Physics Lecture on Newton's Laws of Motion

1. Lecture 13 Newton’s 2nd Law (continued)

2. QUIZ • Find this vector’s magnitude and angle with respect to the positive x axis and draw it • Find the x- and y-components of the vector

3. Which of these is not a force discussed in this chapter? • 1 Tension Force • 2 Orthogonal Force • 3. Normal Force • 4. Thrust

4. Newton’s 2nd Law

5. Units • Weight is a force

6. What is the weight of a 170 lb person in Newtons? • 756N • 38.2 N • 1,666 N • 17.3 N

7. Using Newton’s 2nd Law • Need FNET • Make a free body diagram • Use particle model, object is a point dot located at the origin of coordinate system • Draw vectors representing each force acting on object • Draw vector representing FNET

8. Problem 4:34 • Free body diagram of your physics textbook sliding across the table

9. Free body diagrams • Make diagram of object showing all forces acting on it (as vectors). For now object will be represented by a point. • Decide on the x and y-axes • Find x- and y-components of each force • Find • Show FNET on diagram

10. Newton’s 3rd Law • If object A exerts a force FAB on object B, the object B exerts a force FBA = - FAB on object A. n W

11. Newton’s 3rd • Objects need not be in contact. • Example gravitational force sun exerts on the earth. • One magnet on another.

12. Problem What is a? What force does the 7 kg object exert on the 28kg object? Vice versa? M=28 kg M=7 kg F = 35 N

13. Quiz • An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. Make a free body diagram. • Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-23

14. Special Homework due Monday • Workbook, section 5.2, pages 5-3 and 5-4, problems 7-12

15. Problems due Today • Reading: 4: 5-8 • Problems: 4: CQ1, MC21, 4, 7, 8, 9, 10, 11

16. Problems due Monday • Reading: 5: 1-5 • Problems: 4: 13, 14, 17, 18, 20, 25, 29, 30, 31, 33, 35, 38, 65