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# MECHANICS PHYS 218

Texas A&M

GPA 3.77

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This 54 page Class Notes was uploaded by Miss Elbert Nader on Wednesday October 21, 2015. The Class Notes belongs to PHYS 218 at Texas A&M University taught by Alexei Safonov in Fall. Since its upload, it has received 50 views. For similar materials see /class/225901/phys-218-texas-a-m-university in Physics 2 at Texas A&M University.

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Date Created: 10/21/15

Physics 218 Lecture 4 Kinematics Alexei Safonov Today s Lecture Finish up some math issues ntegrals Problem solving techniques Motion in 1Dimension continued No longer going to use numbers Examples Stopping a speeding car Free fall Catching a speeder Important Equations of Motion If the acceleration is constant XXO Position velocity and Acceleration are vectors More on this in Chap 3 Problem with Derivatives A car is stopped at a traffic light It then travels along a straight road so that its distance from the light is given by xt bt2 ct3 b 240 msz c 012 ms3 Calculate instantaneous velocity of the car at t 0510 s How long after starting from rest is the car again at rest Getting Displacement from Velocity For const acceleration the Equation of motion XX0V0t 12at2 If you are given the vx velocity vs time graph 0 you can find the total distance traveled from the area under the U09 curve 39 x x0 Can also find this from integrating x A car is moving with velocity given by a formula vtabtct2 a b c are known Is this information sufficient to calculate the position of the car at some tT x A Yes position can always be calculated using xt lvt dt X B No because one also has to know acceleration at I C No because one also has to know initial position of the object eg at t0 X D No because acceleration is not constant Definite and Indefinite Integrals How to you calculate the Value of an integral In many ways an integral is an antiderivitz ve 3 wab where bis an arbitrary constant and is added to the right side dct b 2 C dt If I know where my region of integration begins and ends of the equation gt assuming a b and c are constants f cdt ctlt Cb ca Some Integrals Make this more general Iadt 2 at 6 m1 J atm dt a m1c Check quot anti derivitive quot Jatm dtaqm l 0 tm1 1 m1a m10 2 arm Our Example with Const Acceleration XX0 ivdt v0 atdt vot at25 v0tat2 How quickly can you stop a car You re driving along a road at some constant speed V0 and slam on the breaks and slow down with constant deceleration a How much time does it take to stop How far do you travel before you come to a N4 h braid g f When you hit t e brakes Where you stop Free Fall Free fall is a good example for one dimensional problems Gravity Things accelerate towards earth with a constant acceleration ag98ms2 towards the earth We come back to Gravity a lot You throw a ball upward into the air with Throw a Ball up 110 lg aw initial velocity V0 Calculate The time it takes to Its highest point the top Distance from your hand to the top Time to go from your hand and come back to your hand Velocity when it reaches your hand Time from leaving your hand to reach some random height h Q gt lt 6 gtKlt 0 Problem 0 Show that for constant acceleration 2aAX VE V02 Speeder A speeder passes you a police officer sitting by the side of the road and maintains their constant velocity V You immediately start to move after the speeder from rest with constant acceleration a How much time does it take to ram the speeder How far do you have to travel to catch the speeder What is your final speed 4 Police 0 cer Speed 139 X Graphs Describe motion in each point Direction Velocity Acceleration x cm t S Next Week Reading and Lecture Chapter 3 Motion in two or three dimensions Reading Quiz at the beginning of class w clickers Pay attention to Discussion Questions Q3136 amp 0317318 Recitation and Homework HW Chapter 1 HW is due this Monday at noon Recitation Chapter 2 and Quiz One of the problems from Ch2 HW YampF 295 Challenge Problems Catching the bus a student is running at 5ms to catch a bus When it is 40 m to the bus the bus starts moving with constant acceleration a0170 m Questions For how much time and what distance does the student have to run to overtake the bus When she reaches the bus what is the bus velocity Sketch an xt graph for both the student and the bus First question has two solutions What do those mean and what is the speed of the bus at that moment What is the minimum speed the student must have to catch up with the bus Car Crash Test Design You are designing a crash test setup for a car maker You can accelerate a car from rest with a constant acceleration of 100 ms2 so you can make the car crash into a wall This is the last time you Will see numbers in a problem in lecture 1 lfthe path is 200m long what is the velocity of the carjust beforeas it hits the wall 2 For the same acceleration if you want the car to hit the wall with a speed of 30ms about 60 mihr what minimum length must you have YampF Problem 29 A car is stopped at a traffic light It then travels along a straight road so that its distance from the light is given by xt bt2 ct3 b 240 