5 Steps to a 5: AP Physics 1: Algebra-Based 2017 (2016)

STEP 3

Develop Strategies for Success

CHAPTER 8

Strategies to Approach the Questions: Free-Response Section

IN THIS CHAPTER

Summary: The AP Physics 1, Algebra-Based Exam contains question types you probably have not encountered before. This chapter describes types of questions that appear only in the free-response section of the test and the most effective strategies to attack them. Included in this chapter are strategies and advice on how to approach the laboratory question, the qualitative-quantitative translation question, and the free-response questions in general.

Key Ideas

   The free-response section contains five questions to be answered in 90 minutes. Included will be a 12-point lab question, a 12-point qualitative-quantitative translation (QQT), and three 7-point short-answer questions. One of these 7-point problems will require a response in paragraph form.

   There are six simple strategies and tips you can use when answering the lab question to make sure you get all the points you deserve (see list in this chapter).

   If the first part of a QQT requires description, skip it and go to the part of the problem that asks for the calculation. Do the calculation first and then go back to the parts that require qualitative reasoning.

   Remember, you’re only expected to get about 70 percent of the available points to earn a top score. Don’t skip any part of a free-response question—go for partial credit instead. See the list of tips for what to do and what not to do to get the most partial credit possible.

   The free-response section of the exam is read and graded by humans. At the end of this chapter is a list of tips to best communicate to the reader your understanding of the concepts being tested.

Structure of the Free-Response Section

The free-response section contains five questions to be answered in 90 minutes. The five questions will not be all similar in length and style. Instead, the structure will be as follows, but not necessarily in this order:

  • One 12-point question posed in a laboratory setting
  • One 12-point qualitative-quantitative translation
  • Three 7-point short answer questions, one of which will require a response in paragraph form.

The rule of thumb is to spend about two minutes per point answering each question. Start the exam by picking the problem that you can answer the most quickly—that’ll probably be one of the shorter, multipart questions. You probably can do that in less than two minutes per point. That leaves extra time for a problem that might require more of your efforts.

Included in this chapter are separate discussions of the strategies to use in approaching qualitative-quantitative translation questions and laboratory questions. Other than understanding the appropriate pace and the strategies with which to approach these new question styles, no real extra preparation is necessary. The best thing about the free-response section of the AP Exam is that you’ve been preparing for it all year long.

Really? I don’t recall spending much time in class on test preparation.

But think about all the homework problems you’ve done. Every week, you probably answer a set of questions, each of which takes a few steps to solve. I’ll bet your teacher is always reminding you to show your work carefully and to explain your approach in words. 1 That sounds like what’s required on the AP free-response section to me.

How to Approach the Laboratory Question

It is all well and good to be able to solve problems and make predictions using the principles and equations you’ve learned. However, the true test of any physics theory is whether or not it works .

The AP Physics 1, Algebra-Based Exam committee is sending a message to students and teachers that laboratory work is an indispensable part of physics. Someone who truly understands the subject must be able to design and analyze experiments. Not just one, but two of the seven “science practices” listed in the curriculum guide refer explicitly to experimental physics. One of the five free-response questions is guaranteed to be posed in a laboratory setting, in addition to some multiple-choice questions that have experimental elements. Point is, you cannot ignore lab-based questions.

Your ability to answer questions on experiments starts with laboratory work in your own class. It doesn’t matter what experiments you do, only that you get used to working with equipment. You should know what equipment is commonly available to measure various physical quantities, and how that equipment works. You should be comfortable describing in words and diagrams how you would make measurements to verify any calculation or prediction you could possibly make in answering a problem.

Now, “laboratory work” doesn’t necessarily mean a 10-page, publishable report. Describing an experiment should be a three-sentence, not a three-page, process; analyzing data generally means reading a graph, as discussed in the section above. Here is an example of part of a free-response question that asks for a description of an experimental process.

Sample Laboratory Question

In the laboratory, you are given a metal block, about the size of a brick. You are also given a 2.0-m-long wooden plank with a pulley attached to one end. Your goal is to determine experimentally the coefficient of kinetic friction, μ k , between the metal block and the wooden plank.

(a)   From the list below, select the additional equipment you will need to do your experiment by checking the line to the left of each item. Indicate if you intend to use more than one of an item.

(b)   Draw a labeled diagram showing how the plank, the metal block, and the additional equipment you selected will be used to measure μ k .

(c)   Briefly outline the procedure you will use, being explicit about what measurements you need to make and how these measurements will be used to determine μ k .

Six Simple Strategies and Tips for Answering Descriptive Laboratory Questions

Here are the most effective strategies to use to approach a free-response question that asks for a description of an experimental process like the sample question above.

