1. Physics 218, Lecture VII1 Physics 218 Lecture 7 Dr. David Toback

2. Physics 218, Lecture VII2

3. 3 Before we begin Exam Next Tuesday –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

4. Physics 218, Lecture VII4 Overview of Chapter 4 Where we’re going and why –Dynamics vs. Kinematics Force Newton’s Laws of Motion Mass Normal Force Example problems

5. Physics 218, Lecture VII5 Where we’re going and why Moving from: “How things move” Kinematics To: “Why things move that way” Dynamics Why do you care? –You know how to calculate how fast you need to accelerate a car if you want it to go from 0 to 60mi/hr in 6 sec. –What you need to know is how hard the engine needs to work to do it. What FORCE you need. This builds on all the kinematics, vectors and calculus we’ve done before. We’ll develop the concepts, then learn to solve the problems.

6. Physics 218, Lecture VII6 Newton’s Laws Next we’ll do 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 transparencies, they’re in your book. We’re going to translate them into English Big picture: Force

7. Physics 218, Lecture VII7 Before we start What is a Force? Examples: –Push –Pull –Slap –Gravity –Others?

8. Physics 218, Lecture VII8 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”

9. Physics 218, Lecture VII9 Translate that into Engligh:Force To cause an acceleration requires a Force or If there is an acceleration, there must be a Force àForce is a Vector àAdd up all the forces (vectors) to find the Net force

10. Physics 218, Lecture VII10 Newton’s First Law Example of non-zero net forces? –Friction: Causing a rolling ball to stop –Gravity making a ball fall (Net force causes acceleration in the negative direction) Example of zero net force –Car sitting on the pavement No acceleration, must be no net force

11. Physics 218, Lecture VII11 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.”

12. Physics 218, Lecture VII12 Newton’s Second Law Translate this into English: –Net forces cause the velocity to change  If I have a force, what is my acceleration? Need the mass.

13. Physics 218, Lecture VII13 Force to stop a car What constant net force is required to bring a car of mass m to rest from a speed of V within a distance of X F ? X = 0 X = X F V 0 = V V = 0

14. Physics 218, Lecture VII14 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

15. Physics 218, Lecture VII15 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”

16. Physics 218, Lecture VII16 Example Skater pushes on the wall. The wall pushes back, that’s why she flies off with some non-zero speed.

17. Physics 218, Lecture VII17 Examples Force exerted by the person on the ground is equal and opposite to the force exerted by the ground on the person. She pushes on the ground and the ground PUSHES her forward

18. Physics 218, Lecture VII18 Normal Force Consider a pen on a table: –What about Gravity? Didn’t we just say that a pen near the earth would be accelerating due to gravity? –What is the acceleration? –What is the Force? –What keeps the pen from accelerating? Clearly there is a force which keeps it from accelerating. Call this the “normal” force!

19. Physics 218, Lecture VII19 Moving towards Problems We’re now done with the concepts for this chapter This is where the separation between the good students and bad students starts to get noticeable. “I understand the concepts, but I can’t do the problems” Just to be clear: The concepts were only the first half of the lecture. No points on the exams for knowing “concepts you can’t apply”

20. Physics 218, Lecture VII20 Free Body Diagrams This is the trick: 1.Draw all the forces on an object! 2.Then do the summing! 3.Sum all the forces in the X direction and the Y direction separately!!! This is a GREAT way to pick up partial credit on an exam problem. If you don’t know how to draw a force diagram, you don’t know how to solve the problem.

21. Physics 218, Lecture VII21 The weight of a box A box with mass m is resting on a smooth (frictionless) horizontal table. a)Determine the weight of the box and the normal force b)Push down on it with a force of F P. What is the normal force? c)Pull up on it with a force of F P such that it is still sitting on the table. What is the normal force? d)Pull up on it with a force such that it leaves the table and starts rising. What is the normal force?

22. Physics 218, Lecture VII22 Pulling a box FPFP  A box with mass m is pulled along a frictionless horizontal surface with a force F P at angle  as given in the figure. Assume it does not leave the surface. a)What is the acceleration of the box? b)What is the normal force?

23. Physics 218, Lecture VII23 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? 

24. Physics 218, Lecture VII24 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. a)What is the acceleration of the boxes? b)What is the tension of the strings between the boxes?

25. Physics 218, Lecture VII25 Atwood Machine Two boxes with masses m 1 and m 2 are placed around a pulley with m 2 >m 1 a)What is the acceleration of the boxes? b)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.

26. Physics 218, Lecture VII26 Next Time No Exam Thursday Thursday: Make sure you read –Section 3-9: Uniform Circular motion –Chapter 5: More force and friction –There will be a quiz. Finish HW 3 to study for Exam Labs/Rec/Web quizzes as usual

27. Physics 218, Lecture VII27 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.

28. Physics 218, Lecture VII28 Example 1 Force to accelerate a fast car Estimate the net force needed to accelerate a 1000kg car at ½g

29. Physics 218, Lecture VII29 Hockey Puck Which of these three best represents a hockey puck in the real world?

30. Physics 218, Lecture VII30 Example 4-16

31. Physics 218, Lecture VII31 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

32. Physics 218, Lecture VII32 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?

33. Physics 218, Lecture VII33 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