<p>Earth-Dragon: yah the frictional force is actually just the exact same as the other frictional force because of Newton’s Third Law like conker said. so you divide that frictional force by the mass of the slab to get hte acceleration. You could also have used center of mass to solve for the velocity…I’m not really sure how but that’s how my teacher did it. I’ll ask him tomorrow if I remem. He got .572 solving for velocity using something related to center of mass. If anyone did it this way and wants to show their work that’d be nice =)</p>
<p>hmm…I might have done some calculation error ~_~ I have the basic idea everyone has…</p>
<p>Edit: I know what I did wrong, I found the friction between the floor and the SLAB (which was zero)…because then, the weight IS different…right?</p>
<p>By the way, for those of you who didn’t receive your booklets, you can get the questions from the AP site. Here are the links:</p>
<p><a href=“College Board - SAT, AP, College Search and Admission Tools”>College Board - SAT, AP, College Search and Admission Tools;
<a href=“College Board - SAT, AP, College Search and Admission Tools”>College Board - SAT, AP, College Search and Admission Tools;
<p>Now for the E&M answers…I won’t rewrite the questions, since you can access them at the link above.</p>
<p>E&M 1.</p>
<p>a) Arrow points to the -Q at the top right.
b)
i. Electric field is a vector. Since the field due to the two -Q charges cancel each other out, all that needs to be considered is the field due to +Q and -Q.</p>
<p>E = kQ/r^2 + kQ/r^2
E = 2kQ/r^2</p>
<p>r = sqrt[(a/2)^2 + (a/2)^2)] <– determine r
r = sqrt[a^2/2]
r = a/sqrt(2)</p>
<p>E = 2kQ/(a^2/2) <– substitute a^2/2 for r^2
E = 4kQ/a^2</p>
<p>ii. Electric potential is not a vector. You simply add up all the potentials.</p>
<p>V = kQ/r - 3kQ/r
V = -2kQ/r
V = -2sqrt(2)kQ/a <– substitute a/sqrt(2) for r</p>
<p>Ahh…E&M is so much harder to type out than Mechanics. I might do the rest later if I have time. If someone else would like to type their answers, please go ahead.</p>
<p>Yeah, I got 44/45 on the mechanics free response…I forgot a 1/2 in the 1/2at^2. The mech1 was easily the most difficult on that test. Honestly, easy easy mech2, all you needed to do was remember to integrate the force equation, but I’m sure that fooled a lot of people. And then a mech3 like that? They could have at least made it some weird hoop/bar combination like a bicycle wheel or something to see if we knew how to add moments of inertia.</p>
<p>By the way, for the mech1 you might get minimal points for stating that Wfriction=F.d and saying it’s positive, and definitely minimal points for simply saying Wfriction=F<em>d and getting a positive number. Should have been Wfriction=F</em>d<em>cos(theta) where theta is 0. The catch here is that friction usually opposes the force applied, however, since it accelerates the slab in this case, the work done on the slab is positive. And while saying W=F.d and jumping straight to the answer is technically correct, it gives the impression that you don’t know what you’re doing, although less so than substituting the numbers in for force and distance without including some sort of function of theta. If I had to guess, W=F.d and the answer would reluctantly get full credit, as will W=F</em>d*cos(theta), and everything else gets marked down.</p>
<p>Brexford</p>
<p>Oh man. That’s what they meant by using the slope to find A. I just thought it was sort of silly to make a graph instead of calculating values for A directly at each point and averaging them, and didn’t think to use x^2… hopefully they’ll give credit for F/x as well, since it does give a graph with slope A.</p>
<p>
Thanks. Actually, on the test, I wrote the formula as W equals the dot product of F and d. And then I said that was equal to F</em>d and plugged in the numbers. </p>
<p>Again, those of you looking for the FRQ questions for this year’s exam, look 3 posts above.</p>
<p>I dont get why you all thought this year’s test was easy. the first FRQ for mech was pretty confusing. also the MC for both E+M and Mech were killers. I had to skip 10 because there simply wasnt enough time.</p>