2008 Physics B FRQ Discussion

Test Preparation AP Test Preparation
21

<p>I dont know if all my answers are right. Thats just what I had in my calculator.</p>

<p>3a</p>

<p>V = IR</p>

<p>V = emf - (internal resistance)(current)
V = 16 - .5(4) = 12 V</p>

<p>R = resistivity (length) / (area)
R = 1.7 x 10^-8 (L) / (3.5 x 10^-9)</p>

<p>V = IR
12 = 4 (1.7 x 10^-8)(L) / (3.5 x 10^-9)
Solve for L, Im pretty sure its .72 something.</p>

<p>3b, that was discussed in a chat room, I dont quite understand it myself. I guessed up but didnt put a reason. :p</p>

<p>EDIT: Actually, current is to the right, and if magnets produce magnetic fields from south to north, then right hand rule says force is up....but I didnt know which way magnets produce their fields from poles.</p>

22

<p>all u have to do for the 1st one is that find the time it takes to accelerate to the speed and the time it takes to travel the distance it goes. Then there is a constant v and then it easy. you just add the two times. I got 3.6 seconds</p>

23

<p>I thought 3b was down... (right hand rule)</p>

24

<p>i agree with "concretejungle", 3b should be down
the magnetic field flows into the south and out through the north
so if the field is pointing to the right and the current is going into the page (it was a 3d picture), it should be thumb into page and fingers to right, the force would be pointing down.
and for 1a i also used a quadratic and i got 3.333 s and i dont understand WHY thats wrong as you guys have come to establish. please explain</p>

25

<p>3b should be up. The force on the WIRE, according to right hand rule, is down. But it is asking about the magnet. By Newton's 3rd law, it is up.</p>

26

<p>Hold up, guys. In some other thread some guy called John203 was saying that he doesn't have to take the test until 5/23 because of some mix up at his school. It's probably too late now, but maybe we should stop discussing the FRQs in detail for now...?</p>

27

<p>That's late testing, which involves a totally separate FRQ.</p>

28

<p>^^ probably taking the form B test</p>

29

<p>are you sure that for 3(a), your answer is correct? i got 0.93m</p>

<p>because if you do
16V = (4A)(Resistance) - 0.50ohms(4A) </p>

<p>You get R = 4.5 ohms</p>

<p>then you plug into to find l = (3.5 x 10^-9)(4.5ohms) / (1.7 x 10^-8) = 0.93 m</p>

<p>and for 3(b), can you explain why its up?</p>

30

<p>^i got down cos of right hand rule. i am prolly wrong i guess</p>

31

<p>yay! i got all of them</p>

32

<p>3e i put increase in length of the wire = increase in resistance that is not linear</p>

33

<p>Can someone explain me #2. I totally didn't know how to do it on the real test.</p>

34

<p>look at the entire thing as a system.. the force is acting on the entire system, not just the 8 kg or the 2 kg block individually... the entire system is 10 kg.. so F = ma... F = mass (always 10) times acceleration... then plug your numbers away</p>

<p>i forgot that, and that's the only mistake i made on the entire FR (except for 3(e))</p>

35

<p>Im assuming we're suppose to use F=-kx? </p>

<p>F 100N
k = 80</p>

<p>100 = 80 (x)</p>

<p>or am I missing something.</p>

36

<p>part (d) i believe</p>

<p>i was talking about the first couple of parts -- a and b</p>

<p>yes, k = -kx and PE = .5kx^2 were both also used as well.. but the setting up the force diagram is what most people are going to forget (like myself, who woke up at 1 in the morning and had that revelation.. it sucked... lol)</p>

37

<p>but that gest me 1.25 instead of .8N (as mentioned earlier)</p>

38

<p>lol you probably divided wrong because 1.25 is the inverse of .8</p>

39

<p>100 N = 80N/m * x</p>

<p>(part A yes?)</p>

40

<p>For part e of number 2, the first block sticks to the wall so isn't the conservation of energy only done with the moving 8kg block? It would be different if the 2kg block rebounded with no loss of energy, but when it sticks none of its energy is transferred backwards into the spring. Is this incorrect?</p>