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# Physics 218 Lecture 7

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1. Physics 218Lecture 7 Dr. David Toback Physics 218, Lecture VII

2. Before we begin • Exam 1 Next Tuesday • Only on topics through Chapter 4 • Homework and the Mini-practice Exam • When you have 100% on all of them you will have access to a mini-practice exam • Get a 100% on the mini-practice exam BEFORE the in-class exam, get 5 bonus points • Mini-practice exam is the right way to study! Physics 218, Lecture VII

3. Checklist for Today • Things that were due last Thursday: • Chapter 4 reading • Things that are due yesterday (Monday): • WebCT Prelim, Math Quizzes, Ch.2 HW • Things that are due today: • Reading for Chapter 5 • For this week and/or due next Monday: • Read Chapter 6 for Thursday • Recitation: Probs from Chap 3&4 • Lab 3 • All Ch3&4 problems on WebCT due Monday Physics 218, Lecture VII

4. Chapters 5 & 6 • 3 Lectures for Chapters 5 & 6: • One lecture on Chapter 5 • One lecture on Chapter 6 • One lecture on problem solving • Lecture Reading Assignments • Chap 5 due today • Chap 6 due Thursday Today’s lecture not on the exam Physics 218, Lecture VII

5. Overview of Chapter 5 Where we’re going and why • Dynamics vs. Kinematics • Force • Newton’s Laws of Motion • Normal Force • Example problems • Note: It’s important to be good at 2-Dimensional motion at this point Physics 218, Lecture VII

6. Physics 218, Lecture VII

7. 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 0 to 60mi/hr in 6 sec? • New:How much force does your car engine need to exert to do it? • Use all the kinematics, vectors and calculus from Chapters 1-4 • Plan: Do the concepts, then do the problems Physics 218, Lecture VII

8. 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 Physics 218, Lecture VII

9. First Concept What is a Force? • Examples: • Push • Pull • Slap • Gravity • Others? Physics 218, Lecture VII

10. 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” Physics 218, Lecture VII

11. Translate that into English: Force To cause an acceleration (change the velocity) requires a Force or If there is an acceleration, there must be a net Force • Force is a Vector • Add up all the forces (vectors) to find the Net (or total) force Physics 218, Lecture VII

12. Newton’s First Law • Example of non-zero net forces: • Friction: Makes a moving block a slow down • Gravity: Makes a ball fall toward the earth • Example of zero net force • Car just sitting on the pavement • No velocity, no acceleration→no net force • Rocket ship in outer space • Nothing to slow it down → constant velocity → no net force Physics 218, Lecture VII

13. 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” Physics 218, Lecture VII

14. Translate: Newton’s Second Law The acceleration is in the SAME direction as the NET FORCE • This is a VECTOR equation • If I have a force, what is my acceleration? • More force → more acceleration • More mass → less acceleration Physics 218, Lecture VII

15. 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 V within a distance of D? V0 = V VF = 0 X0 = 0 XF = D Physics 218, Lecture VII

16. Getting to Newton’s Third Law How does one apply a force? • Applying a force requires another object! • A hammer exerts a force on a nail Physics 218, Lecture VII

17. 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” Physics 218, Lecture VII

18. Skater • Skater pushes on a wall • The wall pushes back • Equal and opposite force • The push from the wall is a force • Force provides an acceleration • She flies off with some non-zero speed Physics 218, Lecture VII

19. Walking When you walk you push on the ground and the ground PUSHES you forward • Equal and opposite force Physics 218, Lecture VII

20. 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! Physics 218, Lecture VII

21. Moving towards Problems Done with the concepts for this chapter • Overheard… • “I understand the concepts, but I can’t do the problems” • Sigh… No exam points for “concepts you can’t apply” Physics 218, Lecture VII

22. Free Body Diagrams Same tricks as in Chapters 1-4: • Draw a diagram: Draw each force on an object separately! Force diagram! • Break each force into its 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 Physics 218, Lecture VII

23. Pulling a box A box with mass m is pulled along a frictionless horizontal surface with a force FP at angle Q as given in the figure. Assume it does not leave the surface. • What is the acceleration of the box? • What is the normal force? FP Q Physics 218, Lecture VII

24. 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 VII

25. Rest of This Week • Recitation: • Problems from Chapters 3 and 4 • Lab: Tensile Strength • Reading for Thursday: • Read Chapter 6: More on force and friction • HW3&4: Due Monday • Are you ready for Exam 1 next Tuesday? • Use WebCT to study! • Get perfect scores on all Math and HW1-HW4 WebCT mini-practice exam becomes available • Get a 100 on it BEFORE Tuesday’s exam and I’ll give you 5 bonus points on the exam Physics 218, Lecture VII

26. End of Lecture Notes Physics 218, Lecture VII

27. The weight of a box A box with mass m is resting on a smooth (frictionless) horizontal table. • What is the normal force on the box? • Push down on it with a force of FP. Now, what is the normal force? • Pull up on it with a force of FP such that it is still sitting on the table. What is the normal force? • Pull up on it with a force such that it leaves the table and starts rising. What is the normal force? FP Physics 218, Lecture VII

28. Atwood Machine Two boxes with masses m1and m2 are placed around a pulley with m2 >m1 • What is the acceleration of the boxes? • What is the tension of the strings between the boxes? Ignore the mass of the pulley, rope and any friction. Assume the rope doesn’t stretch. Physics 218, Lecture VII

29. How to study for the exam • Do all the HW problems • 4 Problems. Dominated by Chapter 3, almost nothing from Chapter 1. • Make sure you can do all the HW problems easily and by yourself with no numbers • Concentrate on the “hardest” problems. Those will be the most like the ones on the exam. Physics 218, Lecture VII

30. Example 1 Force to accelerate a fast car Estimate the net force needed to accelerate a 1000kg car at ½g Physics 218, Lecture VII

31. Hockey Puck • Which of these three best represents a hockey puck in the real world? Physics 218, Lecture VII

32. Example 4-16 Physics 218, Lecture VII

33. Before we begin • Exam Thursday • Formula sheet to be passed out 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 • However, I’ll try to make it a good review Physics 218, Lecture VII

34. Mass vs. Weight • Mass is a property of the body • Aside: What is mass is a fundamental question at the forefront of physics today. Particle physics! String theory! • Weight is a Force and depends on the mass What is the difference between: • Your mass on the moon and the earth? • Your weight on the moon and the earth? • Your weight in a space ship and the earth? Physics 218, Lecture VII

35. Next Time • Exam Thursday: Here during regular time • Bring a calculator and ruler • Formula sheet will be provided (same as on the web) • Will cover: • Chapter 1 (1-6) • Calculus 1 • Chapter 2 (1-7) • Chapter 3 (1-8, 10) • Finish HW 3 to study for Exam • Labs/Rec/Web quizzes as usual Physics 218, Lecture VII