I didn’t really screw up, just half of a part but they don’t carry over mistakes into the next parts so I am guessing that’s ;like a 1-2 point deduction and that is it
Can someone post a link to the frqs?
The questions, or answers?
Here are the free responses: https://secure-media.collegeboard.org/digitalServices/pdf/ap/ap15_frq_physics_c-m.pdf which have just been released.
I can definitely share my answers for any which you’d like me to.
3 please on FRQ form O.
Okay @ClassicalCollege , I’d be happy to redo my work for that one.
So part a) I said the moment of inertia is defined as the integral of r^2 dm. In this case our bounds are 0 and L (as it’s rotating about its end). We replace the dm with M/L dx (ie. the unit mass times an infinitesimal length). Thus, we have M/L * the integral from 0 to L of r^2 dx. Evaluating this integral, we get M/L *[r^3/3] evaluated from 0 to L which becomes M/L * L^3/3=ML^2/3- the desired answer.
Okay, part b) I used energy analysis. I set the initial gravitational potential energy equal to the sum of final gravitational potential and rotational energies. I let my reference line lie at the bottom; thus initial gravitational potential is MgL and final is MgL/2. Our rotational kinetic energy is 1/2Iw^2=1/2ML^2/3w^2. Thus, our conservation of energy equation is MgL/2=1/2ML^2/3w^2. Solving for w, we get sqrt (3g/L). As velocity is w*L, our velocity at point B is sqrt(3gL).
Part c)and d) I graphed length along the horizontal and velocity^2 along the vertical.
Part e) I really don’t remember what I got- I just remember that my final slope was around 26.7. As v^2=3gL, the slope is 3g, so the experimental value for g would be around 8.9 m/s^2.
The last part I ran out of time for on the actual exam, but some possibilities I thought of would be to reduce the starting angle and reduce the length of the bar (I’m sure there are many others).
Are my answers similar to those of others? Please let me know if I did anything wrong/if you got anything different…
thanks @Mathinduction I really messed up part b and e so I ended up skipping them, but for part a, I went through the same derivation process and got the right answer. Running out of time, I just matched up units, and graphed the length vs. velocity^2 like you did - i imagine we got very similar graphs. On part e, I got the right slope but as I did not do part b correctly, I wasn’t able to go any further (I could have probably divided by 3, but I was rushing). For part ii on e, I said something about decreasing surface area but I honestly don’t know.
@ClassicalCollege - I wouldn’t worry too much; just from the work you listed, it sounds like you still have the potential to have gotten a good 7-8+ points.
what angle did you get for #2?
also, if I messed up the sign conventions on question 1–and my answers to a(i) (ii) and (iii) were all different because of that, how many points would I lose?
Hi @Chalmydia - I don’t remember exactly; however, without redoing all of the computations, I think I remember getting around 24 degrees.
What sign convention did you use? I listed v0 as a negative (so for b- I had gsintheta*t-v0 (I hope that’s correct!)). I think at worst if each is 2-3 points, they’d remove 1 point from each answer; however, at best they’d just remove 1 point the first instance you used the wrong sign convention and then give you full credit from there on (assuming your computations were correct). I think the second option is more likely.
damn i got like 75 degrees or something
and i like put -gsintheta as acceleration for example
@Chalmydia - I know what you might have done for the 2nd one. After you’ve solved for h using the energy equation, that height isn’t a side in your triangle. 1.2 m is along the hypotenuse, but your height is not the h you solved for but rather 1.2-h. If that’s the case and you showed energy analysis, you’d definitely get most of the credit. There’s also a good chance that I got it wrong, so definitely don’t trust my answer completely!!
You guys are lucky you had form O. I was the only one in my school with form M and, when looking at the release form O questions, all I can say is that form M was 10x as difficult…
oh…i think i made a calculator mistake: i did the exact problem again, and i got 24.9 degrees, lol. i had the right process too, oh well
YES. 24.9 degrees for the win. I screwed up 3 a,b, and e. i. though… Also the angle in #2 decreases with increasing mass right? I said increases initially, but people were trying to convince me otherwise today when we went over them
I got 23.99 degrees. Probably a difference in g value, as I used g=10.
For 2e, T=2pi(sqrt(l/g)), therefore the desired time is pi(sqrt(1.2/10)), right? Also, can anyone describe their graph for 1e?
@theboss262 , I think the angle for #2 might increase with increasing mass (that’s what I said). As velocity, etc. remain the same the momentum would increase meaning that it would have a greater impact, thus causing an increase in the angle. I’m not 100% sure though.
Form M is literally Form M. It’s a different version of the test (if there were only 1 form, form O, why would they even bother naming it “Form O” in the first place?)
No, you did not get an AP Physics 1 lmao. Otherwise, it would be easier and you would see electricity/waves on it. Also, it’s normal to not see any calculus. On Form O, there were only a couple of instances with Calculus, so due to sampling variability there very well could have been none.
To conclude, you should not be suspicious or worried about anything (except maybe for the fact that you didn’t do too well? if that was the case).