  1. Follow the directions.Sounds simple, doesn’t it? When the test says, “Draw a diagram,” it means you need to draw a diagram. And when it says, “Label your diagram,” it means you need to label your diagram. You will likely earn some points just for these simple steps.

Exam Tip from an AP Physics Veteran

On the 1999 AP test, I forgot to label point B on a diagram, even though I obviously knew where point B was. This little mistake cost me several points.

  1. Use as few words as possible.Answer the question, and then stop. You can lose credit for an incorrect statement, even if everything else in your answer is perfect. The best idea is to keep it simple.
  2. There is no single correct answer.Most of the lab questions are open-ended. There might be four or more different correct approaches. Don’t try to “give them the answer they’re looking for.” Just do something that seems to make sense—you’re likely to be right.
  3. Don’t assume you have to use all the stuff they give you.It might sound fun to use a light sensor when determining the net torque on a meterstick, but really? A light sensor?
  4. Don’t overthink the question. You’re not supposed to win a Nobel Prize for your work. Free-response problems should never take more than 25 minutes to complete, and usually take much less time. Don’t expect to design a subatomic particle accelerator, expect to design a quick measurement that can be done in your classroom.
  5. Write for an audience at the same level of physics as you.That means, don’t state the obvious. You may assume that basic lab protocols will be followed. There’s no need to tell the reader that you recorded your data carefully, and you do not need to remind the reader to wear safety goggles.

Answering Lab Questions

Now it’s time to pull it all together. Here are two possible answers to the preceding sample question. Look how explicit I am about what quantities are measured, how each quantity is measured, and how μ k is determined. But, like many physicists, I would have flunked out of art school. Your diagrams don’t have to look beautiful because AP readers believe in substance over style. All that matters is that all the necessary components are there in the right places.

Answer #1

In the laboratory, you are given a metal block, about the size of a brick. You are also given a 2.0-m-long wooden plank with a pulley attached to one end. Your goal is to determine experimentally the coefficient of kinetic friction, μ k , between the metal block and the wooden plank.

(a)   From the list below, select the additional equipment you will need to do your experiment by checking the line to the left of each item. Indicate if you intend to use more than one of an item.

(b)   Draw a labeled diagram showing how the plank, the metal block, and the additional equipment you selected will be used to measure μ k .

(c)   Briefly outline the procedure you will use, being explicit about what measurements you need to make and how these measurements will be used to determine μ k .

Use the balance to determine the mass, m , of the metal block. The weight of the block is mg . Attach the spring scale to the bulldozer; attach the other end of the spring scale to the metal block with string. Allow the bulldozer to pull the block at constant speed.

The block is in equilibrium. So, the reading of the spring scale while the block is moving is the friction force on the block; the normal force on the block is equal to its weight. The coefficient of kinetic friction is equal to the spring scale reading divided by the block’s weight.

Answer #2

In the laboratory, you are given a metal block, about the size of a brick. You are also given a 2.0-m-long wooden plank with a pulley attached to one end. Your goal is to determine experimentally the coefficient of kinetic friction, μ k , between the metal block and the wooden plank.

(a)   From the list below, select the additional equipment you will need to do your experiment by checking the line to the left of each item. Indicate if you intend to use more than one of an item.

(b)   Draw a labeled diagram showing how the plank, the metal block, and the additional equipment you selected will be used to measure μ k .

(c)   Briefly outline the procedure you will use, being explicit about what measurements you need to make and how these measurements will be used to determine μ k .

Determine the mass, m , of the block with the balance. The weight of the block is mg . Attach a string to the block and pass the string over the pulley. Hang masses from the other end of the string, changing the amount of mass until the block can move across the plank at constant speed. Use the motion detector to verify that the speed of the block is as close to constant as possible.

The block is in equilibrium. So, the weight of the hanging masses is equal to the friction force on the block; the normal force on the block is equal to its weight. The coefficient of kinetic friction is thus equal to the weight of the hanging masses divided by the block’s weight.

The Qualitative-Quantitative Translation (QQT)

While physics is not about numbers (see Chapter 2 ), physicists routinely plug values into relevant equations to produce numerical predictions such as “the reading on the scale will be 550 N.” In AP language, that’s quantitativereasoning.

Just as routinely, though, physicists use equations as the basis for a less-specific conceptual prediction. Without using any numbers, it’s usually possible to look at the features of an equation—what variables are in the numerator or denominator, what’s squared or square rooted, what is added or subtracted—and determine something like “the reading on the scale will increase.” In AP language, that’s qualitative reasoning.

