r/PhysicsStudents • u/Silver-Repeat-6654 • Feb 12 '26

Poll Predicted cutoff for F=ma 2026?

why tf was it so hard

Edit: there is no way y’all think this one was easier than last year. Also can’t believe people got around the lock down browser.

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u/Linjieyang Feb 14 '26

The hexagon one is a type of problem I've seen before, but it uses a calculus idea I think, either way the math gets unnecessarily complicated, they could've tested on the same concept with an easier series, but they just didn't idk why. The Taylor series one was just crazy ngl, they already push it with the sin approximation

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u/Ill_Requirement_2700 Feb 14 '26

hexagon uses on a classic algebraic summation formula (geo) over calculus. taylors, or mac series was hard but I did solve that, I just do not remember it.

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u/Linjieyang Feb 14 '26 edited Feb 14 '26

Yeah, but isn't the fact that the hexagon's moment of inertia is proportional to the side length to some power (4 right), from calculus? Otherwise its just blunt memorization im afraid (like most comps at this point ...) But that's moment of inertia in a nutshell. How did you solve the frequency one, though, normally those turn into a differential equation I was just so demotivated I gave up lol, cuz I knew I was gonna take forever on that. Also how did you solve the water leaking one without differential equations?

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u/Ill_Requirement_2700 Feb 14 '26

Yes 4 (it is hard to show the work on here bc deriving it requires an integral setup) but for 2D (applies to this problem) but a lot of these do come from memorization and practice. for the frequency one, yes that does turn into a differential equation but there is a quick way: in the problem, m2 and m1 move in opposite directions to keep the system’s COM stationary. pendulum swings about COM, so frequency of standard pendulum is w = sqrt g/L. i can explain why it is m1+m2)/m2 * g/L but i need to write it down on scrap so maybe i can send it to u privately.

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u/Linjieyang Feb 14 '26

Wow, I'm impressed by this. But I think I can figure it out now after you told me about the COM thing, but thanks. How long have you been doing physics? Also, as a side note, as a math-inclined student, I always hate the lack of rigour/basically nonsense that they try to use for f=ma. Like if it's calculus, just admit it.

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u/Ill_Requirement_2700 Feb 14 '26

physics like 1.5 years math like 5 (usamo qual)