The Official AP Physics C Thread

Test Preparation AP Test Preparation
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<p>Oh damn, we didn’t do orbits & oscillations in class.
What are Kepler’s Laws? Is it just (period)^2 / (radius)^3 = the same for another mass?</p>

<p>And second law is just that the centripetal force is equal to its universal gravitational force, right? Is there a third one too?</p>

<p>If an oscillation/orbits/pendulum/spring thing shows up on FRQ, I’m literally dead.</p>

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<p>^how do you calculate escape velocity? and the kinetic energy needed?</p>

123

<p>

</p>

<p>you need conservation of momentum because there is friction
haha but I really don’t feel like doing this problem</p>

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<p>For the spring, consider what happens when you put two springs of a certain spring constant x in series. Set that equal to k and solve for x.</p>

<p>The “special” spring was basically just noticing F = x^3, so U = \int_0^3 x^3 dx</p>

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<p>

set the kinetic energy equal to the potential of the object. This is true because think about the amount of energy you need to totally escape (go infinitely far away), this is the same as potential energy</p>

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<p>O.O.
What would a gravitation FRQ ask?</p>

<p>I mean, other than one of those graphing questions, I don’t see how they could extend a gravitation problem into a legit FRQ. Unless it’s just an uber easy #1.</p>

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<p>what about this one guys?</p>

<p>Assume that a cloud consists of tiny water droplets of mass m suspended (uniformly distributed,and at rest) in air, and consider a raindrop of mass M falling through them. What
is the acceleration of the raindrop? (Assume that when the raindrop hits a water
droplet, the droplet’s water gets added to the raindrop. Also, assume that the
raindrop is spherical at all times.)</p>

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<p>

</p>

<p>Yea, I thought they were hard too. Did anyone think that the second PR test was a lot easier though than the first one?</p>

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<p>A gravitation FRQ would be something involving the Law of Universal Gravitation and it asks you stuff about the masses, radii and the distances…stuff like that.
I think there was one the 2007 exam. Check on collegeboard.</p>

<p>Yeah, PR mechanics FRQs are pretty hard i just noticed. x_x
Barron’s isn’t easy either.</p>

130

<p>afacool1:</p>

<p>Is that not just g?</p>

131

<p>2nd law is just angular momentum is conserved
3rd law is T^2 / R^3 is a constant ratio for a mass in orbit</p>

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<p>Both prep books are much harder than the practice tests I’ve taken. I averaged 85-95% on the actual AP MCs that my teacher gave us, but I can only get 70ish % of the Barron’s and PR questions. =/. They’re good practice for conceptual review though</p>

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<p>

Yea…I couldn’t do any of it…</p>

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<p>

Nope because notice the drop is running into stationary droplets. also its accumulation of mass with respect to velocity is not constant since the more mass it accumulates, the larger its volume, and the more droplets it hits in a region of space. Haha, its not an actual AP problem, here’s the link to the solution if you’re interested</p>

<p><a href=“http://www.physics.harvard.edu/academics/undergrad/probweek/sol5.pdf[/url]”>http://www.physics.harvard.edu/academics/undergrad/probweek/sol5.pdf&lt;/a&gt;&lt;/p&gt;

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<p>HOLY ****. That’s ridiculous, mate. Definitely didn’t think about that, lol.</p>

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<p>Lulz, thats a crazy solution to such a simple problem.
Physics C doesn’t need you to define three-dimensional vectors and all…its all good people.</p>

<p>Btw, do we need to memorize the moment of inertias for basic shapes or will they give it to you?</p>

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<p>If it shows up in a free response, it’ll either be extremely simple to solve, or they will give it to you.</p>

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<p>just integrate!
Although for most AP problems I’ve seen, they usually give it to you at the start of the problem.</p>

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<p>another question:</p>

<p>A helium nucleus (charge + 2q and mass 4m) and a lithium nucleus (charge +3q and mass 7m) are accelerated through the same electric potential difference V0(the “0” is supposed to be a subscript). What is the ratio of their resultant kinetic energies, K lithium/ K helium?</p>

<p>A) 2/3
B) 6/7
C) 1
D) 7/6
E) 3/2</p>

<p>I HAVE NO IDEA HOW TO DO THIS???</p>

140

<p>a gravitation frq came out in 2005, #2.</p>

<p>moment of inertia came out in 2004, #3a. It would’ve been incredibly simple if you knew the moment of inertia of a rod by memory; if not, its not too hard to solve.</p>