﻿ 8 Strategies for the Free-Response Questions - STEP 3 Develop Strategies for Success

# STEP 3 Develop Strategies for Success8 Strategies for the Free-Response Questions

IN THIS CHAPTER

Summary: The AP Physics 2 exam contains a variety of problem types unique to the free-response section of the exam: laboratory design, qualitative-quantitative transition (QQT), paragraph-length argument, and student-contention questions. (If you took the AP Physics 1 exam last year, you have seen these already.) This chapter gives strategies and advice on how to approach these unique questions.

Key Ideas

The 90-minute free-response section contains four questions: a 12-point lab question, a 12-point qualitative-quantitative transition (QQT) question, and two 10-point short-answer questions. One of the 10-point questions will require a paragraph-length argument.

There are simple tips and a strategy you can use to maximize your score on the lab question.

This is not an English exam! Keep your paragraph-length argument short and sweet. Get to the point and then stop writing.

Student-contention questions are a unique type of question that can pop up on the exam. They are not that hard once you see how they work.

The QQT question asks you to use qualitative reasoning and then asks you to use equations and numbers. If you are stronger with numbers, work the problem in reverse.

You can get partial credit on the free-response portion of the exam. Remember that you need roughly only 70%—75% or so to earn a top score and only about 40%—45% or so to earn a qualifying score. Make sure to attempt every part of each problem to get the maximum partial credit possible.

The Structure of the Free-Response Section

The free-response section contains only four questions to be completed in 90 minutes. Piece of cake!

• One 12-point laboratory-based question

• One 12-point qualitative-quantitative transition (QQT) question

• Two 10-point short-answer questions of which one will contain within it a requirement for a paragraph-length argument

This gives you about two minutes per point or about 25 minutes to answer the longer questions and 20 minutes for the shorter ones.

The best thing about the free-response section of the AP exam is this: you’ve been preparing for it all year long! “Really?” you ask, “. . . I don’t remember spending much time preparing for it.”

But think about the homework problems you’ve been doing throughout the year. Every week, you probably answer a set of questions, each of which might take a few steps to solve, and I’ll bet that your teacher always reminds you to show your work. This sounds like the AP free-response section to me!

The key to doing well on the free-response section is to realize that, first and foremost, these problems test your understanding of physics. The purpose is not to see how good your algebra skills are, how many fancy-sounding technical terms you know, or how many obscure theories you can regurgitate. So all I’m going to do in this chapter is give you a few suggestions about how, when you work through a free-response question, you can communicate to the AP graders that you understand the concepts being tested. If you can effectively communicate your understanding of physics, you will get a good score.

Getting Off to a Good Start

Every year there are horror stories of students getting stuck on a problem and not finishing the free-response questions. “I got stuck on the first one and then I had to rush. But I still ran out of time!” Rule No. 1: Get off to a good start! When you begin the free-response section, remain calm and keep your writing utensil on your desk. Take a quick inventory of the four questions and ask yourself: “Which of these looks the easiest?” Start there. This will ensure that you start on a positive note and have as much time as possible to finish the exam. Practice this strategy in class during your exams. Always work your way from what seems easiest to what seems hardest.

Lab Questions

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

The AP development committee is sending a message to students that laboratory work is an important aspect of physics. To truly understand physics, you must be able to design and analyze experiments. Thus, each free-response section will contain one question that involves experiment design and analysis.

Here is an example:

In the laboratory, you are asked to investigate the relationship between the temperature and volume of a fixed amount of gas. The following equipment is available for use:

___ a cylinder with a movable piston

___ a cylinder with a fixed volume

___ a pressure sensor

___ a container large enough to hold a cylinder with extra room

___ a source of hot water

___ a source of ice water

___ a ruler

___ a thermometer

___ a stopwatch

___ a set of masses that will fit on the movable piston

(a) Put a check in the blank for each of the items above that you would need for your investigation.

(b) Outline an experimental procedure you would use to gather the necessary data to answer this question. Make sure your procedure is practical and contains sufficient detail and information so that another student could follow your directions.

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

Exam Tip from an AP Physics Veteran

On the 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 points!

Zack, college senior and engineer

2. Use as few words as possible. Answer the question, then stop. You can lose credit for an incorrect, or inconsistent, statement, even if the other 15 statements in your answer are correct. The best idea is to keep it simple.

