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March 28, 2024, 07:29:48 pm

Author Topic: HSC Physics Question Thread  (Read 1030376 times)  Share 

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DrDusk

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Re: HSC Physics Question Thread
« Reply #3840 on: November 07, 2019, 10:06:31 pm »
0
Hi,
Could I please receive help for this multiple choice question?
Which fundamental quantity required that its unit of measurement be redefined following acceptance of the theory of special relativity?
A: luminous intensity
B: length
C: mass
D: time
I thought B, C and D were all redefined after special relativity, but the answer is C. Why?
Thanks
Oh no there was a massive discussion on this question already. Out of curiosity where are you getting it from? because the answer is incorrect. The answer IS (B), not C.

Also it cannot be (D), because the question asks that the "Unit of measurement was redefined". Our unit of time was NOT redefined, just how we think of it. We still use seconds and what not to define the unit of time. However the unit of length WAS redefined because now we measure a meter on the distance light travels in a certain period of time.

not a mystery mark

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Re: HSC Physics Question Thread
« Reply #3841 on: November 10, 2019, 01:50:25 pm »
0
Hey guys: Thoughts on this emf question???
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louisaaa01

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Re: HSC Physics Question Thread
« Reply #3842 on: November 10, 2019, 04:22:30 pm »
+4
Hey guys: Thoughts on this emf question???

Hey not a mystery mark,

I've done this question a few times in my studies and I keep getting the same answer - one that isn't even an option. I'll show you my working/approach - let me know if your answer is similar!

Magnetic flux transitions from a maximum to a zero every 90o, i.e. every quarter of a rotation.

The coil completes 46.8 rotations/second - so 1 rotation is 1/46.8 s and thus 1/4 rotation occurs every 1/(4 x 46.8 ) = 5/936 s

Maximum flux = BA = 0.05 x 0.162 = 0.00128 Wb

So, there is a change in flux of 0.00128 Wb every 5/936 s

Using Faraday's Law, to find the induced emf, we multiply the rate of change of flux with the number of turns in the coil.



So induced emf = 48 V (we ignore the negative as this just accounts for Lenz's Law) - which isn't listed as an option.

I've had a look at the official answer (which I believe is A from memory) and I think the issue lies in the fact that, to get 12V, this relies on magnetic flux changing from a maximum to zero over one complete rotation - this actually isn't the case as the flux will be maximum when the coil is perpendicular to the field lines and minimum when it is parallel. In one complete rotation, the net change in magnetic flux is actually zero as the coil returns to the same position!
« Last Edit: November 10, 2019, 04:29:55 pm by louisaaa01 »
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not a mystery mark

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Re: HSC Physics Question Thread
« Reply #3843 on: November 10, 2019, 04:39:36 pm »
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Hey not a mystery mark,

I've done this question a few times in my studies and I keep getting the same answer - one that isn't even an option. I'll show you my working/approach - let me know if your answer is similar!

I've had a look at the official answer (which I believe is A from memory) and I think the issue lies in the fact that, to get 12V, this relies on magnetic flux changing from a maximum to zero over one complete rotation - this actually isn't the case as the flux will be maximum when the coil is perpendicular to the field lines and minimum when it is parallel. In one complete rotation, the net change in magnetic flux is actually zero as the coil returns to the same position!

YESS!! It's exactly the same, I couldnt find a way to get 12V. Ahaha - bro problems with the new syllabus. It's already so confusing - stuff like this is drilling my brain harder. 

Thanks so much!!
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not a mystery mark

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Re: HSC Physics Question Thread
« Reply #3844 on: November 10, 2019, 06:38:39 pm »
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Okay, I'm writing one last question for today.
Can somebody explain this solution??

Normally I just use but this has something completely difference. I then assumed that it would be because of friction, but the working out doesn't use it at all? So now I'm just mega confused.

And I'm pretty sure their pytharoas's theorem isn't for real. Eughhhhhhh

Pls help
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louisaaa01

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Re: HSC Physics Question Thread
« Reply #3845 on: November 10, 2019, 06:49:47 pm »
+4
Okay, I'm writing one last question for today.
Can somebody explain this solution??

Normally I just use but this has something completely difference. I then assumed that it would be because of friction, but the working out doesn't use it at all? So now I'm just mega confused.

And I'm pretty sure their pytharoas's theorem isn't for real. Eughhhhhhh

Pls help

Hey not a mystery mark,

Funnily enough I had exactly the same problem with this question and turns out their solution is completely wrong! They actually issued an amendment to this solution (which still had errors). The main reason that they're wrong is that they combine equations for motion up and down an inclined plane (from Year 11) with circular motion around a banked track - which you can't do.

Keep in mind that the equation you gave for v only works when no friction is acting. In this case, we have both friction (down the slope) and normal force (perpendicular to the track and thus at θ to the vertical). It may help to draw a force diagram. We begin by resolving forces vertically and horizontally:

Vertically: Ncosθ - Fsinθ = mg

Horizontally: Nsinθ + Fcosθ = mv2/r

We have the values for F, θ, m and g (given in parts (a) and (c)) so we can substitute these into the vertical equation to find N.

Knowing N, we rearrange the horizontal equation and make v the subject. We can substitute all our known values to find v. It should end up being 17.62ms-1.

