<p>is the answer E?
2345 are all correct since conservation of momentum?</p>
<p>There’s another thread in this forum with exam curves.</p>
<p><a href=“http://talk.collegeconfidential.com/ap-tests-preparation/700107-consolidated-list-ap-exam-curves.html[/url]”>http://talk.collegeconfidential.com/ap-tests-preparation/700107-consolidated-list-ap-exam-curves.html</a></p>
<p>They won’t be 100% accurate for this year, but if you can score comfortably above that cutoff then you should be fine.</p>
<p>IRT #52:
Mechanical energy is not conserved because whatever energy was stored that caused the explosion is now released in the form of kinetic energy and radiation.<br>
Linear momentum is conserved because the explosion is an internal force.
Position of center of mass is not conserved because the asteroid is already moving…
Velocity center of mass is conserved, same as linear momentum.
Angular momentum is also conserved, also because there are no external forces.</p>
<p>Hmmm next question:</p>
<p>A shuttle of mass 10kg is launched perpendicular to the Earth’s surface with rockets that supply a force such that dF/dt = 1000F(50-F), where t is in seconds and F is in kilonewtons, with the initial force at the moment of launch being 0.
(a) How long does it take the shuttle to lift off?
(b) If the Earth’s atmosphere is 200km high (I’m just making this up) and its mass is 6x10^24 kg, how long does it take to escape the atmosphere?
(c) If the rocket runs out of fuel after 15 days, how long does it take the shuttle to fall back to the earth? (assume there are no other forces exerted, e.g. by other planets)</p>
<p>if the elevation is 2r, does that mean the total radius from center of the earth is 3r? if so it would be B.</p>
<p>Oh right Wcfx, you lose 1/4th for every wrong answer. Forgot about that.
Yea the explanation was weird, sry.</p>
<p>Yes feuxfollets, that is correct.
The answer is three. Only linear momentum, angular momentum and velocity of the center of mass are conserved. Kinetic energy and the position of center of mass are NOT.
Funny how you can guess the wrong three things and still get the answer right.</p>
<p>Ok for your question:
a) Solve the differential equation for t, using 100 N as the force. Right?
b) Law of Universal Gravitation, or Energy? I lost myself.
c) Depends on answer to part b), whether or not you’re over the atmosphere after 15 days.</p>
<p>Is this a real FRQ question (cause it doesn’t look like 15 pts lol)?</p>
<p>And yea manyman, the total distance is 3r.</p>
<p>ok…
feuxfollets, your problem is hugely difficult to do, i’m just gonna post my attempt (probably wrong)</p>
<p>only did a and did not work</p>
<p>first, separation of variables on the dF/dt and integrate using the fraction method. Next, I got </p>
<pre><code>50e^(50000t)
</code></pre>
<p>F=---------------------------
1000+e^(50000t)
which doesn’t actually yield an answer for F=0.1</p>
<p>for part b i assume that you use v=sqrt(2gh) first?
then find the balanced force?</p>
<p>for part c, using F=ma and kinematics?
or is energy better here?</p>
<p>idk, this problem is waaaaaay beyond my levels, lol</p>
<p>hope i dont see this on the actually ap</p>
<p>The answer to the pendulum was B (classic problem: they give you elevation instead of distance).</p>
<p>OK, here’s a hard mechanics problem, no calculus required.</p>
<table><thead>
<tr>
<th>---------</th>
</tr>
</thead><tbody>
<tr>
<td>—a-----</td>
</tr>
<tr>
<td>---------</td>
</tr>
<tr>
<td>---------</td>
</tr>
</tbody></table>
<p>=======================table================</p>
<p>Block a (16 kg) is moving on a frictionless table, being pushed to the right by a force F (not shown) on block a’s left side. On the right of block a is block b (4 kg), which is not touching the table but being held in place by friction between a and b (coeff = 0.5).</p>
<p>The smallest possible value of F is:</p>
<p>A) 50 N
B) 100 N
C) 200 N
D) 400 N
E) 800 N</p>
<p>^ is it B? 100 N?
I’m getting 80 N as my answer, but it asks for least possible value, so I’d say 100 N?</p>
<p>Hmmm, interesting…</p>
<p>Acceleration will be F/20, and the force acting on b is given by F - Fb = 16 * F/20
So Fb = F - 4/5 F = 1/5 F</p>
<p>Then the frictional force is 1/5 F * 1/2 = 1/10 F.</p>
<p>This has to be equal to the gravitational force, 4 * 10 = 40.</p>
<p>So F = 400 (D).</p>
<p>I thought there is no applied force acting on B. Its acting on (A + B) as a whole.</p>
<p>Applying a force on A causes A to exert a force on B. By Newton’s Third Law this is the same magnitude as the force exerted by B on A.</p>
<p>So you’re pushing A with a force of F, and block B is pushing against A with a force of Fb. The sum of those two forces is the net force of A.</p>
<p>eh?
this is the force diagram i have…</p>
<pre><code> Ffriction
i
i
----------------Fnormal by a on b
i
i
Fgravity of b
</code></pre>
<p>for a
Force normal by ground
i
i
Force normal by b on a------------------F
i
i
Fgravity of a</p>
<p>i got 80N?</p>
<p>edit: the force diagram is skewed, but you can figure it out</p>
<p>Ah yes yes, thats what I wrote down too. I thought you were referring to Fb as the applied force on B. Its actually the normal force exerted by B on A. Gotcha.</p>
<p>Edit: Yes hahgooman, I got 80 N too. feuxfollet’s method seems to make more sense now (I forgot to take into account a + b as a system, its F/20).</p>
<p>Answer was: D (400 N).</p>
<p>Good luck on the test!</p>
<p>is it just me or 1999 Physics Mech. C frq is extremely hard?</p>
<p>Just got back a while ago from the test. Goddamn, was the Mech. FRQ hard. :/</p>
<p>how have u already taken it its an afternoon exam lol</p>
<p>yeah how did you take the test already?</p>
<p>He could have taken the international test. I think that would explain it.</p>
<p>Test in two hours, guys!!! Good luck!!</p>
<p>4 more hours now. well for me at least. I hope everyone on here does well. there can’t be more than several thousand people taking it right? so lets say there are a couple hundred on here. Who’s to say we can’t all be part of the top 22%? </p>
<p>I think everyone on here worrying will have put in the extra time to study so will be prepared and probably get a 5. <em>knocks on wood</em>.</p>