3. There is no single correct answer. Most of the lab questions are open-ended. There might be four or more different correct approaches. So don’t try to “give them the answer they’re looking for.” Just do something that seems to make sense—you probably will be right!

4. Don’t assume you have to use all the stuff they give you. It might sound fun to use a force probe while determining the index of refraction of a glass block, but really? A force probe?

5. Don’t over-think the question. They’re normally not too complicated. Remember, you’re supposed to take only about 25 minutes to write your answer. You’re not exactly designing a subatomic particle accelerator. The lab you design should be simple and elegant. When your lab design is overly complicated, let’s be honest, it’s probably not a very good lab.

6. Don’t state the obvious. You may assume that basic lab protocols will be followed. So there’s no need to tell the reader that you recorded your data carefully, nor do you need to remind the reader to wear safety goggles.

7. Find a physics relationship/equation to help you. Determine what physics relationship is being investigated and find an equation that models that behavior. The example above is obviously concerning gases and gas laws. That leads us to the ideal gas law: PV = nRT.

8. Let your physics relationship guide you through the lab. Now that we have PV = nRT, where does it lead us? Well, we are looking for a relationship between temperature and volume. So we will need to change one of these variables and measure the change in the other. The equation also tells us that pressure and number of molecules are involved in the relationship. But, we are not investigating those quantities, so we must keep them constant.

9. Put your plan together. Here is one possible plan:

• Use the movable piston and put a mass on top of the piston to maintain constant pressure. The amount of gas in the cylinder should remain constant.

• Measure the temperature with the thermometer. Use a ruler to measure the height of the gas in the cylinder and multiply by the area of the piston to find the volume.

• Starting at room temperature, place the cylinder in the large container with ice water.

• Record the temperature as it decreases and measure the volume of the gas at every 10°. Collect the data in a table and plot the relationship.

Note that choosing the equipment came after finding a physics relationship and developing a plan. To make sure your explanation is clear, it is helpful to draw a picture even if the question doesn’t ask for one. Make sure you describe what measurements you will take. Explain how you will be finding any calculated values, like we just did for volume.

Practice Designing a Lab

In the following table is a list of common lab-based questions that ask you to design a lab. Next to each lab is a list of useful equations that you might use to design your lab around. For example: if you are tasked with finding the density of a fluid, you might use the basic definition of density by measuring the mass and volume of the fluid: . Or, you might want to use the buoyancy force by floating an object of known mass on the fluid and measuring the volume displaced: Fb = ρVg. Take some time to review the following list of labs and associated equations. There will be a lab question on the free-response section. Practice designing labs. Ask your teacher to help you perform some of them. Remember, practice makes perfect!

Paragraph-Length Arguments

You are in the middle of the free-response section and see these fateful words: “In a coherent paragraph-length answer, describe . . . .” This is followed by a question and a full page of blank white paper just begging you to write all over it. Your brain automatically switches into English-essay mode. STOP! Before you write anything, remember: Less is better than more. This is not an English exam. The humans grading the free response are physics people. They do not want fluff. They are looking for specific information that tells them you understand the physics of the situation.

Here is an acronym that will help: CLEVeR.

State your claim and support it with evidence and reasoning. Get to the point. Answer the question and stop writing. You usually earn credit for just having a sequential and logical answer that does not have anything contradictory, wrong, or irrelevant! If you keep writing, you are likely to contradict yourself or use incorrect physics and lose points. If you can answer the question in three sentences, do so. Be CLEVeR! Keep it short and sweet.

“How do I maximize my score on paragraph-length argument questions?” First off, there is only one per AP exam. Second, they are usually worth 5 to 7 points, which means they will only be a part of a 10-point question. So don’t get stressed out about paragraph-length response questions. Botching your answer won’t keep you from passing the exam.

Yeah, yeah . . . but “How do I maximize my score on paragraph-length response questions?” Here is what the graders are looking for:

1. Answer primarily written in prose form. (That means you must use words as your main method for explaining your ideas.)

2. Handwriting must be readable! (Sloppy handwriting that cannot be read by a reasonable person in a reasonable amount of time with reasonable effort will not receive any points. It is your responsibility to put your ideas on paper so they can be understood by others.)

3. The answer follows an organized and logical thought process. (One idea leads to another, and it should make sense after being read through the first time.)