msz b 012 ms3ct Calculate instantaneous velocity of the car at t 0510 s How long after starting from rest is the car again at rest Physics 218 Lecture 7 Dynamics Alexei Safonov Before we begin Exam 1 Next Monday Only on topics through Chapter 3 Homework and the Minipractice Exam HW2 was due today HW3 due Monday If you have 7Q on each weekly HW assignment you can get bonus pomts by working on a minipractice exam Mini practice exam is now available on MP it is completely optional and has to be completed BEFORE the inclass exam in order to receive points To get any bonus points you need to get 80 on the mini practice exam so that you will get between 4 ad 5 points If you do it with less than 80 you won t get any points Minipractice exam is a good way to test yourself before the exam Try it as if it were a real inclass test Set aside about an hour and a half and try to get it done within that time frame Chapters 4 amp 5 3 Lectures for Chapters 4 amp 5 One lecture on Chapter 4 One lecture on Chapter 5 One lecture on problem solving Lecture Reading AssignmentsClicker Quizzes Chap 4 today Chap 5 Wednesday Overview of Chapter 4 Where we re going and why Dynamics vs Kinematics Force Newton s Laws of Motion Mass Normal Force Example problems Note It s important to be good at 2 Dimensional motion at this point Where we re going and why Moving from How things move Kinematics To Why things move that way Dynamics Why do you care Different questions Old What acceleration do you need to go from O to 60mihr in 6 sec New How much force does your car engine need to exert Use all the kinematics vectors and calculus from Chapters 13 Plan Do the concepts then do the problems Newton s Laws You shouldn t memorize them rather you need to be able to understand and use them Don t write them down from the overheads they re in your book We re going to translate them into English Big picture Force Our First Concept What is a Force Examples Push Pu Slap Gravity Others Newton s First Law Every body continues in it s state of rest or of uniform speed in a straight line unless acted on by a non zero net force Translate that into English Force To cause an acceleration change the veloc1tv reqwres a Net Force or If there is an acceleration there must be a net Force 9Force is a Vector 9Add up all the forces vectors to find the Net or total force Newton s First Law Example of nonzero net forces Friction Makes a moving block slow down Gravity Makes a ball fall toward the earth Example of zero net force Carjust sitting on the pavement No velocity no acceleration gt no net force Rocket ship in outer space Nothing to slow it down gt constant velocity gtno net force Newton s Second Law The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass The direction of the acceleration is in the direction of the net force action on the object Translate Newton s Second Law The acceleration is in the SAME direction as the NET FORCE gt This is a VECTOR equa on lfl have a force what is ZFX max Fy may my acceleration e gt More force gt more welght W mg acceleration gt More mass gt less acceleration Vector Equation Force to stop a car You are a car designer You must develop a new braking system that provides a constant deceleration What constant net force is required to bring a car of mass m to rest from a speed of Vwithin a distance of D Getting to Newton s Third Law How does one aggly a force Applying a force requires another object A hammer exerts a force on a nail Newton s Third Law Whenever one object exerts a force on a second object the second exerts an equal and opposite force OR To every action there is an equal and opposite reaction Skater Skater pushes on a wall The wall pushes back Equal and opposite force The push from the wall is amme Force provides an acceleration She flies off with some nonzero speed Walking round on the Person FPerson on the Ground FG She pushes on the ground and the ground PUSHES her forward Equal and opposite force Normal Force Consider a pen sitting on a table Is the force of gravity acting on it Is the pen accelerating What is the Force What is the difference between a force and the Net Force What keeps the pen from accelerating Clearly there is a second force that keeps it from accelerating Call this the normal force Free Body Diagrams Same tricks as in Chapters 13 1 Draw a diaqram Draw each force on an object separately Force diagram 2 Break each force into the X and Y components THEN sum Show your TA that you know the difference between a force and a component of force GREAT way to pick up partial credit Pulling a box A box with mass m is pulled along a frictionless horizontal surface with a force FP at angle as given in the figure Assume it does not leave the surface aWhat is the acceleration of the box bWhat is the normal force 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 doesn t stretch and is massless aWhat is the acceleration of the boxes bWhat is the tension of the strings between the boxes Next Time Reading for next time Read Chapter 5 More on force and friction Lecture Reading Assignment for clickers quiz Hints Q59515 amp Q524530 HW3 Recitation prep due Full HW Due Monday Are you ready for Exam 1 Complete all HW up to and