Physics

I hope you’re all having a nice weekend, but especially Matt.

Here’s the problem set.

(I tried adding an answer sheet to make the grading a little easier for me.)

Please do these problems for Wednesday, January 4, 2011.

Here are the solutions to the concept practice problems we worked on in class today.

Click here to get the AP problem due next class. Please show ALL work in order to get credit. I will be grading these according to the official AP rubric, in which most of the credit is awarded for the process and not the final answer.

1.) –2.6×106^ J

3.) (a) −3.38×10^3 J (b) 3.38×10^3 J

4.) (a) 2980 J (b) 3290 J

12.) 39 m/s

13.) 6.4 x 10^5 J

14.) 2.5 x 10^7 J

34.) 1.5 m

35.) 196 J, 196 J, 392 J

40.) 4.43 m/s

67.) (a) 1.5 x 10^2 J (b) 7.07 m/s

Draw a v vs t graph for each of the situations described in the previous post. Remember that velocity is the slope of the x vs t graphs that you already created.

Sketch an x vs t graph of the following situations:

1. A student leaves class and begins walking down the hall. After a few seconds she runs to catch up with a friend, then resumes walking.
2. A soccer player stands motionless for a few seconds then charges the ball at top speed.
3. A dog walks, stops to sniff, walks, stops to sniff, walks, stops to sniff, then runs after a squirrel.
4. Three men compete in the 100 yard dash. The three men have different top speeds, which they reach very soon after the starting gun is fired. Draw the x vs t graph for each runner, but put them all on one set of axes. On your graph, indicate where the finish line is and which graph corresponds to first, second and third place runner.
5. A child leaves his house and runs down the street to the bus stop. Halfway to the bus stop he realizes he has forgotten his toy truck for show and tell and runs back home to get the toy. After grabbing the truck he runs all the way to the bus stop. By the time he is running to the bus stop for the second time he has tired and is running slower than before.
6. A car merges onto the highway, steadily increasing its speed from 30 mph to 65 mph.
7. A car sees that the road has flooded and slams on the brakes to avoid going into the water.
8. An airplane accelerates to a high speed and takes off. The plan then cruises at this high speed for a long time. Once the plane has reached its destination it slows slightly to land, then comes to a stop very quickly.

As promised, here are my responses…

1.) Why did you sign up for AP Physics?

Since starting at JSR seven years ago I’ve been interested in teaching AP Physics, mostly because of the challenge it provides to both the students and the teacher. I appreciate a good challenge. I started the AP Physics program three years ago when the school asked me if I would be willing to give it a try.

2.) What do you hope to get out of this class?

Teaching AP Physics allows me to work with students who are both talented and interested. At least I assume that is your disposition if you’ve signed up for an AP class. Students like you challenge me to think about ideas more deeply, and I enjoy that.

3.) What have you heard about this class?

I have heard that the class is difficult, but I have also heard that students enjoy coming to class and enjoy learning about the way things work.

4.) How comfortable are you with being confused?

I haven’t always been comfortable with being confused, but I have come to see confusion as an essential part of the learning process. If you aren’t confused when you’re learning something, it probably means you already know it. Confusion is an essential part of real learning. We will all need to work on being comfortable with confusion so that we can work through it. Last year most students handled the confusion aspect of class very well and I heard very little complaining. I hope to see that trend continue this year.

5.) What do you do when you don’t know what to do?

I’ve heard problem solving defined as what you do when you don’t know what to do. After all, if you knew what to do there wouldn’t be a problem. So how do I solve a problem? I try something: draw a picture, make a graph, write down what I know. I Start. If you read a problem and you don’t immediately know what to do, sitting there and staring at it won’t help. Start working. Start coming up with a plan. If you get stuck and you come see me for help I want to hear you say, “I tried this and this and this and here’s what happened. What should I try next?” Simply saying, “I have no idea what to do!” will rarely help you solve a problem or learn.

6.) What are one or two things that you, as a student, are planning to work on next year? How are you going to do that? (So you know what I mean by goals, here is an example. As a teacher one of my goals next year is to conduct more labs and hands-on activities in class. Your goals might be related to skills or they could be something more attitudinal, like trying not to get frustrated.)

