December 11, 2019, 02:23:56 am

### AuthorTopic: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions  (Read 1565 times)

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#### Joseph41

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##### HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« on: October 09, 2019, 04:30:14 pm »
Post all exam discussion, questions, and sample solutions for Physics here!

#### Husky

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #1 on: November 11, 2019, 07:09:58 pm »
I don't even know what to say... that paper was way harder than I thought it would be. Walked in expecting a band 6 and now I'm hoping to scrape a mid band 5 aligned lol. Honestly though, multiple choice was the worst part of the paper it was way too hard and took away my chance of a high mark.

The worst short answer questions had to be that stupid fan crap and the protractor. Completely skipped that protractor one didn't even know where to start in terms of the derivation, it was worded so poorly as were all the questions.

#### r1ckworthy

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #2 on: November 11, 2019, 07:21:46 pm »
I don't even know what to say... that paper was way harder than I thought it would be. Walked in expecting a band 6 and now I'm hoping to scrape a mid band 5 aligned lol. Honestly though, multiple choice was the worst part of the paper it was way too hard and took away my chance of a high mark.

The worst short answer questions had to be that stupid fan crap and the protractor. Completely skipped that protractor one didn't even know where to start in terms of the derivation, it was worded so poorly as were all the questions.

Me too, I bullshitted the fan and protractor question

Don't worry, your mark will get aligned so it's just a waiting game to see if you manage to get that band 6 or not! Others (me included) found it terrible as well, so we will just have to wait and see
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#### Husky

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #3 on: November 12, 2019, 05:26:50 am »
Here’s the paper

#### blasonduo

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #4 on: November 12, 2019, 01:37:02 pm »
I've decided to run through multiple choice. I am a little uncertain on 12 and 19 since I’ve never actually covered these in my studies. Feel free to pick up on any mistakes if you see them in here. It has been a while for me.

Spoiler
1) D - At the peak of the height, the ball has no vertical velocity, so it can only have horizontal velocity.

2) B - The spectra are slightly shifted, indicating a difference in relative speed. The lines are consistent for both spectra, so the composition is the same.

3) C - They shot alpha particles in their experiment and were expecting them to pass right through with minimal disturbance

4) C - Main sequence is the “stable phase” so P is earlier on, R is a red star, and S has the lowest luminosity.

5) D - we need a constant change to have a non-zero reading, but we only have a brief one.

6) A - λ = b/T can be turned into y = b/x which is a hyperbola with asymptote as T/x = 0, only A shows this.

7) D - Lenz’s law wants to keep the magnet, so a North pole needs to be made, using right-hand grip rule to find what why the current needs to flow shows D as the correct answer.

8.) A - Hubble’s equation used parsec, and he found a linear relationship between them.

9) D - This is just using the formula (or intuition). The orbital period is T^2 = r^3, so bigger radius is a longer period. Potential energy is U = -1/r, so bigger the radius, the closer the answer is to 0. We want the greater total energy, so we want the one closer to 0. The minus in this equation DOES count.

10) A - I = I_max*cos^2(30), since we don’t care what I_0 is, set it to 1. cos^2(30) = 0.75, 0.75/1 = 0.75. It showed light was a wave.

11) C - r^3/t^2 = r^3/t^2 let one of them be earth, the other be the dwarf planet.

12) A - This is a neutron to a proton transformation which is a Beta- decay, which emits an electron

13) B - E = hf -> E = h c/λ = 3.06*10^-19. # of photons = power/E = 0.03/(3.06*10^-19) = B

14) B - v = sqrt(GM/r) double r, we get 1/sqrt(2) = 0.707

15) C - d is the distance between slits, theta angle from slits, so just use trig.

16) D - F = Eq -> mg = Eq -> E = mg/q. However, this is to just make the particle move horizontally (since net force is now 0) so times everything by 2, to mimic mg in the opposite direction. E = 2mg/q

17) D - This one is pretty difficult, and this is the best answer I could come up with. Resistance SLOWS the electrons. Since nothing is changing apart from the speed, it’ll take longer for the electrons to hit the screen, meaning more influenced by gravity, so it should fall down very slightly.

18) B - When the wire enters the field, Lenz’s law states it will try to oppose that change, so the loop with try to make a magnetic field out of the page, this translates to current flowing anti-clockwise - opposite to the battery’s flow, meaning the lightbulb should get dimmer, eliminating A and C. When the loop leaves the field, it’ll do the same thing, but in the opposite direction, so the lightbulb will get brighter than originally. D never gets brighter than that, so B is left.

19) A - From what I’ve gathered from the internet, (Y+Z) - (W+X) will always be positive (unless Z is negative, which can’t happen) meaning that no matter what Z is, this reaction will give off energy. It has nothing to do with binding energy. Again, this one probably is wrong, because I actually have no clue

20) B - A is wrong since the cube is still in a circular motion, so it is accelerating. C is wrong because if this were true, the cube would not be stationary on the blackboard. D is wrong because of the formula F = mu*N, yes they are perpendicular, but it doesn’t make them independent. A more in-depth answer is that the formula for frictional force is a cross product, not a dot product.
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#### Joseph.Ryan

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #5 on: November 12, 2019, 04:52:19 pm »
The exam was much harder than i expected.