including HW3 with 70 and do the minipractice exam on MP with gt80 total score If you do it BEFORE the exam next week you can get up to 5 bonus points Physics 218 Lecture 1 A Short Introduction Alexei Safonov Today Lecture Structure of the class Organizational issues Instructors textbooks meeting times Course requirements and grading Practical details Exams Homework and quizzes Reading assignments Recitations Physics 218 Basics Content Introduction to Classical Mechanics Concepts Problem Solving and Labs Requires high school level calculus Structure Lecture 2 per week ENPH 202 here Recitation 1 per week HELD 118 Lab 1 per week HELD 205 Instructors Lectures ENPH 202 MW 545pm Alexei Safonov me Email safonovphvsicstamuedu ENPH 428 9798451479 Office hours T 200300 PM Tentatively likely to change Recitation and labs in HELD building TA s will provide contact info at your first meeting next week First meeting on the upcoming week Online Info about PHYS218 PHYS218 main website httpphvsics218phvsicstamuedu My webpage httpfacultvphvsicstamuedusafonov218 Will include syllabus schedules due dates homework info these notes and all future announcements Will try to keep up to date but sometimes it will be behind so view this as a courtesy Materials Get them right now or the bookstore may run out of them But and Register your clicker see your handouD Register for Mastering Physics ASAP MP packet either comes with the textbook or available separately in the bookstore also can do online Textbook and lab manual see syllabus Grading Grading will be based on the following scales Exam1 75 points Exams 2amp3 100 points each Final 200 points Lab 100 points Recitation 100 points lnclass Quizzes and Lecture Assignments 50 points Homework 75 points Total 800 points I will curve all scores AFTER the final but will give you feedback along the way If your final score is higher than average of exams 1 3 final will be reweighted to have 275 max and exams 13 will be reweighted to give 200 max Lecture Structure You will have to routinely read material of each new Chapter before lecture Clicker Quizzes lnclass quiz at the beginning of lectures using clickers 23 multiple choice questions about the material of the lecture Several miniquizzes during lecture Quizzes contribute 50 points to your final score Typical average from last year 25 Homework Weekly assignments on Mastering Physics website check often Need to get 70 or more on every weekly assignment or you fail the entire course Each late portion of HW gets 10 reduction in score so if it s 3 days late and you did not yet start working on you assignment you are done with this class Important There is an option to give up on a problem in MP Don t use it If you give up on a couple of problems that can bring your score below 80 Instead move on to another problem and go back to this problem later Start working on HW problems early Last year average student s score about 67 out of 75 Recitations Quizzes You have to do most of the work on your HW before the recitation At recitations you will have a chance to ask questions about the HW problems Your instructors will walk you through 23 problems that attracted most questions In the last 10 minutes you will take a quiz Usually one of the problems in the current HW typically one of the difficult ones Average score from last year 80 out of 100 Labs Lab policies are uniform and set by the Department Implementation is in the hands of your TAs and they will inform you of all the details at your first session As a general rule no labs are to be dropped and you need an officially recognized excuse to redo a lab You need to get at least 70 for labs or you fail the entire course If you are retaking this course and have received 80 for labs on your last attempt I may accept your old score You need to submit a written request talk to people at Physics Front Office Window 4th floor of the office wing of ENPH Even if approved you still need to attend recitations do quizzes etc ll Average score for lab is about 85 points out of 100 last yeaD Lab Score Transfer TEXAS AampM UNIVERSITY COLLEGE OF SCIENCE DEPARTMENT OF PHYSICS LAB GRADE T0 Currenl Professor STUDENTS NAME STUDENT 1m PREV SEMYEAR COURSE PREV PROF Eml lddrul GRADE SIGNED BY DATE How To Do Well in PHYS218 Be prepared that this is one of very hard and time consuming classes Need typically 10 hours a week besides classes Best strategy Maximize scores in all service categories Labs recitations HW clicker quizzes some very limited extra credit opportunities Do your best on the exams They are what really matters most to what grade you get You won t do well on exams unless you train yourself in solving HW problems Do problems yourself if someone explains you the solution and you understood it that s not good enough lfyou are stuck ask for help with a similar problem if you had to get help with a HW problem solve 5 more similar ones from the book yourself Really helps to go beyond HW assignments and do more problems from the book You likely will have to struggle through some of the problems but that s the only way you will notice that soon things will start coming to you easier

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