PH 211 Winter 2014

1. PH 211 Winter 2014 Monday February 10

2. Lab calendar update • Tuesday 2/11 Lab 5 • Thursday 2/13 Lab 4 • Tuesday 2/18 NO LAB • Wednesday 2/19 MIDTERM 2 • Thursday 2/20 Lab 5

3. Midterm pickup Pick up exams in office hours Tuesday. Show ID! • Compare your exam and solutions. • Come to office hours for questions. • If asking for re-grading, use form from web-site, and follow procedure.

4. Google moderator Which topic would you like to discuss in class on Wednesday? http://www.google.com/moderator/#16/e=20e46c

5. Google moderator Which topic would you like to discuss in class on today? http://www.google.com/moderator/#16/e=2173db

6. Which force needs to be added so the net force is zero?

7. Which force needs to be added so the net force is zero? 0 of 208 1

8. Free-body diagrams and acceleration

9. Box against the wall problem (no friction):Net force zero

10. Box against the wall problem (no friction):Net force zero y x

11. Why choose x-y directions this way?? y x x: - Fp +Fn=0 y: Fp-FG=0

12. Book problem 8.53: • A 100 g ball on a 60 cm long string is swung in a vertical circle about a point 200 cm above the floor. The string breaks when it is parallel to the ground and the ball is moving upward. The ball reaches a height of 600 cm above the floor. What was the tension in the string the instant before it broke?

13. v=0 m/s 6m 2m

14. vi = 8.85 m/s T = 13 N

15. Normal force • We know it is perpendicular, but… • What IS it? • Molecular model • Let’s investigate what it depends on: • Experimental observations

16. What did the normal force depend on in this example problem?A. The mass of the boxB. ‘g’C. the push forceD. all of the above

17. What did the normal force depend on in this example problem? • The mass of the box • The value of ‘g’ • The push force • All of the above 0 of 208 :01

18. Which of the following is the normal force (between two objects in contact) DIRECTLY dependent on: (a directly dependent variable MEANS that variable is present ALWAYS when we need to find the normal force!) • The mass of one of the objects • The mass of both of the objects • The gravitational constant ‘g’ • The angle of the incline for the objects • External pushes on the objects in contact • None of the above

19. Which of the following is the normal force (between two objects in contact) DIRECTLY dependent on: (a directly dependent variable MEANS that variable is present ALWAYS when we need to find the normal force!) • The mass of one of the objects • The mass of both of the objects • The gravitational constant ‘g’ • The angle of the incline for the objects • External pushes on the objects in contact • None of the above 0 of 208 1

20. Normal force If object does not move into surface: reaction force equal and opposite to net force pushing into surface!

21. Example problem • AXIS CHOICE for free-body diagrams (why it helps with normal force AND with motion) • What’s touching + gravity – only forces acting ON the object • A very large and heavy box is being moved by two people on 10 degree incline ( no friction!). One is pulling it by a rope from the front at an angle of 15 degrees above the incline. The other is pushing down on the back at an angle of 20 degrees below the incline. • Draw the free body diagram

22. y x 15° 10° 20°

23. A box is being pulled to the right at steady speed by a rope that angles upward. In this situation: n > mg. n = mg. n < mg. n = 0. Not enough information to judge the size of the normal force. Slide 6-83

24. A box is being pulled to the right at steady speed by a rope that angles upward. In this situation: • n > mg. • n = mg. • n < mg. • n = 0. • Not enough information to judge the size of the normal force 143 of 208 19