<p>The countdown has begun, and it's less than three weeks until May. I figured I would start a Physics B practice thread, where you ask questions (MC, FR, or w/e) and answer other people's questions. </p>
<p>How much force is required to vertically lift an object of mass m with acceleration g?</p>
<p>A) mg
B) 2mg
C) mg^2
D) 2mg^2
E) m/g</p>
<p>This should be an easy one..</p>
<p>wait, are you serious? This was on an AP exam. </p>
<p>A.</p>
<p>Oh, it was? Are you sure? Which one?
Also, apparently the answer is B…</p>
<p>O.O What. How is it? There a force going down caused by gravity and to lift it in constant velocity, you have to lift it at the same but opposite force. </p>
<p>Care to explain plz. Oh wait, now, i see, there’s the normal force, which is mg too. So, if you lift it, that mg is added it to mg of the normal force and you get 2mg. Oh i see. Well, that was stupid of me.</p>
<p>Was this on a previous AP exam or did you get it from a prep book?</p>
<p>A person stands on a scale in an elevator. He notices that the scale reading is lower than his usual weight. Which of the following could possibly describe the motion of the elevator?</p>
<p>A. It is moving down at a constant speed.
B. It is moving down and slowing down.
C. It is moving up and slowing down.
D. It is moving up and speeding up.
E. It is moving up with constant speed.</p>
<p>Actually I want someone to explain for me. I myself was pretty sure the answer is A, just like you described. But the answer in the answer key is B (no explanation).
Is it really from an old AP exam? If so, tell me which one (date), and I’ll see if I have the exam to make sure of the correct answer.</p>
<p>EDIT: That question seems pretty ambiguous… The elevator is obviously moving upward, but you can’t really describe how fast it’s moving.</p>
<p>since g is already pulling down, you need F=2mg</p>
<p>@Malfunction I don’t know if that question you ask was a AP Exam question, so that is why I asked you.</p>
<p>Oh, I almost felt for B, but it’s C if you set up the equation.</p>
<p>A and E are wrong because weight does not change in constant speed. If it’s D, you’re gaining weight (kinda funny to say). </p>
<p>B: mg-ma since moving down is losing weight. Since slowing down is negative acceleration, mg-m(-a)=mg+ma.</p>
<p>C: mg+ma since moving up is gaining weight. Since slowing down is negative acceleration, mg+m(-a)=mg-ma.</p>
<p>So, C, am I right?</p>
<p>
And what happened to the force of gravity? Shouldn’t it like, cancel with the normal force, and so you’re left with only one mg? Man I really feel stupid now…</p>
<p>And no, that question was from a website, and I highly doubt it was from a real AP exam, which is why I was surprised when you said it was from an old AP.</p>
<p>That is what I initially thought too. So, you only need 1mg to lift it since mg of the normal force counteracts with the mg of gravity. However, I guess if you’re lifting it, the force of lifting it has the same direction as the normal force, so you have to add them. Well, for normal force, the ground that the object is “lifting” the object.</p>
<p>And where will the force of gravity go then. -_-
Whatever I’m going to sleep now.</p>
<p>Gravity is there, and the question asks “How much force is required to vertically lift.” So, you have to add the lifting force only…I think. And I’ll be studying Euro History since I really need to.</p>
<p>For the elevator question, Jerry is right about it being C.</p>
<p>As for the first problem, the wording is very confusing, but I see exactly how it makes sense. It says that it wants you to lift the object at g, or at 9.8 meters per second squared. It insists that the net acceleration be 9.8 meters per second squared. Therefore:</p>
<p>F - W = mg, where F is the lifting force, W is the object’s weight (which is mg), and the mg to the right of the equals sign is the mass and net acceleration of the system.</p>
<p>Therefore, F - mg = mg, and thus:</p>
<p>F = 2mg</p>
<p>OH! That makes sense. Thx Keasbey Nights.</p>
<p>No problem! Here’s a question from a review book that I have, although I tailored a bit to make it a bit harder, but it’s still pretty simple.</p>
<p>Two loops of wire exist. One is a circle with radius 4. The other is an ellipse with radii of 8 and 2. Both loops are held so that their plane is perpendicular to a uniform magnetic field. How would ΦC, the magnetic flux through the circular loop, compare to ΦE, the magnetic flux through the elliptical loop?</p>
<p>(A) ΦC = 16ΦE
(B) ΦC = 4ΦE
(C) ΦC = ΦE
(D) ΦE = 4ΦC
(E) ΦE = 16ΦC</p>
<p>You got to be kidding me. You’re expecting us to find the area of an ellipse?</p>
<p>The area of an ellipse is pi * major radius * minor radius. It’s actually the same thing as a circle’s area, but because a circle has identical major and minor radii, it shortens to pi*r^2.</p>
<p>I found the area of the ellipse on the internet. I can’t believe the area was that simple: pi x A x B.</p>
<p>My answer is C. My reasoning.
Φ=BA, B is uniform.</p>
<p>ΦC=B<em>pi</em>4^2=16Bpi
ΦE=B<em>pi</em>8*2=16Bpi</p>
<p>Very good! Do you have any you’d like to post?</p>
<p>Not yet, but I will. I just need to review light and optics first and then do a past MC exam to evaluate myself. After that, I will post a MC and FR question to share.</p>
<p>And thx for the MC question.</p>