Official 2011 AP Physics C Thread

[salzahrah]

<p>i still dont see how magnetic field is related to dE/dt lol</p>

<p>and thanks for that graviton!!</p>

[Wonderer1]

<p>@ Gravitron, thank you so much. My physics is really bad and even after a lot of reading and attempts, i can only get like 10-15 right on the mc and a like only 10 points max from the FR. I’m only aiming for a 3, you made me feel a little better, thank you so much</p>

[Abrayo]

<p>50/90 is not quite failing. I’m worried because I consistently get low 20s in the multiple choice and my FR is scattered. </p>

<p>I noticed that you get points for writing down equations.
Let’s say that there’s a problem when you have to use conservation of momentum, but I tried it as conservation of energy. If I wrote down BOTH formulas, but only ended up using the energy formula, do I still get the point for writing the momentum one down? Or does it only count if I actually use it?
Otherwise people would just write down a bunch of formulas, right?</p>

[Graviton]

<p>There are obviously a lot of ways to solve problems, and they are going to grade the route you take, so no, I don’t think you would get points for that. If energy and momentum were both conserved and you wrote that down and then solved whatever problem using energy instead of momentum, then you would still get credit for the first problem.</p>

[Abrayo]

<p>I meant in the case where energy was not conserved, so using that would be wrong.
But okay, what if I did this:
Stated conservation of momentum, put in some values, got stuck, and just stopped because I didn’t know what to do.
Then I did conservation of energy (without crossing anything out), and did the problem incorrectly.</p>

<p>Would I get anything for trying conservation of momentum? Would I get nothing because I got “two different answers” (even if I stopped in the middle of one)?</p>

[NYEM]

<p>For those of you worrying about the curves, it seems as though the amount needed for a 5 has decreased over the years, with an increase in test difficulty. Here’s some data on the cutoff scores:</p>

<p>Electricity and Magnetism
1988

• [<em>]5 - 51/90
  [</em>]4 - 34/90
  [<em>]3 - 24/90

1993

• [</em>]5 - 56/90
  [<em>]4 - 40/90
  [</em>]3 - 29/90

1998

• [<em>]5 - 49/90
  [</em>]4 - 35/90
  [<em>]3 - 26/90

2004</em>

• [<em>]5 - 48/90
  [</em>]4 - 34/90
  [<em>]3 - 28/90

2009</em>

[ul]
[<em>]5 - 53/90
[</em>]4 - 39/90
[li]3 - 32/90[/li][/ul]</p>

<p>Mechanics
1988

• [<em>]5 - 56/90
  [</em>]4 - 43/90
  [<em>]3 - 31/90

1993

• [</em>]5 - 59/90
  [<em>]4 - 46/90
  [</em>]3 - 34/90

1998

• [<em>]5 - 55/90
  [</em>]4 - 43/90
  [<em>]3 - 32/90

2004</em>

• [<em>]5 - 52/90
  [</em>]4 - 41/90
  [<em>]3 - 32/90

2009</em>

[ul]
[<em>]5 - 50/90
[</em>]4 - 39/90
[li]3 - 31/90[/li][/ul]</p>

<p>*These curves were adjusted by the college board to accomodate the elimination of the guessing penalty. My guess is that the actual curves were much lower without this adjustment!</p>

[scholarshipFTW]

<p>Help with E&M?

1. <a href=“http://tinyimage.net/images/25790944428468568236.png[/url]”>http://tinyimage.net/images/25790944428468568236.png&lt;/a&gt;
   I don’t even… The answer is B… What should I review??</p>

<p>2)<a href=“http://tinyimage.net/images/81684598844478119651.png[/url]”>http://tinyimage.net/images/81684598844478119651.png&lt;/a&gt;
Why can’t I use the formula V^2/R for this one?? answer C btw ( I did get it right, but just because I didn’t see the answer choice I needed had I used V^2/R…)</p>

<p>3)<a href=“http://tinyimage.net/images/14929814429358178541.png[/url]”>http://tinyimage.net/images/14929814429358178541.png&lt;/a&gt;
How do I approach questions like 52? The answer is E… (I get 51 though)</p>

<p>4)<a href=“http://tinyimage.net/images/89661487184785817963.png[/url]”>http://tinyimage.net/images/89661487184785817963.png&lt;/a&gt;
Umm… I don’t remember learning this… What is this an application of?</p>

<p>5)<a href=“http://tinyimage.net/images/05849919813601766271.png[/url]”>http://tinyimage.net/images/05849919813601766271.png&lt;/a&gt;
57:B -> Why is R2 ignored? I put A originally…
58:A -> I thought there is no current after a significant amount of time b/c capacitor takes up all the charge? no?</p>

[Abrayo]

<p>I think I can get a 4 on this exam, but a 5 is a huge reach for me. . . </p>

<p>Do you think it’s likely that there will be a torque FRQ?</p>

<p>Also are we supposed to have a ruler?
To draw the line of best fits? Or are we not allowed one?
Would we be penalized for a line that isn’t completely straight?</p>

[actpro]

<p>

[quote]

Could someone help me with these Mech questions?</p>

<p>AP Physics C Mechanics Practice Tests_°Ù¶ÈÎÄ¿â</p>

<h1>34, doesn’t the escape velocity depend on the mass of the planet since Ve = sqrt(2GM/R)?</h1>

<p>why is the answer not B?</p>

<h1>32, I don’t know how to do this one.</h1>

<h1>28, if U =0.5kx^2, and k = F/x = 9, then 0.5<em>9</em>3^2 = 81/2? no? Why is the asnwer 81/4?</h1>

<ol>
<li>Again, I dont get this one</li>
</ol>

<p>25,26, not sure how angular acceleration varies with theta</p>

<p>Thanks!