Any one know the correct way of doing 30 (a)?

#### blasonduo

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #6 on: November 12, 2019, 05:13:39 pm »
The exam was much harder than i expected.

Any one know the correct way of doing 30 (a)?

$\Delta PE = \Delta KE \\ \\ \\ mg \Delta h = 0.5m \Delta v^2 \\ \Delta h = \frac{0.5 \Delta v^2}{g} \\ \Delta h = 0.115 m$

3 marks for this?
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#### DrDusk

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #7 on: November 12, 2019, 05:21:14 pm »
Question 35 really should not be there.

A lot of students who take Hsc Physics also take Maths ext 2. This question is a standard 4u Mechanics question that every 4u student would've gotten out in 30 seconds. They should've put in a much more interesting Classical Mechanics question.
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#### DrDusk

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #8 on: November 12, 2019, 05:23:16 pm »
As for the rest of the paper. I'm kind of disappointed, it's really not much different to the old syllabus. Instead of writing essays on society and what not your writing essays on Physics. Either way it's still essays which is not what Physics is or should be even at the high school level.
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#### blasonduo

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #9 on: November 12, 2019, 05:39:22 pm »
Question 35 really should not be there.

A lot of students who take Hsc Physics also take Maths ext 2. This question is a standard 4u Mechanics question that every 4u student would've gotten out in 30 seconds. They should've put in a much more interesting Classical Mechanics question.

Personally, I disagree. This question basically came directly from one of the dot points in the syllabus, and the derivation is just equating the forces, which I thought every class should've done for this particular dot point. Even if students couldn't derive it, some marks were to describe the motion. I thought it was a fair question.

As for the rest of the paper. I'm kind of disappointed, it's really not much different to the old syllabus. Instead of writing essays on society and what not your writing essays on Physics. Either way it's still essays which is not what Physics is or should be even at the high school level.

This though, I 100% agree with. I am really surprised by how many essays writing they were required to do, given the change in syllabus, but I am happy that it was reduced, and the higher marked questions were more based on conceptual understanding than history / society based questions.
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#### Joseph.Ryan

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #10 on: November 12, 2019, 05:44:02 pm »
$\Delta PE = \Delta KE \\ \\ \\ mg \Delta h = 0.5m \Delta v^2 \\ \Delta h = \frac{0.5 \Delta v^2}{g} \\ \Delta h = 0.115 m$

3 marks for this?

Yes

#### Joseph.Ryan

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #11 on: November 12, 2019, 05:52:14 pm »
One more question, what was the correct answer for q. 35?

#### blasonduo

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #12 on: November 12, 2019, 06:16:30 pm »
One more question, what was the correct answer for q. 35?

The ball's horizontal force will only be due to centripetal force, this vertical force is just mg.

$\text{This means that } \frac{mv^2}{r} \text{is our horizontal force and mg is our vertical force} \\ \text{Since these are at right angles to each other, we can use trigonometry to find theta} \\ tan(\theta) = \frac{F_c}{F_g} \\ tan(\theta) = \frac{\frac{mv^2}{r}}{mg} \\ \theta = tan^{-1} (\frac{v^2}{rg})$

Have a look at this if you need a visual aid to help https://atarnotes.com/forum/index.php?topic=164552.msg1073048#msg1073048
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#### DrDusk

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #13 on: November 12, 2019, 06:18:52 pm »
One more question, what was the correct answer for q. 35?
The equation you would need to show would be

$\tan(\theta) = \dfrac{v^2}{rg}$

Using a scenario where say the car is banked at an angle which means the angle on the protractor is the angle of the bank that the car is travelling on. Basically argue that by deriving the equation above by balancing the forces.

EDIT: You can do it as suggested above or the way I suggested which is the standard '4u' method that most 4u students might've gone with. Either way you'll get the right answer
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#### DrDusk

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##### Re: HSC Physics - 11/11/19 - Discussion, Questions & Potential Solutions
« Reply #14 on: November 12, 2019, 06:25:48 pm »
Personally, I disagree. This question basically came directly from one of the dot points in the syllabus, and the derivation is just equating the forces, which I thought every class should've done for this particular dot point. Even if students couldn't derive it, some marks were to describe the motion. I thought it was a fair question.
That's not what I mean. I mean they should've asked something better. This is a very Standard scenario that a lot of student's would've done many times. I literally gave my students this exact scenario as a question many times.

I mean its Hsc Physics so I get that they can't really ask many different types of mathematical questions but they could've asked a one with a graph where you had to use the gradient or something for the cars motion. Those tend to be harder and it's at the end of the paper so really it's probably a bit too easy.
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