Physics 203 College Physics I Fall 2012

1. Physics 203College Physics IFall 2012 S. A. Yost Exam, Chapter 4 Newton’s Laws – Part 2

2. Exam 1 • Average: 51%, High: 92% • (normalized to 85 points) F D C B A

3. Survey: Question 1 • Indicate your current status • A Freshman (4th class) • B Sophomore (3rd class) • C Junior (2nd class) • D Senior (1st class) • E Other

4. Survey: Question 2 • Select the highest high school physics courses you have completed. • A One semester of HS Physics • B Two semesters of HS Physics • C One semester of AP Physics • D Two semesters of AP Physics • E No Physics

5. Survey: Question 3 (one of two) • If the highest math course you completed is in this list, select it. (There will be another list following.) • A MATH 104 • B MATH 105 • C MATH 106 • D MATH 107 • E None of these

6. Survey: Question 4 (two of two) • If the highest Citadel Math course you have completed is in this list, select it. • A MATH 119 • B MATH 131 • C MATH 132 • D STAT 160 • E None of these

7. Survey: Question 5 • Select Citadel Biology Courses you have completed. • A BIOL 101 only • B BIOL 101 and BIOL 102 • C BIOL 130 only • D BIOL 130 and BIOL 140 • E None of the above

8. Survey: Question 6 • Select Citadel Chemistry Courses you have completed • A CHEM 103 only • B CHEM 103 and CHEM 104 • C CHEM 151 only • D CHEM 151 and CHEM 152 • E None of these

9. Survey: Question 7 • The figure shows a right triangle. Estimate the length x indicated. • A 11.0 cm • B 6.5 cm 4.0 cm • C 5.5 cm 7.0 cm • D 8.1 cm • E 7.4 cm x

10. Survey: Question 8 • An auto travels East from home at 65 mi/hr for 3.0 hours. Then it returns West at 50 mi/hr for 4.0 hours. How far is the auto from home? • A105 miles East • B 5 miles East • C14 miles East • D20 miles West • E5 miles West

11. Survey: Question 9 • The quantity Z depends on X and Y according to the equation • 1 1 1 • Z X Y . • What is the value of Z if X = 4 and Y = 6? • A0.1 B6.4C2.4 • D1.0 E10.0 = +

12. Schedule • This week: Chapter 4 (continued). Finish reading it by next time. • The topic is Newton’s Laws. • A problem set will be up later today and due a week from today.

13. Newton’s First Law • If an object does not interact with any other object, there exist reference frames in which the object has zero acceleration. • Such a reference frame is called an inertial frame. • In an inertial frame, an isolated object moves with a constant velocity. • Any frame moving at a constant velocity with respect to an inertial frame is also inertial.

14. Newton’s Second Law • Forces act as vectors. If the vector sum of forces on an object is zero, the object experiences no acceleration. • If the total force vector acting on an object is F, then the acceleration of the object is determined by the net force and the object’s mass: • F = ma • Conversely, if the forces on an object balance, so that the net force is zero, it has no acceleration, and moves in a straight line at constant speed. → → →

15. Newton’s Third Law • All interactions occur in pairs. Forces always exist between a pair of objects. • If object 1 acts on object 2 with a force F21, then object 2 acts on object 1 with a force F12that is equal in magnitude and opposite in direction: • F21 = – F12. • Unlike the second law, this holds in any reference frame, intertial or not, regardless of any motion. → → → →

16. Car on the Highway • A car is accelerating onto the highway. • What force is responsible for this? • Friction between the tires and road. F F

17. Car on the Highway • As you accelerate, coffee cup slides backwards off the dashboard. • What force is responsible for this? • None: without friction, the cup would maintain a constant velocity while the car accelerates around it. • The coffee cup does accelerate backward relative to the car, but the car is a non-inertial reference frame, so no force is responsible for this acceleration.

18. Push and Fall Newton’s Third Law Mike pushes Johnny with a force of strength F and Johnny falls. The force Johnny exerts on Mike in the process must be A) less than F B) greater than F C) equal to F FMJ Mike pushes → F FJM Johnny falls → FMJ → → = –FJM

19. Units, Weight • Forces are measured in Newtons. • Weights are forces, and can be given in units of Newtons. It is incorrect to measure weights in kg, because this is a mass, not a force. • The weight of a mass m is W = mg. • 1 kg weighs 9.8 Non Earth. • The units follow from Newton’s 2nd Law: • 1 N = 1 kg m/s2

20. Units, Weight • Technically,poundsare a unit of weight, but are commonly used to measure mass on Earth. • The correct English unit of mass is called the slug, which weighs about 32 lb on Earth. The English pound is1 lb = 1 slug ∙ ft/s2. • 1 kg weighs2.20 lbon Earth, but this would be different elsewhere. The weight of 1kg on Earth is 9.8 N, so 1 lb = 9.8 N / 2.20 = 4.45 N(anywhere). • Roughly, a Newton’s about a quarter pound.

21. Weight and Normal Force If you are standing on a floor, the floor pushes up on you with a force opposite your weight to prevent you from falling through the floor. The perpendicular component of the contact force between two objects is called the normal force. (Here, “normal” means “perpendicular”, not “ordinary”.) N W N+ W = 0

22. Weight and Normal Force Are the weight and normal force an interaction pair, which must be equal and opposite by the third law? No: The normal force is between the person and the floor, while gravity is between the person and the Earth. N W N+ W = 0