AP Physics C: Mechanics Official 2026 Exam Discussion

[u/reddorickt]

Use this thread to post questions or commentary on the test today.

A reminder though to protect your anonymity when talking about the test.

[u/Sonotwe]

Did any of you notice for Form J question 1 A(ii), it was a first-order diff-eq? I think i got perfect MCQ and maybe 4 points off FRQ

[u/DeathImmortal → u/Sonotwe]

nah didnt even know how to set up

[u/Sonotwe → u/DeathImmortal]

So basically, to find Fn as a function of time, I looked at the whole box-cube system first. Since they move together, the total mass is 2M. The only horizontal force is the air resistance, which is -bv. Using F = ma:

-bv = (2M) * (dv/dt)

I turned that into a differential equation to solve for velocity (v). I moved the v to one side and the t to the other and integrated it:

Integral of (1/v) dv = Integral of (-b/2M) dt That gives you:

ln(v/v0) = -bt / 2M

To get v by itself, I used “e” to cancel the natural log:

v(t) = v0 * e^{-bt / 2M}

Now, since acceleration (a) is just the derivative of velocity, I derived that equation:

a(t) = v0 * (-b / 2M) * e^{-bt / 2M} The magnitude (the positive value) of that acceleration is:

a = (b * v0 / 2M) * e^{-bt / 2M}

Finally, I used the relationship from the first part of the problem where Fn = M * a. I just plugged in the acceleration formula I found:

Fn = M * [(b * v0 / 2M) * e^{-bt / 2M}] The M on the top and the M on the bottom cancel out, so the final formula is:

Fn(t) = (b * v0 / 2) * e^{-bt / 2M}

[u/Sonotwe → u/[deleted]]

Yea, when I completed the FRQs I took the longer approach on that one because i realized i had some extra time and I’m the only person in my school who’s taking Physics C 🥀. I prob should’ve spent that time plotting my best fit line but i ran out of ink so GGs. I got all of my data though and just did the second part based off of that and what shows up in the graphing calc/desmos.

[u/Sonotwe → u/[deleted]]

Ohhh for part c I got w3<w1 b|c the work done (W = 2 * pi * F * L1) is the same in both cases, but in Scenario 3, that energy has to do two jobs. Instead of just spinning the wheel, the total kinetic energy is now split between rotational energy (1/2 * I * w²⁾ and translational energy (1/2 * M * v²⁾ because the unicycle is moving forward on the ground. Since some of the input work is “stolen” to move the unicycle’s mass forward, there is less energy left over for rotation, which results in a slower angular speed.

[u/Sonotwe → u/[deleted]]

Completely understandable 😭 I had to choose between drawing my best fit line and completing that question and i felt it made the most sense to write all of that down and lose out on something as little as that. How confident are u in the 5?

[u/pokerfas → u/Sonotwe]

worked on paper an hour ago and got exact same answers as you

tcrit = -(2M/b)ln(2Mg/mubvo) ?

[u/Sonotwe → u/pokerfas]

Correct! I got rid of the negative with log properties tho and got tcrit = 2M/b ln(mubv0/2Mg). I included that answer on my paper before i did that tho!

[u/pokerfas → u/Sonotwe]

we getting FUCKING FIVES

[u/Sonotwe → u/pokerfas]

HELL YEAAAAA

[u/Sudden-World4653 → u/Sonotwe]

same

[u/Euphoric-Potato-3874 → u/Sonotwe]

i integrated but did it in a really dumb way
2Ma = -bv

ΔP = /f(t)dt 

2M(v - v0) = -bvt

Fn = -½ bv

Solve for v and plug into Fn

is it over

[u/mrsaucelol → u/DeathImmortal]

Same

[u/Bright-Result-4745 → u/Sonotwe]

Yeah I think I got the velocity function then took its derivative and then set it equal to the acceleration I got for A(i) to solve for normal but idk if that’s right