The mathematics involved actually resembles what we do in Maths Ext 2 - due to the error in the question, it ends up being harder than intended. I hope this clarifies things though!
« Last Edit: November 10, 2019, 06:51:59 pm by louisaaa01 »
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not a mystery mark

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Re: HSC Physics Question Thread
« Reply #3846 on: November 10, 2019, 06:58:30 pm »
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Hey not a mystery mark,

Funnily enough I had exactly the same problem with this question and turns out their solution is completely wrong! They actually issued an amendment to this solution (which still had errors). The main reason that they're wrong is that they combine equations for motion up and down an inclined plane (from Year 11) with circular motion around a banked track - which you can't do.

Keep in mind that the equation you gave for v only works when no friction is acting. In this case, we have both friction (down the slope) and normal force (perpendicular to the track and thus at θ to the vertical). It may help to draw a force diagram. We begin by resolving forces vertically and horizontally:

Vertically: Ncosθ - Fsinθ = mg

Horizontally: Nsinθ + Fcosθ = mv2/r

We have the values for F, θ, m and g (given in parts (a) and (c)) so we can substitute these into the vertical equation to find N.

Knowing N, we rearrange the horizontal equation and make v the subject. We can substitute all our known values to find v. It should end up being 17.62ms-1.

The mathematics involved actually resembles what we do in Maths Ext 2 - due to the error in the question, it ends up being harder than intended. I hope this clarifies things though!

You are always there to save the day!!! TYSMM, you clarified that so well dang. Also yea the Ext 2 squad - one of the best subjects.
Thank you again for saving my hsc ahaha
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mani.s_

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Re: HSC Physics Question Thread
« Reply #3847 on: November 14, 2019, 02:29:41 pm »
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Hi, I have a question, say I have a conical pendulum and the length of the string is kept constant. If I increase the radius of the pendulum, will period increase or decrease???

Thanks :)

DrDusk

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Re: HSC Physics Question Thread
« Reply #3848 on: November 14, 2019, 02:49:27 pm »
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Hi, I have a question, say I have a conical pendulum and the length of the string is kept constant. If I increase the radius of the pendulum, will period increase or decrease???

Thanks :)
Depends if your just increasing radius and decreasing density to keep mass constant or increasing radius and mass at the same time.

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Re: HSC Physics Question Thread
« Reply #3849 on: November 14, 2019, 03:12:08 pm »
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Depends if your just increasing radius and decreasing density to keep mass constant or increasing radius and mass at the same time.
The mass also stays constants, so only increasing the radius.

DrDusk

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Re: HSC Physics Question Thread
« Reply #3850 on: November 14, 2019, 04:07:07 pm »
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The mass also stays constants, so only increasing the radius.
What are your thoughts on the matter?

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Re: HSC Physics Question Thread
« Reply #3851 on: November 15, 2019, 05:08:20 pm »
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What are your thoughts on the matter?
I did an experiment with a conical pendulum where the string length and the mass of the pendulum were kept constant. I increased the radius in intervals of 10 cm up to 50cm. At each radius, the period seemed to decrease. It wasn't by much like around 0.2-0.3 seconds. But not sure if that's supposed to happen. Using the period I'm going to find the centripetal force and relate it to the radius.

DrDusk

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Re: HSC Physics Question Thread
« Reply #3852 on: November 15, 2019, 06:56:33 pm »
+2
I did an experiment with a conical pendulum where the string length and the mass of the pendulum were kept constant. I increased the radius in intervals of 10 cm up to 50cm. At each radius, the period seemed to decrease. It wasn't by much like around 0.2-0.3 seconds. But not sure if that's supposed to happen. Using the period I'm going to find the centripetal force and relate it to the radius.
Sorry I meant have you done any calculations. Have you got any ideas on what kind of approach you should take to solving the problem mathematically?
Perhaps you could balance the forces on the mass or something and go from there. Have you tried that? =)
« Last Edit: November 15, 2019, 07:01:20 pm by DrDusk »

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Re: HSC Physics Question Thread
« Reply #3853 on: December 19, 2019, 12:42:46 pm »
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Hi Everyone, ;D

I have been given a physics depth study recently, and I am wondering what exactly differentiates a Band 5 depth study from a Band 6 depth study? ???

Is it more in depth research? or including formula's within your explanations?

Any advice is much appreciated! :D

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louisaaa01

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Re: HSC Physics Question Thread
« Reply #3854 on: December 21, 2019, 07:43:20 pm »
+5
Hi Everyone, ;D

I have been given a physics depth study recently, and I am wondering what exactly differentiates a Band 5 depth study from a Band 6 depth study? ???

Is it more in depth research? or including formula's within your explanations?

Any advice is much appreciated! :D

Coolmate 8)

Hey Coolmate,

It really depends on the parameters of your depth study and the associated marking criteria but everything you've mentioned certainly differentiates a Band 5 from a Band 6. Here are a couple more pointers:

- If you need to assess the validity + accuracy + reliability of the primary/secondary investigation, you really need to know the difference between these terms, and not just acknowledge them under one 'umbrella'. Critically evaluate all aspects of your depth study.
- Clearly established inquiry question and a valid investigation (primary OR secondary) that addresses this question. Conclusion made that clearly reflects your findings.
- As you mentioned, include all appropriate Physics formulae. Perhaps derive such formulae if it is relevant to your depth study (e.g. if it is investigating circular motion)
- Your depth study should also be well-structured and clear.
- Research should indeed be in-depth, as you've pointed out, but try not to digress from the question at hand.

This list isn't exhaustive, but should point you in the right direction :) I'd highly recommend reading your marking criteria if you have access to it and pay attention to the modality used in each mark bracket (e.g. a Band 5 might be "thorough" but a Band 6 might be "extensive")

Hope I could be of some assistance!
« Last Edit: December 21, 2019, 07:45:49 pm by louisaaa01 »
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