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 section 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 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: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:
Applying Newton’s second law, F (^) net = 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/s^2 .
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.
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: