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9/19/2013PHY 113 C Fall 2013-- Lecture 81 PHY 113 C General Physics I 11 AM-12:15 PM MWF Olin 101 Plan for Lecture 8: Chapter 8 -- Conservation of energy.

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Presentation on theme: "9/19/2013PHY 113 C Fall 2013-- Lecture 81 PHY 113 C General Physics I 11 AM-12:15 PM MWF Olin 101 Plan for Lecture 8: Chapter 8 -- Conservation of energy."— Presentation transcript:

1 9/19/2013PHY 113 C Fall Lecture 81 PHY 113 C General Physics I 11 AM-12:15 PM MWF Olin 101 Plan for Lecture 8: Chapter 8 -- Conservation of energy 1.Potential and kinetic energy for conservative forces 2.Energy and non-conservative forces 3.Power

2 9/19/2013 PHY 113 C Fall Lecture 82

3 9/19/2013PHY 113 C Fall Lecture 83 Comments on preparation for next Thursdays exam

4 9/19/2013PHY 113 C Fall Lecture 84 Comments on preparation for next Thursdays exam -- continued

5 9/19/2013PHY 113 C Fall Lecture 85 Comments on preparation for next Thursdays exam – continued What you should bring to the exam (in addition to your well-rested brain): A pencil or pen Your calculator An 8.5x11 sheet of paper with your favorite equations (to be turned in together with the exam) What you should NOT use during the exam Electronic devices (cell phone, laptop, etc.) Your textbook

6 Advice: 1.Keep basic concepts and equations at the top of your head. 2.Practice problem solving and math skills 3.Develop an equation sheet that you can consult. Equation Sheet Problem solving skills Math skills 8/27/2013PHY 113 A Fall Lecture 16

7 9/19/2013PHY 113 C Fall Lecture 87 iclicker question Which of the following best describes your opinion about the equation sheet: A.I have not prepared my equation sheet yet but hope to do so soon B.I have a preliminary equation sheet but have not used it for doing homework C.I have a preliminary equation sheet and have used it for doing homework D.I am dreaming about preparing the equation sheet, but have not done it yet E.In my opinion, the equation sheet is not really necessary

8 9/19/2013PHY 113 C Fall Lecture 88 Review of energy concepts:

9 9/19/2013PHY 113 C Fall Lecture 89 Some questions from webassign Assignment #7 In 1990 Walter Arfeuille of Belgium lifted a kg object through a distance of 17.1 cm using only his teeth. (a) How much work was done on the object by Arfeuille in this lift, assuming the object was lifted at constant velocity? T mg T yfyf yiyi

10 9/19/2013PHY 113 C Fall Lecture 810 Some questions from webassign Assignment #7 In 1990 Walter Arfeuille of Belgium lifted a kg object through a distance of 17.1 cm using only his teeth. How much work was done on the object by gravity? T mg T yfyf yiyi

11 9/19/2013PHY 113 C Fall Lecture 811 Some questions from webassign Assignment #7 What is the work done as the particle moves from x=0 to x=3m?

12 9/19/2013PHY 113 C Fall Lecture 812 Some questions from webassign Assignment #7 -- continued What is the work done as the particle moves from x=8 to x=10m?

13 9/19/2013PHY 113 C Fall Lecture 813 Some questions from webassign Assignment #7 A 4.34-kg particle is subject to a net force that varies with position as shown in the figure. The particle starts from rest at x = 0. What is its speed at x = 5.00 m?

14 9/19/2013PHY 113 C Fall Lecture 814 Work and potential energy Note: It is assumed that F is conservative

15 9/19/2013PHY 113 C Fall Lecture 815 Work and potential energy -- continued mg y y ref Example – gravity near the surface of the Earth:

16 9/19/2013PHY 113 C Fall Lecture 816 Work and potential energy for gravity (near Earths surface) mg yfyf yiyi y ref

17 9/19/2013PHY 113 C Fall Lecture 817 Work and potential energy continued iclicker question Why is there a strange - sign in the definition of potential energy? A.Physicists like to be annoying B.No reason at all C.There is a somewhat good reason

18 9/19/2013PHY 113 C Fall Lecture 818 Work and potential energy continued For the case that the total force acting on the system is conservative, we can use the definition of potential energy with the work-kinetic energy theorem

19 9/19/2013PHY 113 C Fall Lecture 819 Summary of work, potential energy, kinetic energy relationships

20 9/19/2013PHY 113 C Fall Lecture 820 Example A block, initially at rest at a height h, slides down a frictionless incline. What is its final velocity? h h=0.5m i f

21 9/19/2013PHY 113 C Fall Lecture 821 Energy diagram E yiyi yfyf K U U, K, E (J) m

22 9/19/2013PHY 113 C Fall Lecture 822 k

23 9/19/2013PHY 113 C Fall Lecture 823 Energy diagrams

24 9/19/2013PHY 113 C Fall Lecture 824 Example: Model potential energy function U(x) representing the attraction of two atoms

25 9/19/2013PHY 113 C Fall Lecture 825 Comment on relationship between potential energy and (conservative) force:

26 9/19/2013PHY 113 C Fall Lecture 826 Example: Mass sliding on frictionless looping track i iclicker exercise: In order for the ball completes the loop at A, what must the value of h? A.h=R B.h=2R C.h>2R D.Not enough information.

27 9/19/2013PHY 113 C Fall Lecture 827 i Example: Mass sliding on frictionless looping track 00

28 9/19/2013PHY 113 C Fall Lecture 828 Another example; first without friction Mass m 1 (=0.2kg) slides horizontally on a frictionless table and is initially at rest. What is its velocity when it moves a distance x=0.1m (and m 2 (=0.3kg) falls y=0.1m)? iclicker exercise: What is the relationship of the final velocity of m 1 and m 2 ? A.They are equal B.m 2 is faster than m 1. C.m 1 is faster than m 2.

29 9/19/2013PHY 113 C Fall Lecture 829 Another example; first without friction Mass m 1 (=0.2kg) slides horizontally on a frictionless table and is initially at rest. What is its velocity when it moves a distance x=0.1m (and m 2 (=0.3kg) falls y=0.1m)? T T m 2 g

30 9/19/2013PHY 113 C Fall Lecture 830 Another example; now with friction Mass m 1 (=0.2kg) slides horizontally on a table with kinetic friction and is initially at rest. What is its velocity when it moves a distance x=0.1m (and m 2 (=0.3kg) falls y=0.1m)? T T m 2 g f

31 9/19/2013PHY 113 C Fall Lecture 831

32 9/19/2013PHY 113 C Fall Lecture 832 iclicker exercise: Assume a mass m starts at rest at A and moves on the frictionless surface as shown. At what position is the speed the largest? A.A B.B C.C D.none of these.

33 9/19/2013PHY 113 C Fall Lecture 833 Power

34 9/19/2013PHY 113 C Fall Lecture 834


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