Monday, May 23, 2011

Announcements:

• Exam1 papers are now graded and can be picked up.
• With extra credit, the average score on this exam will be ~70%, (highest score will be ~98%, and the lowest is 35%)
• Answers to multiple-choice questions on Exam1 (version 101):
• Version 101:  B, C, B, A, C, A, B, D, C, B, A, B, B, B, C, A, C, C, B, B, A, A. B, A, B, C
• Version 102:  B, C, B, C, A, C, B, A, D, B, B, A, B, C, B, B, C, A, C, B, A, A, B, C, A, B
• Answer key for graphing question.
• Compare your graphs to the answer key, and if your score on Blackboard is more than 1 point different than what you believe you should have, let me know, and I will re-grade your graphs.
• Estimated course grades will soon be posted on Blackboard.

Assignments:

• Read Chapter 8 and submit HW8 by midnight Tuesday.
• Speeding Ticket Real-World Problem is due Wednesday morning.

Chapter 8 - Potential Energy

Conservation of Energy:  Ei = Ef = mechanical energy + Wnc
Gravitational potential energy:  mgh
Elastic potential energy:  0.5kx^2
Work done by non-conservative forces cannot be easily converted to mechanical energy:  friction, heat, sound, etc.
Demo:  Spring launcher  - How can we find the spring constant k for this spring? - How high can the spring travel when it is stretched some distance x from equilibrium?
 - What is the maximum speed of this spring when it is launched?

Conceptual questions
Book problems:  7.68, 7.69

Responses to Minute Papers from Friday: (*indicates most important comments)

• *I am concerned about improving my grade since I did not do well on the first exam. - Remember that this exam is only 15% of your overall grade, and there is plenty of time for improvement (and many students have made significant improvements in the past).
• *Is it possible for the final exam to count more if we show improvement? - Yes, I may add an overall "exam improvement bonus" if your three exam scores have a positive slope (when I have done this in the past, the bonus has been 10% of the slope).
• *What opportunities are there for extra credit? - There are many:  Web Project (<3%), original real-world problems for exams (<3%), correct answers to clicker questions (<2%), extra class participation (<1%).
• *How much should we focus on learning concepts versus working problems? - I recommend about an even split, since this is how I make my exams (half conceptual and half quantitative).
• *I do not understand why my performance on the exam is so much lower than on Mastering Physics? - This is true for most students.  The difference is that Mastering Physics provides hints, multiple attempts, and unlimited time.  For more authentic test preparation, you should set aside 50 minutes to take the practice test and see how well you do.
• I am concerned about falling behind and trying to grasp new material before fully understanding earlier concepts. - You are not alone, and unfortunately, this is a problem for many students who take this course during a regular semester as well.
• I am having difficulty knowing which formula to apply to each problem. - This mostly comes with practice.  Remember to try to first understand which physics principle applies to a given problem before choosing a formula.
• When will we get our RWP back? - This is still being graded and should be returned in a couple of days.  I told my grader to grade the exam (graphs) first and then the Bank Robbery problem since the exam is higher priority.  Remember that my solution to the Bank Robbery problem is already posted on the RWP webpage.
• Can we use energy to solve projectile motion problems in the future (like on exams)? - You can solve a problem any way you like as long as you do so correctly (so that you get the right answer).
• I like your funny comments and entertaining demonstrations; they help keep my attention! - Thanks; I like to have fun too!
• I like your enthusiasm and class demonstrations! - Thanks!
• I am having trouble with circular motion problems; any suggestions? - The main idea here is to determine the direction of the acceleration, and make sure the net force is in this direction.  You will see more of these problems when we get to the chapter on rotational dynamics.
• I had trouble understanding how to find h in the pendulum problem. - This is difficult for many students, which is why I used it as an example in class.  Try to convince yourself by drawing your own picture and solving for the difference between the length of the string and the longest leg of the right triangle.
• Why does tension do no work for a pendulum? - The tension force acts at a right angle to the displacement, so this force does no work; only gravity does work on the bob.
• I am still confused about how to find static and kinetic friction problems. - Try solving more problems in your book and check your answers to confirm your understanding.
• Why are both cal and kcal used; why not just one or the other? - For the same reason that we use meters and kilometers; big numbers are more easily conveyed in the larger units.
• SI has been a very helpful resource, and the TA has been very helpful. - I am glad to hear this!  Matt won our Outstanding TA Award this year, and it sounds like he deserved it!
• I still do not understand uncertainty for the labs. - Tell me about it; I spent five years studying this subject for my dissertation research, and I still do not understand it completely!
• My bladder wishes we would have had a break today! - Sorry about that; I will try to remember to have a break like we usually do.