My answers with explanations
Hi guys!
Firstly i would like to say, AHAHAHAHA LOL this is the funniest forum thread i have ever read, some of you are hilarious, so direct, 'if you picked this or this you're an idiot'.....ehehe.
Anyway, I tried to read the whole thing as thoroughly as I could. And I would like to add my 2c.
Being my first post, before i start i would just like to add, physics is my favourite subject and probably my best, i certainly HATE English and thus the board of studies, but I never have to do it again, until it bites me back in the butt when i get my results back. I was so enthusiastic about physics in year 11, but then got glandular fever during the transition of yr11 to yr12. It screwed me up bigtime, major fatigue. In every class, most of the time i just rested on the desk usually with a headache, i tried to take things in as much as i could. When it came down to studying, well i did very small amounts, it was so hard. Being an insomniac doesn't help either. I need a UAI of 75 to get into electrical engineering at UTS. Me being too tired I didnt do that misadventure thing or whatever to get more marks because of damn glandular, and I thought I could get at least 75 without it. I just have to wait and see...
Anyway, what I'll do is for every question explain as simply as possible why I think the answer is whatever it may be and also try to explain why i dont think the other 3 are right. Well for most of the Q's anyway, some i had no idea, like all of the history questions, yeah couldnt have been worse doing quanta to quarks. In blue ill put the answer/s i thought were right, I didnt necessarily choose those in the exam, if i put the wrong one ill say so next to the one.
Q1. A
a) The velocity changes since its a composition of both X and Y and since Y changes, the overall velocity changes.
b) The acceleration is always g = 9.8m/s/s, since its parabolic, it must be.
c) Only the Y component of velocity is zero, X is always constant and does exist unless it was thrown directly upward, the picture shows it had X component.
d) As per b, the acceleration doesnt change, g = 9.8m/s/s
Q2. D
d) This one is a little obvious, Atmosphere creates drag and slows it down so it can no longer sustain an orbit and falls to earth. This is similar to Q4.
Q3. B
b) All a, c and d are in the formula for escape velocity, mass of probe is not
Q4. Either C or D
Lots of controversy.
a)There is no way it will move away from the earth if it is slowed down. It would have to fire boosters toward the earth to do that. forward boosters fire perpendicular to the earth so just affects the speed of the craft, (which will affect the altitude due to downward acceleration).
b) It would definitely make a difference to the orbit as the new slower velocity cant maintain the same orbit. (see orbital velocity formula)
c) I picked this in the test because i liked the idea of the spaceship crashing into the earth, well partially, my argument for c is that because if the ship slows down it cant maintain the same orbit so decays toward the earth, the question is does it take up a lower orbit or does it crash to the earth? According to the orbital velocity formula a slower ship has a greater orbital radius, and since its now closer to the earth, ie lesser orbital radius, then it cant be in orbit and hence falls to the earth.
d) (Probably the better answer) Since the ship is falling it does speed up (slingshot effect) and so might be able to maintain a stable orbit, however I am not sure whether it will gain enough speed to maintain the orbit (since it already lost half), plus drag of the atmosphere will slow it down more. So it would keep losing speed and thus altitude and then fall to the earth slowly. (I hope its not this for my sake, lol, it crashes into the earth damnit!)
Q5. C
a)b)d) It has been proved previously to be wrong.
c) Only logical answer, just throw a ball as hard as you can straight ahead then time it to hit the ground, then throw it as high as you can, it will take much longer to hit the ground.
Q6. B
a) Length WILL affect the current due to the formula posted previously.
b) Since it states in the question that it is 'of very low resistance' you can assume that it is negligible thus thickness of the wire wont make a difference (or the LEAST, which is what the questions asks).
(Where on earth did someone pull out the explanation that thicker wire made it heavier and so they rotated it more slowly? that made me crack up ehehe, its a good observation but isnt relevant for this question, and plus that would mean that its still the speed that affects the current, not the thickness, but due to the thickness, you can assume the experiment was controlled)
c) Speed is a change in mag field so more of it will make more current.
d) The wire is assumably using the earth's magnetic field, and since they go from top of the world to bottom, it will matter which way they are holding the wire.
