Most students preparing for AP Physics 1 spend their time on multiple choice, because multiple choice is comfortable. You can do it on your phone, you find out immediately whether you were right, and being right feels like progress.
The free response is where the exam is decided, and it is the part almost nobody practices honestly. Scoring your own work against a real rubric is unpleasant — it makes visible exactly how much of what you wrote earned nothing. That discomfort is exactly why the hour is worth more than any other hour you’ll spend prepping.
Below is a complete worked example: an original problem in the AP Physics 1 style, the rubric, a wrong answer of the kind I see constantly, and a diagnosis of what the wrong answer reveals.
The problem is mine, not a released College Board question, and the point allocation is my own modeled on how their rubrics distribute credit. Use released exams from AP Central for the real thing. This is here to show you what the scoring actually rewards.
The problem
A block of mass m slides along a frictionless horizontal surface at speed v₀. It collides with a second block of mass 2m that is initially at rest. The two blocks stick together and continue moving.
The combined blocks then slide up a frictionless incline.
(a) Determine the speed of the combined blocks immediately after the collision, in terms of v₀. (2 points)
(b) Determine the maximum vertical height h that the combined blocks reach on the incline, in terms of v₀ and g. (3 points)
(c) A student claims that h can be found by setting the initial kinetic energy of the first block equal to the gravitational potential energy of the combined blocks at maximum height:
½ m v₀² = (3m) g h
Is the student correct? Justify your answer. (3 points)
Try it before reading on. Twenty minutes, no notes, and write part (c) in full sentences as you would on the exam.
The rubric
Part (a) — 2 points
- 1 point: For correctly applying conservation of linear momentum to the collision. Writing m v₀ = (3m) v_f earns this point, even if the algebra that follows is wrong.
- 1 point: For the correct answer, v_f = v₀ / 3.
Part (b) — 3 points
- 1 point: For applying conservation of mechanical energy after the collision, using the post-collision speed rather than v₀.
- 1 point: For a correct energy statement, ½ (3m) v_f² = (3m) g h.
- 1 point: For the correct answer, h = v₀² / (18 g).
Note the structure. The first point is for choosing the right principle at the right moment. You can lose the arithmetic and keep it. You cannot lose the principle and recover it downstream.
Part (c) — 3 points
- 1 point: For stating that the student is incorrect.
- 1 point: For identifying that the collision is inelastic, so kinetic energy is not conserved through it.
- 1 point: For a complete justification connecting the two — that some of the initial kinetic energy is converted to other forms during the collision, so the energy available to lift the blocks is less than ½ m v₀², which means the student’s method overestimates h.
The first point is worth almost nothing on its own, and students who write only “No, the student is wrong” get exactly one of three. The exam is not testing whether you can spot that something is off. It is testing whether you can say why.
A wrong answer, of the kind I see constantly
(a) m v₀ = 3m v_f, so v_f = v₀/3.
(b) Using energy: ½ m v₀² = 3mgh, so h = v₀² / (6g).
(c) Yes, the student is correct, because energy is always conserved.
This earns 2 out of 8. Both points in part (a), nothing in (b), nothing in (c).
Read part (a) again. The student did it perfectly. They know momentum conservation, they set it up correctly, they solved it correctly. Then in part (b) they abandoned their own result and went back to v₀.
That is the whole failure, and it is worth sitting with, because it is not a knowledge problem. A student who cannot do part (a) does not know the physics. This student clearly knows the physics and then does not use it thirty seconds later.
What the wrong answer is actually telling you
Three things stand out, and none of them is “study harder.”
First, the parts were treated as unrelated. Part (b) was started from scratch instead of built on part (a)’s answer, and on a multi-part AP question the parts are almost always a chain. The single cheapest habit to build is asking what did I just find, and does this part need it? before writing anything new.
Second, an equation was reached for before the situation was understood. Conservation of energy is a true and beautiful statement, and it does not apply across an inelastic collision in the form this student used it. The student pattern-matched on “block moving, block goes up a hill, energy problem” and never stopped to ask what happened during the collision itself. That’s the same failure mode I described on the physics page, which is why the habit I build first is a setup ritual that happens before any algebra.
Third, “energy is always conserved” is a real misconception wearing the costume of a correct statement. Total energy is conserved. Mechanical energy is not, when two blocks stick together — some of it turns into heat and sound at the moment of impact. The student isn’t ignorant here. They have half a true idea and no sense of where its boundary sits, and that is more dangerous than not knowing at all, because it feels exactly like knowledge.
There is also a check that would have caught this for free. The wrong answer gives h = v₀²/(6g); the right one gives v₀²/(18g). The wrong answer is three times larger. If you have just been told that the blocks stick together, and your method predicts they rise higher than the actual physics allows, something is wrong with the method — an inelastic collision should lose energy, not conserve all of it.
The response that earns full credit on part (c)
No, the student is not correct.
The collision is perfectly inelastic, since the blocks stick together, so kinetic energy is not conserved during the collision. Some of the initial kinetic energy of the first block is converted into other forms, primarily thermal energy, at the moment of impact.
This means the kinetic energy of the combined blocks immediately after the collision is less than ½ m v₀². Only that smaller amount is available to be converted into gravitational potential energy on the incline. Because the student uses the full initial kinetic energy, their method overestimates h.
Three sentences of setup, one sentence of consequence. Nothing rhetorical, no restatement of the question, no hedging. The graders are looking for specific physical claims, and every sentence above is doing work.
For the record, two thirds of the initial kinetic energy is lost in this collision. Worth verifying yourself.
How to use this
The mechanical part is straightforward and almost nobody does it:
- Do the problem timed, with no notes.
- Score yourself against the rubric before you look at any solution. Be strict — if a point requires a justification and you wrote a conclusion, you did not earn it.
- For every point you missed, write one sentence on whether you missed it because you did not know something, or because you knew it and did not use it.
That third step is the one that matters. Sort a few exams’ worth of lost points that way and a pattern shows up fast, and the two categories need opposite responses. Points lost to not knowing mean you go back and learn the content. Points lost to knowing and not using mean the content is fine and the method is not, and no amount of additional studying will fix it.
The second kind is the more common one at this level, and it is the faster one to fix.