One of the five free-response items on the AP Physics 1 Exam is called a “qualitative-quantitative translation question,” abbreviated as QQT. It will generally ask you first for qualitative reasoning, next for quantitative reasoning, and then it will expect you to explain in words how the different aspects of your solutions relate to each other. If you think of algebra as the language of physics, the QQT will ask you to translate from algebra to English.

An Important Strategy for Solving QQTs

The fact is, by the time they’re through a full year of physics, most first-time physics students are far more comfortable with numerical calculation than they are with verbal description of physics concepts. Partly that’s because most physics classes emphasize calculation more than writing; partly it’s because people are taught to calculate in elementary school but are rarely asked to write in words how and why a calculation worked.

Play to your strengths. If the first part requires description, skip it and go to the part of the problem that asks for the calculation. Do the calculation. Write out every step carefully, annotating your solution with words explaining the point of each step. Then go back to the parts that require qualitative reasoning. Now, since you’ve approached the situation the “easier” way, you should have a good clue as to why you did the mathematical steps you did.

The actual qualitative-quantitative translation question will be one of the longer questions on the exam, a 12-point question. Usually only part of the question will directly ask about translating calculation into words, and vice versa. Here’s an example of the actual translation portion of such a question.

Sample QQT

Two blocks are connected over a light, frictionless pulley, as shown. Block A of mass 2 kg is on a frictionless surface; Block B of mass 1 kg hangs freely. The blocks are released from rest. (A) After the blocks are released, is the tension in the rope greater than, less than, or equal to the weight of Block B? Justify your answer in words, without equations or calculations. (B) Calculate the tension in the rope after the blocks are released.

Answering the QQT

Understand that you might see several other parts, say, asking for a free-body diagram of each block, or asking about the behavior of the blocks after Block B hits the ground, or whatever. For now, though, let’s just focus on the true translation in Parts (A) and (B).

Even though Part (A) specifically forbids equations and calculations, try approaching it like a calculation problem, anyway. Part (B) asks for the actual value of the tension in the rope. Take care of that first.

Treating the two blocks as a single system, the net force is the 10-N hanging weight. The mass of this two-block system is 3 kg, so by F net = ma , the system’s acceleration is 10 N/3 kg = 3.3 m/s per second.

Next, consider a system as just Block A. The only force acting on Block A is the tension in the rope, so that’s the net force. The tension then is the mass of Block A times its acceleration, (2 kg)(3.3 m/s per second) = 6.6 N.

There’s the answer to Part (B), with a nicely explained calculation. What about Part (A)? Well, we at least know the answer from our calculations—the tension is less than the 10-N weight of Block B. But now we need to justify the answer without doing the calculation. How do we do that? Describe why the calculation came out as it did, without doing the actual calculation. Be specific about what values are bigger, smaller, or the same throughout the calculation.

Correct Answer #1: “Block A has the same acceleration as the two-block system but less mass than the whole system. Block A experiences a smaller net force than the whole system. The tension is the net force on Block A; the weight of Block B is the net force on the whole system. The tension is less than Block B’s weight.”

To take a different approach, we know that force problems are best approached with free-body diagrams before plugging in any numbers to equations. An alternate explanation might be to look at a free-body diagram of just Block B.

Correct Answer #2: “The tension pulls up on Block B, and the weight pulls down. Since Block B has a downward acceleration—it speeds up and moves down—the downward forces must be bigger than the upward forces. So the tension is less than the weight.”

In either case, you’re using words to describe the way you would solve the problem if you were doing a calculation. That’s translating from quantitative reasoning to qualitative reasoning—the very definition of a QQT.

What Do the Exam Readers Look For?

The key to doing well on the free-response section is to realize that, first and foremost, these problems test your understanding of physics; next, they test your ability to communicate that understanding. The purpose of the free-response questions is not to see how good your algebra skills are, or how many fancy-sounding technical terms you know, or how many obscure ideas you can regurgitate. You already know from Chapter 1 that the free-response section of the AP Physics 1 Exam is graded by human readers, not a computer. All I’m going to do in this section is give you some important suggestions about how you can best communicate to the reader that you understand the concepts being tested.

All free-response questions are graded by physics teachers who must carefully follow a “rubric” for each question. A rubric is a grading guide—it specifies how points are awarded, including the elements of an answer necessary for both full and partial credit.

You Cannot “Game” the Rubric

It’s tempting to try to find that “One Weird Trick” that will guarantee you an extra point or two on the exam. You and your teacher might look at previous years’ rubrics 2 and think you see a pattern. But I warn you: Tricks don’t work to solve AP Physics 1 problems.

Each rubric is unique. It’s created originally by the author of the test question, revised by the exam development committee, adjusted by table leaders based on actual student responses and feedback from colleagues, and finalized mere hours before the reading begins. The College Board does not have hard and fast rules about how a rubric should be written. Each table leader has wide latitude to adjust the rubric so that it awards credit for good physics, and so it does not award credit for bad physics.

Rubrics Provide Ample Opportunity for Partial Credit

Recall that you’re only expected to get about 70 percent of the available points to earn a top score. You’re not supposed to answer every problem perfectly; you’re expected to communicate as much physics understanding as possible. Rubrics are designed to award partial credit for answers that are correct but incomplete or that are essentially correct with only minor mistakes.

Thus, your strategy should always be to make a reasonable attempt at each part of every problem. Don’t fret that you don’t know how to do everything perfectly; just give the best answer you can, and expect to earn credit in proportion to how well you’re explaining what you do know.

Here are some hints regarding partial credit:

  • If you can’t solve Part (a) of a multipart problem, don’t skip parts (b) through (e)! Sometimes you’ll get everything else perfect if you just move along.
  • If the answer to Part (b) depends on the answer to Part (a), it’s okay to say “I didn’t get Part (a), but pretend the answer was 25 m/s.” As long as your answer isn’t absolutely silly,3 you will get full or close to full credit. Rubrics are generally designed not to penalize the same wrong answer twice.
  • If you get a right answer using a shortcut—say, by doing a calculation in your head, or just remembering a demonstration in your class—you’re likely to get credit for your answer but not for the reasoning behind your answer.
  • However, if you get a wronganswer using a shortcut, you’re not likely to get any credit at all. AP readers can read only what’s written on your test. They cannot read your mind, and they are not allowed to assume that you know what you’re talking about. The moral of the story is this: Communicate with the readers so you are sure to get all the partial credit you deserve.

Exam Tip from an AP Physics Veteran

When I first started physics class, I became frustrated that I didn’t get full credit on questions where I thought I understood the right answer. I tried to argue with my teacher after a test: “Here, let me tell you what I was trying to say, so you can give me these points that I deserve.” My teacher told me, “So, are you allowed to go to Kansas City where the AP Exam is graded, and go along with your test from reader to reader to tell them what you really meant?” From then on, I started being more careful to explain my answers thoroughly on homework problems and on tests. I got an easy 5 on the AP Exam.

You should also be aware of some things that will not get you partial credit:

  • You cannotearn partial credit if you write multiple answers to a single question. If AP readers see that you’ve written two different answers, they are instructed to grade the one that is incorrect, even if the other is correct. If you’re not sure of the answer, you can’t hedge your bets.
  • You cannotearn “extra” credit. Readers are not allowed to say, “Wow, that’s the most complete answer I’ve ever heard. Here’s +1.” Don’t include unnecessary information. It won’t help, and if you make a misstatement, you will actually lose points. Answer the question fully, and then stop.

Final Advice About the Free-Response Questions

Here are some final tips and advice regarding the free-response section of the test:

  • Annotate any calculations with words. Explain why you’re using the equation you’re using, and show the values you’re plugging in.
  • If you don’t know exactly how to solve part of a problem, it’s okay to explain your thinking process as best you can. For example, “I know the centripetal force points toward the center of the satellite’s orbit, and I know it’s a gravitational force. But the centripetal acceleration cannot be calculated because I don’t know the value of this centripetal force.” Such an answer might earn partial credit, even if you were supposed to do a calculation.
  • Don’t write a book. Even a question that asks for a paragraph response should be answered in a few sentences, not a few pages. Get straight to the point.
  • If you make a mistake, cross it out. If your work is messy, circle your key points or your final answer so that it’s easy to find. Basically, make sure the readers know what you want them to grade and what you want them to ignore.
  • If you’re stuck on a free-response question, try another one. Question 5 might well be easier for you than Question 1. Get the easy points first, and only after that, try to get the harder points with your remaining time.
  • Put units on every numerical answer.
  • Don’t be afraid to draw—diagrams, graphs, or whatever—in response to a question. These may be useful elements of an explanation, especially on the occasions when you’re forbidden from using numbers or equations. Be sure to label diagrams and graphs.
  • If your approach is so complicated that it’s not doable in 15 to 20 minutes with minimal calculator use, you’re doing it wrong. Look for a new way to solve the problem, or just skip it and move on.

1 If your teacher doesn’t expect you to show work clearly and explain your answer in words, do it anyway—that’s good AP exam preparation, see?

2 You can find rubrics to old AP Physics B and C exams on the College Board’s AP Central website. Although the principles of grading to a rubric will not change with the switch to AP Physics 1, the style of the rubrics will likely be different in future years.

3 Like saying that a person not named “Clark Kent” was running 25 m/s.