Q7. A
Again, lots of controversy.
a)OK there are actually TWO right hand rules relating to induced forces. ONE of them is for MOTOR effect and the other is for GENERATORS. You have the mag field and the direction of rotation, and you need to find the current direction.
- Ok the motor effect is where your fingers are the magnetic field. Your thumb points in the direction of the conventional current, and your palm points in the direction of force on the wire. This question is NOT that one, resulting in D.
- The generator one is where fingers are mag field, your thumb points in the direction that the wire is moving, and the palm points in the direction of current.
Take the diagram for a, in your head rotate the coil 90 degrees clockwise and picture your fingers pointing downward with the field, then since its clockwise point your thumb to the right, your palm points into the page so thats the direction of current. Thus answer is A.
b,c) Visual trick but not very effective. Only possible answers are a and d.
d) This would be true if this was a MOTOR, and you knew the current and mag field but not the direction of rotation.
Q8. C
a) Yes its a DC current (batteries are DC), but its the CHANGE in mag field that induces current. And closing the switch causes current to go from ZERO to MAX, therefore there is a change and an induced current.
b) Once the current flows and stops changing, there is no induced current so cant be continuous.
c) Since the change is only as soon as you turn it on and stops shortly after you leave it on the induced current stops as illustrated.
d) I am a complete moron, in the exam i chose this because I thought the question said the switch was closed and opened again straight away, just exam stress i guess. Well yeah the switch definitely stayed on so cannot be this answer.
Q9. B
I am definite, I saw it with my own eyes! (it was inverted as in the copper was a big tube and we put a super magnet down it, it went slowly, then a tube with a slit cut through the whole thing, the magnet went slowly but quicker than the solid tube, then we got a plastic pipe and mag dropped right through, this is basically the same experiment)
Ring P is plastic so no eddy currents slowing it down, falls at 9.8m/s/s
Ring Q is copper with completed loop, allows lots of eddy's to slow it down.
Ring R is copper with slit, eddy CAN STILL EXIST, just less of them and so it slows but not as much. Definitely slower than ring Plastic
Q10. Not Sure, I picked C, here's why:
I was stuck in the exam, but then thought which way does mag field go around a wire, its right hand cork screw. So put my hand horizontal and it goes in the Y plane, so i thought putting sheets perpendicular will lessen them. And so therefore B would not. However I have reason to believe that the answer is B, after seeing a picture posted on a link in a previous post.
a) der
d) I have never seen this in my life so if it worked i probably would have seen it. I would have thought eddys spin in in the direction of the circles.
Q11. B
This is one of the pracs you were supposed to do. Its just a matter of remembering what this particular one proves. Straight lines is the Maltese cross one.
Q12. - Beats the crap out of me.
Like I said before, Im bad with history, probably because remembering names is boring so i didnt bother, even after studying it. I can't even remember what I put in the exam. hopefully guessed right. Never liked this black body stuff anyway.
Q13. Again, dunno. But i picked C, for some unknown reason. Phosphorus caught my eye, i might have worked it out in the exam with a good reason for picking C but cant be bothered right now, its over.
Q14. B
Pretty straight forward formula substitution work. Using E=hf
Q15. D seemed the only logical one to me.
I don't know how this thing works, but if holes are opposite charge to electrons it makes sense they would be forced in opposite directions.
HMMMM thats it.......pretty interesting. Ill sum it up:
1. A
2. D
3. B
4. C -> prolly D
5. C
6. B
7. A
8. C
9. B
10. Maybe C
11. B
12. -
13. Some say A or C
14. B
15. Most say D
I am Pretty confident with the answers I gave a single answer to.
I conclude Q4 the most interesting.
So yeah any problems with my explanations please reply.
Wow nearly 2am, better get to bed.
CYA!