Exam 2 2015

[schooliskool]

16 is D. The tensions are vectors, not magnitudes, so by the definition of an equilibrium, the sum of the forces will equal 0, so A.

Wait wut m8 A or D lolol

[GeniDoi]

Wait wut m8 A or D lolol

Woops fixed. I meant it is A.

[alchemy]

Just realised I screwed up question 13.... D:
For 9 I got C and for 16 I got D.

[jazzycab]

As has been correctly pointed out, I had my inequalities the wrong way around for Q9 MCQ (for options B AND C) and the correct answer is indeed C.
Also, Q3e) I have doubled the area - I missed the 2 in the denominator in my working for some reason.
For those of you getting 47 for the angle in 4a) ii) - I also initially got this, but after rereading, I'm fairly sure my solutions are correct. You wanted the angle of elevation from O, but O is not (0, 0, 0) it is actually (50, 50, 0) and after 150 seconds, the j-component of the position vector is STILL 50, so the base of the right-angled triangle created is 25m

[boingo]

As has been correctly pointed out, I had my inequalities the wrong way around for Q9 MCQ (for options B AND C) and the correct answer is indeed C.
Also, Q3e) I have doubled the area - I missed the 2 in the denominator in my working for some reason.
For those of you getting 47 for the angle in 4a) ii) - I also initially got this, but after rereading, I'm fairly sure my solutions are correct. You wanted the angle of elevation from O, but O is not (0, 0, 0) it is actually (50, 50, 0) and after 150 seconds, the j-component of the position vector is STILL 50, so the base of the right-angled triangle created is 25m

But your working should show tan^-1(60/25) according to the triangle you drew. That's what I got too. So I think you may have flipped them around in your solutions.

[althepal]

Interesting. I'm not sure if r(0) can be taken as O though, because the question actually says r is the position vector from O, meaning that r(0) is taken in relation to O which is (0.0.0)

[jazzycab]

Right you both are, which means this is a compound Pythagoras/right-angled trig question and the angle of elevation is indeed 47

[GeniDoi]

The origin is definitely 0i + 0j + 0k, not the initial position of the helicopter at t=0.
 

[drmockingbird]

That was a tough one...do you reckon 108/120 would be enough to get raw 40 with 90+ sacs? I'm worried the A+ cutoff will be like last years

[lena36]

That was a tough one...do you reckon 108/120 would be enough to get raw 40 with 90+ sacs? I'm worried the A+ cutoff will be like last years

i think it'll be closer to 2013's grade distribution so 140/160

[GeniDoi]

If you had forgotten to specify direction in the first two inclined plane questions would you still have gotten the mark?

[jazzycab]

I think you will be fine on both counts GeniDoi. In the first case, you are asked to find the force required to prevent the trailer from moving down the ramp, implying that the direction is up the ramp.
In the second case, the question asks you to find the acceleration of the trailer down the ramp, so the direction is definitely implied in the question.