 1 and m 2 are placed on a frictionless horizontal surface and pulled with a Force F P. 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? M2M2 M1M1

Physics 218, Lecture VIII18 Is it better to push or pull a sled? You can pull or push a sled with the same force magnitude, F P, 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? FPFP

Physics 218, Lecture VIII19 Further Applications Now put it all together! Friction: Static and Kinetic Dynamics of Uniform Circular Motion Highway Curves, Banked and Unbanked

Physics 218, Lecture VIII20 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 VIII21 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 VIII22 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 VIII23 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)0 b)20 N c)40 N

Physics 218, Lecture VIII24 Next Time Reading: Section 3-9 and Chapter 5 Lecture: Lots of problems HW4 Due, Labs/Rec/Web quiz as usual