One of my biggest goals for this year is to make inquiry a bigger part of AP Physics. In short, inquiry is students conducting experiments of their own design to answer a question they they came up with on their own. This is very difficult because student-directed inquiry is very time consuming and we don’t have a lot of time. My goal is to focus our effort on the concepts and topics that are most important and eliminate any topics that are not essential for the AP Exam in May, thus freeing up time for you to conduct experiments in class.

7.) What grade are you planning to get in this class? What grade (1-5) are you planning to get on the AP exam in May? How are you going to do that?

You should be able to achieve a 90% or better in AP Physics if you are willing to work for it. It will not be easy, but it is not impossible. Most students’ grades fall between 80% and 93%.

On the AP Exam most of my students students earn a passing grade of 3 or better. UNH will give you credit for Physics 401 if you score a 3 or better; they will give you credit for Physics 401 and 402 if you earn a 4 or 5. JSR has offered AP Physics B for three years now and the scores break down as follows:

2 people scored a 5
9 people scored a 4
15 people scored a 3
2 people scored a 2
0 people scored a 1

In other words, 93% of the JSR students who have taken the AP Physics B exam are eligible to earn credit at UNH. Actual awarding of credit will, of course, depend on your specific school and major. Talk to me in class if you have more questions about getting credit for your work.

8.) I will probably assign at least an hour of regular homework every class. This homework is going to be a really important part of you learning how to do physics. How are you going keep up with this work?

I will do all of the homework I assign. I need to so that I can answer your questions thoroughly. You also need to do all of the homework. Refer to #5. You must genuinely try every problem. Leaving it blank and asking me in class will not help you learn. You have to try something, even if it doesn’t work.

9.) Give me your reaction to this idea: I’m not going to grade your daily homework, but I still expect that it will be done well and done thoroughly. (This is something I’m considering for next year but I haven’t come to a final decision yet.)

I should note that I did not collect or check homework last year. The people in class who worked hardest on homework even though it was not collected ultimately had the highest grades in the course.

10.) What are the one or two things that I could do next year that would most help you learn and work to the best of your ability?

I could give you the usual teacher list of things a good student does. You’ve heard it before. I think if I had to narrow my focus to three things they would be: Be Interested, Do the work and Keep it positive.

11.) Is there anything else I should know about you as a person or a student?

I will refer you to a great article in the Boston Globe by a writer named Jonah Lehrer. The article is called “The Truth about Grit.” The article debunks the idea that greatness comes solely through natural ability. Mr. Lehrer argues that, in reality, people who achieve great things have to work really hard at it.

The first example of this that the article describes has to do with Isaac Newton, a man whose ideas we will spend about half the year talking about. The mythology surrounding Newton describes him as a super-genius who saw the universal truths of physics in a flash as he sat beneath an apple tree. Mr. Lehrer points out that that just isn’t how it went down…

Unfortunately, the story of the apple is almost certainly false; Voltaire probably made it up. Even if Newton started thinking about gravity in 1666, it took him years of painstaking work before he understood it. He filled entire vellum notebooks with his scribbles and spent weeks recording the exact movements of a pendulum. (It made, on average, 1,512 ticks per hour.) The discovery of gravity, in other words, wasn’t a flash of insight – it required decades of effort, which is one of the reasons Newton didn’t publish his theory until 1687, in the “Principia.”

In case you aren’t good with math 1687 – 1666 = 21 years of work. The more time I spend learning and teaching the more I believe that being smart isn’t even half of the battle. I urge you to take a few minutes to read the whole article and tell me what you think in the comments (first names only please).

1. I made a short video on vectors which you can see here. (Note: I made this video last summer, so the beginning part about the pages being in the wrong order is not true for this year.)
2. The introduction to motion video I mentioned in the packet is here. As you watch you’ll notice that the video is broken into sections. You only need to watch up through Part 4: Acceleration. (Just to be clear — watch part 4, but do not watch part 5.) As you watch you will probably want to make sure you understand the equations explained in Part 4 as these will be most helpful when trying to solve problems.
3. Remember to email me with your survey responses.

Have a great summer.

Please complete the following AP Problems for class on Monday (or Tuesday if there’s a snow day).

• Problem 1 – Omit part (c)
• Problem 2