</p>

<p>Anyone? :(</p>

[Abrayo]

<p>^ sorry, not sure what the questions are.</p>

<p>For every FRQ, is there always a calculus based part? Or not necessarily?</p>

[salzahrah]

<p>mostly always^
that one MC question about the spring getting cut in half and the new spring constant being 2K is like intended for students to get wrong…=/</p>

[Abrayo]

<p>All right.</p>

<p>What question are you talking about? Or did you just bring that up randomly?</p>

[TheMysteriousOne]

<p>For #28, the F is not -kx or -kx^2 (I suck at springs), but if you are not hasty and look at the chart, it is rather x^3. Therefore, take the integral of that (sign does not really matter because work is positive, generally speaking) and you get U(s) = 1/4x^4. Sub in 3 and you get 81/4</p>

[WongTongTong]

<p>

mostly always^
that one MC question about the spring getting cut in half and the new spring constant being 2K is like intended for students to get wrong…=/

</p>

<p>Hmm, I can see how it could trip people up, but if you remember that k is an indicator of how stiff the spring is, then the answer, intuitively, should be twice as stiff if you cut it in half.</p>

[actpro]

<p>Abrayo, here it is:</p>

<p>[AP</a> Physics C Mechanics Practice Tests_°Ù¶ÈÎÄ¿â](<a href=“AP Physics C Mechanics Practice Tests - 百度文库”>AP Physics C Mechanics Practice Tests - 百度文库)</p>

<p>@Mysterious: Ahhh, I get it now. That was a tricky one haha, and yeah, I was rushing through it.</p>

[TheMysteriousOne]

<ol>
<li>Hopefully I don’t explain this incorrectly. My physics teacher told me when in doubt, it’s generally safe to assume momentum is held constant, though I guess this is one of those cases where you can’t make that assumption.
L= Iw
I for a point-mass = MR^2
w = v/R
L = MvR
IIRC, when doing stuff with rotation, you define it as the perpendicular distance, which in this case should be the horizontal distance between the ball and P which remains constant.
Thus, you have only v to worry about. Since v is accelerating (at a constant g), you have a linear velocity and MR can be treated as a constant. Thus, L=kv which fits C</li>
</ol>

[scholarshipFTW]

<p>Anyone for any of the questions :(</p>

<p>1) <a href=“http://tinyimage.net/images/25790944428468568236.png[/url]”>http://tinyimage.net/images/25790944428468568236.png&lt;/a&gt;
I don’t even… The answer is B… What should I review??</p>

<p>2)<a href=“http://tinyimage.net/images/81684598844478119651.png[/url]”>http://tinyimage.net/images/81684598844478119651.png&lt;/a&gt;
Why can’t I use the formula V^2/R for this one?? answer C btw ( I did get it right, but just because I didn’t see the answer choice I needed had I used V^2/R…)</p>

<p>3)<a href=“http://tinyimage.net/images/14929814429358178541.png[/url]”>http://tinyimage.net/images/14929814429358178541.png&lt;/a&gt;
How do I approach questions like 52? The answer is E… (I get 51 though)</p>

<p>4)<a href=“http://tinyimage.net/images/89661487184785817963.png[/url]”>http://tinyimage.net/images/89661487184785817963.png&lt;/a&gt;
Umm… I don’t remember learning this… What is this an application of?</p>

<p>5)<a href=“http://tinyimage.net/images/05849919813601766271.png[/url]”>http://tinyimage.net/images/05849919813601766271.png&lt;/a&gt;
57:B -> Why is R2 ignored? I put A originally…
58:A -> I thought there is no current after a significant amount of time b/c capacitor takes up all the charge? no?</p>

[theRADtomato47]

<p>1984 #31) Mass M1 is moving with speed v toward stationary mass M2. The speed of the center of mass of the system is what?
a) (M1/M2)v<br>
b) (1+M1/M2)v<br>
c) (1+M2/M1)v<br>
d) (M1/ M1+M2)v </p>

<p>Can someone explain why the answer is d?</p>

[TheMysteriousOne]

<p>I accidentally figured something like that out during a mock physics test
VCM = SUM(mi*vi)/SUM(mi) (just pretend it’s a summation, lolz)
V = (M1v)/(M1+M2)</p>

[WongTongTong]

<p>

2)<a href=“http://tinyimage.net/images/81684598844478119651.png[/url]”>http://tinyimage.net/images/81684598844478119651.png&lt;/a&gt;
Why can’t I use the formula V^2/R for this one?? answer C btw ( I did get it right, but just because I didn’t see the answer choice I needed had I used V^2/R…)

</p>

<p>I’m not taking E+M, but since it said as a function of ‘I’, I think it’s better to have ‘I’ in your equation.</p>

<p>Twinkle twinkle little star power equals I squared R; R= pL/a; therefore, the Power is directly proportional with p and L.</p>

<p>

1984 #31) Mass M1 is moving with speed v toward stationary mass M2. The speed of the center of mass of the system is what?
a) (M1/M2)v
b) (1+M1/M2)v
c) (1+M2/M1)v
d) (M1/ M1+M2)v</p>

<p>Can someone explain why the answer is d?

</p>

<p>Hmmmm, interesting. I’m not sure this is quite right, but if you consider the most radical case where the two objects inelastically collide, then solving for the resulting velocity (using conservation of linear momentum), you get (M1/ M1+M2)v.</p>

<p>Edit: Nvm, Mysterious’ explanation sounds more legit.</p>