4. The answer sites physics principals and ideas by name. (For example: “Charge will move until equilibrium is established, which is when the electric potential is the same for both metal spheres.” Here is another example: “Because there is only one pathway in the circuit, Kirchhoff’s current rule tells us that the current is the same for both resistors.) Always make sure you invoke the key physics principles and ideas in your argument!

“Wait a minute. Lots of stuff in physics is hard to explain without an equation, graph, diagram, or sketch! Can’t I use them?!?!” YES, you can use equations, graphs, diagrams, and sketches, BUT your answer must be primarily WORDS. I always tell my students to write out the equation they are using and then describe how the equation is important in your argument. The same goes for graphs, figures, and diagrams.

One last piece of advice. The AP Physics readers consistently tell us that students need to be more precise in their explanations. Remember, you are not writing poetry that is up to the interpretation of the reader. You need to use “physics language” and avoid the use of pronouns. Instead of saying “It gets pushed away.” Be more precise! Say something more like this: “The electron receives a force to the right due to the electric field.” Or, “The electric force pushes the electron to the right.” Speak in “Physics” and your grade will improve.

Student-Contention Questions

The student-contention question is an interesting type of question that sometimes shows up on the exam. It is not hard, just different. Here is an example:

A student makes this claim about the electrons making the two transitions shown in the energy-level diagram:

“The electrons will both emit a photon of light during their transition. The photon emitted by transition A will be a shorter wavelength than transition B because the electron begins at a higher-energy level.”

(a) What aspects of the student’s statement do you agree with and which do you disagree with? Qualitatively explain your reasoning.

(b) Derive an algebraic expression for the emitted photon.

In these questions, simply and directly explain your reasoning using physics principles. Here is a sample solution to this problem using the CLEVeR method:

(a) I agree that photons will be emitted in both cases (your claim). The electrons are losing energy as they drop to lower energy levels and will emit a photon (your evidence and reasoning).

I disagree that A will have a shorter wavelength (your claim). B has a larger drop in energy, which will produce a higher-energy photon of shorter wavelength (your evidence and reasoning).

(b) , Therefore: .

(c) The equation shows the wavelength and photon energy to be inversely related (your claim). Therefore, the larger energy transition in B will produce a smaller wavelength (your evidence and reasoning).

Qualitative-Quantitative Transition (QQT) Questions

Look back at the student-contention question we just completed. Notice how parts (a) and (c) have no numbers. This is a qualitative reasoning question asking you to use words to make your argument. Part (b) asks for an algebraic derivation. This is a quantitative reasoning question asking you to use a symbolic equation to make your argument. Quantitative reasoning would also include finding a number answer similar to if you had been asked to calculate the wavelength of the emitted photon.

The QQT question will ask you to consider a situation and explain it first qualitatively (with words) and then quantitatively (with equations and numbers). You probably have been solving lots of number problems in your AP Physics 2 class and use your calculator a lot. As a consequence, you may be more comfortable with number problems. If this is the case, play to your strengths and solve the number part of the problem first, and then answer the conceptual part. Here is an example:

Two moles of gas is taken from its original state at point A through point B to a final state at point C as shown in the graph. A student makes this statement: “The net heat added or subtracted from the gas is zero because the final temperature of the gas is the same temperature as its starting temperature.”

(a) Do you agree or disagree with the student’s statement? Explain your reasoning with words.

(b) Justify your argument with a calculation.

Which do you feel more comfortable answering (a) or (b)? If you are a numbers person, start by solving the calculation first and then put it into words. Here’s how it would work for this question:

Solving part (b) with equations:

PAVA = PCVC = nRT, since the PV value at points A and C are the same, the temperature change in the process is equal to zero.

, since the change in temperature during the process is zero, the change in internal energy is also zero.

WABC = —PΔV = —(800,000 Pa)(0.0004 m3 — 0.0001 m3) = —240 J, there is no work done moving from point B to C as there is no change in volume.

ΔU = 0 = Q + W, therefore Q = —W = —(240 J) = 240 J.

Now that you have the math worked out, put it in words to answer (a). Remember to be CLEVeR with your response!

I agree the temperatures at A and C are the same because the pressure times volume is the same at both locations. I disagree that the heat is zero. Since the temperature does not change, the change in the internal energy of the gas is zero. There is negative work done by the gas during its expansion. Therefore, heat must be added to the system.

