How to Approach the Free-Response Section for AP Physics B & C
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 we bet that your teacher always reminds you to show your work. This sounds like the AP free-response section to us!
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 we're 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.
What Do the Graders Look For?
Before looking at a single student's exam, the high school and college physics teachers who are responsible for grading 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:
A student pulls a 1.0-kg block across a table to the right, applying a force of 8.0 N.
The coefficient of friction between the block and the table is 0.20.
- Determine the force of friction experienced by the block.
- If the block starts from rest, what is 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:
- Applying Newton's second law, Fnet = ma, to find the block's acceleration.
- 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.
- Using a correct kinematics equation with correct substitutions to find the final velocity of the block; i.e., vf = vo + at, where vo = 0 and a = 6.0 N/1.0 kg = 6.0 m/s2.
- Obtaining a correct answer with correct units, 9.0 m/s.
Now, we're not suggesting that you try to guess how the AP graders will award points for every problem. Rather, we want you to see that the AP graders care much more about your understanding of physics than your ability to punch numbers into your calculator. Therefore, you should care much more about demonstrating your understanding of physics than about getting the right final answer.
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