Monday, May 16, 2011

Announcements:

• Exam 1 will be held in class on Thursday instead of Wednesday (based on class vote today).
• If you prefer to take this exam on Wednesday, let me know when you want to do so.
• ConceptTest (clicker) questions are posted on Blackboard under "Course Documents".
• Class Participation scores will be posted on Blackboard (hopefully later today):  
• If your score is less than 100% and you have not missed any days of class, let me know.
• The maximum possible Class Participation score is 110%.
• Remember to take advantage of SI sessions (at 5:00 pm) and my office hour after class if you have questions.
• Responses to student questions and comments from Friday's class can be found below.
• There will be a review session for Exam1 on Wednesday after class (9:30 - 10:00).

Assignments:

• Submit HW4 before midnight tomorrow.
• RWP3 - Bank Robbery  is due Wednesday morning (hand in during class).
• Read Chapter 5 and submit HW5 by midnight Wednesday.

Chapter 3:  Review with ConcepTests

Note regarding CE 3.10:  Angle measurements for a compass heading are not the same as polar coordinates that correspond to a Cartesian coordinate system!

Chapter 4:  Motion in Two Dimensions

Same physics as 1D motion.  Simply treat each component independently and add vectors.

Tips for solving kinematics problems:

• Displacement is a vector, distance is a scalar and is always bigger than |displacement|
• In free-fall, horizontal velocity is constant (gravity only affects vertical component)
• Use final velocity components to find final speed (resultant)
• Speed is a scalar and therefore is always positive

Assumptions for Projectile Motion:

• g = constant
• air resistance is negligible (valid for dense objects and speeds less than ~20 mph)
• Earth's rotation is ignored

Chapter 4 ConcepTests

Demos:
Simultaneous ball drop
Howitzer on track

Student questions and comments from Friday's Minute Paper (2009 and 2011) - Instructor responses: (most important comments have *)

• The course is face-paced, but this is to be expected. - I agree!  I was thankful for the weekend; how about you?!
• *How should we prepare for exams? - See Physics Study Tips and Practice Exams in the Resources section of the course syllabus.
• *Are we going to have a review session before the test? -  I will stay after class in room 215 and answer questions until 10:00 AM on Wednesday, and don't forget about SI sessions!
• *What is the best way to prepare for the exam? - Review class notes and homework.
• *Not sure what to expect for exams. - Remember to look at my Practice Exams from 2003 to get a sense of what you can expect.
• *I wish that we could have more than one day to work on the homework. - The assignments will usually be available at least two days before they are due, and you can still submit them after the deadline with only 10% penalty per day (prorated by hours and minutes, so only 1% penalty for 2 hours late).
• *What is required for the real-world problems for a test? - See the instructions in the syllabus.
• *I am having trouble knowing what equation to use and where to begin some problems. - This is where the physics lies; the rest is math.  If you already knew how to solve all the problems, there would be no reason for you to take this course.  In fact, the definition of a "problem" is one that you do not know exactly how to approach.  After some experience, a "problem" can become an "exercise."  Your goal is to prepare sufficiently for each exam so that most of the questions become exercises and not problems for you.
• *I feel like I am struggling to keep up and learn everything we need to know. - You are not alone!  Many students feel this way; just do your best to focus on the main ideas, and use your time effectively.
• *I am concerned that we have not seen examples of more complex vector problems in class. - I generally try to work simpler problems in class to make the most of the time we have, but occasionally I will work a more difficult problem.  My exam problems are easier than most of the problems found in the textbook.
• *The order-of-magnitude estimates are difficult for me. Are we supposed to know trivia like the U.S. population for tests? - If needed, I would give you this value, but then you might be expected to estimate the length of a human chain without the aid of additional information. 
• I feel like I am temporarily understanding concepts in class and then forgetting them. - It may help you to study with someone else so that you can talk through the material and clarify your understanding.  Teaching someone else is one of the best ways to learn!
• The online homework is better than expected (fair, gives partial credit, etc.). - I am glad!
• It is probably best to read the chapter before coming to class. - Definitely!  I expect you to rely on the textbook for most of the course content; then class time can be used to clarify the more difficult aspects.
• Class time is great, even though it is early in the morning. - I am glad to hear this!
• I am still confused about when to use + g or -g. - Choosing up to be positive and using "-g" for the acceleration due to gravity is the best bet.
• How can you tell something about speed from an acceleration graph? - Velocity is the area under the acceleration graph, just as displacement is the area under the graph of v(t).
• I understand vector addition, but I still have difficulty finding components using sine and cosine. - This will become clearer with practice!
• I do not understand how C = A + B as shown in class. Doesn't this conflict with Pythagorean's Theorem? - Recall that Pythagorean's Theorem only applies to the special case of right triangles.
• If the wind is blowing directly opposite the direction that a plane is heading, do you just subtract the two? - Yes, the plane's ground speed is: v = v(air) - v(wind) 
• How should we represent the magnitude and direction of a vector? - Any notation that is clear to the reader (or computer!) is acceptable.
• I do not understand the unit vector notation. - Do not worry about this since it is not emphasized in your book and I will not include it on my exams.
• Is Vmax the same as Vf? - In general, these are not the same, but they could be for certain problems.
• How to find the initial velocity when throwing a ball.  Is it always 10 m/s? - The initial velocity is related to the maximum height and time of flight; it was only 10 m/s for the in-class problem.
• I am curious about the role that air resistance plays in equations we have studied so far. - Air resistance has mostly been ignored, but it will be included in Chapter 5.
• I have questions about the lab. - You can ask me or your lab instructor.
• I am not sure about uncertainties in lab. - There is no easy answer here because each case is unique and depends on how the measurement or calculation is obtained.
• How did you decide to study/teach physics? - I have always been curious about the physical world, and I enjoy sharing my interest with other people.