Choose to approach the QQT question with equations first and words second, or words first and equations second. It doesn’t matter. The key is to play to your strengths so that you earn as many points as you can. Speaking of earning points, let’s talk about how the exam is graded.

What the Exam Reader Looks For

Before grading a single student’s exam, the high school and college physics teachers who are responsible for scoring the AP free-response section make a “rubric” for each question. A rubric is a grading guide; it specifies exactly what needs to be included for an answer to receive full credit, and it explains how partial credit should be awarded.

For example, consider part of a free-response question from AP Physics 1:

A student pulls a 1.0-kg block across a table to the right, applying a force of 8.0 N. The coefficient of kinetic friction between the block and the table is 0.20. Assume the block is at rest when it begins its motion.

(a) Determine the force of friction experienced by the block.

(b) Calculate the speed of the block after 1.5 s.

Let’s look just at part (b). What do you think the AP graders are looking for in a correct answer? Well, we know that the AP free-response section tests your understanding of physics. So the graders probably want to see that you know how to evaluate the forces acting on an object and how to relate those forces to the object’s motion.

In fact, if part (b) were worth 4 points, the graders might award 1 point for each of these elements of your answer:

1. Applying Newton’s second law, Fnet = ma, to find the block’s acceleration.

2. Recognizing that the net force is not 8.0 N, but rather is the force of the student minus the force of friction (which was found in [a]), 8.0 N − 2.0 N = 6.0 N.

3. Using a correct kinematics equation with correct substitutions to find the final velocity of the block; i.e., v = vo + at, where vo = 0 and a = 6.0 N/1.0 kg = 6.0 m/s2.

4. Obtaining a correct answer with correct units, 9.0 m/s.

Partial Credit

Returning to part (b) from the example problem, it’s obvious that you can get lots of partial credit even if you make a mistake or two. For example:

• If you forgot to include friction, and just set the student’s force equal to ma and solved, you could still get 2 out of 4 points.

• If you solved part (a) wrong but still got a reasonable answer, say 4.5 N for the force of friction, and plugged that in correctly here, you would still get either 3 or 4 points in part (b)! Usually the rubrics are designed not to penalize you twice for a wrong answer. So if you get part of a problem wrong, but your answer is consistent with your previous work, you’ll usually get full or close to full credit.

• That said, if you had come up with a 1000 N force of friction, which is clearly unreasonable, you probably will not get credit for a wrong but consistent answer, unless you clearly indicate the ridiculousness of the situation. You’ll still get probably 2 points, though, for the correct application of principles!

• Notice how generous the partial credit is. You can easily get 2 or 3 points without getting the right answer and 40%—75% is in the 3—5 range when the AP test is scored!

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

• You will not get partial credit if you write multiple answers to a single question. If AP graders see that you’ve written two answers, they will grade the one that’s wrong. In other words, you will lose points if you write more than one answer to a question, even if one of the answers you write is correct.

• You will not get partial credit by including unnecessary information. There’s no way to get extra credit on a question, and if you write something that’s wrong, you could lose points. Answer the question fully, then stop.

• Always show your work. If you use the correct equation to solve a problem but you plug in the wrong numbers, you will probably get partial credit, but if you just write down an incorrect answer, you will definitely get no partial credit.

• If you don’t know precisely how to solve a problem, simply explain your thinking process to the grader. If a problem asks you to find the radius of a charged particle in a magnetic field, for example, and you don’t know what equations to use, you might write something like this: “The centripetal force points toward the center of the path, and this force is the magnetic force.” This answer might earn you several points, even though you didn’t do a single calculation.

• If you make a mistake, cross it out. If your work is messy, box your answer so that it’s easy to find. Basically, make sure the AP graders 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 3 might be easier for you than question 1. Get the easy points first, and then try to get the harder points only if you have time left over.

• Always remember to use units.

• It may be helpful to include a drawing or a graph in your answer to a question, but make sure to label your drawings or graphs so that they’re easy to understand.

• No free-response question should take you more than 20—25 minutes to solve. They’re not designed to be outrageously difficult, so if your answer to a free-response problem is outrageously complicated, you should look for a new way to solve the problem, or just skip it, move on, and come back to it later.

• Use “physics language” and avoid the use of pronouns. Instead of saying “It gets pushed to the right.” Say something like “The electron receives a force to the right due to the electric field.” Speak in “Physics”!

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