HSC Physics Question Thread

[blasonduo]

Hi guys i have a couple HSC questions where I have trouble wrapping my head around:

https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-physics.pdf (Q19)
B was my answer because if the astronaut was in orbit, it wouldnt "fall" down, it would just go around and around the planet.

Thanks so much

Hello!!

This is MUCH easier if we rule out the incorrect ones, For A, the force of gravity is NOT negligible, as that's what keeps objects in orbit! so it is wrong. While B is true, this is not answering the question, it is just a statement.

We now have it to C or D, for C, IF the forces were to be the same, (ie F = ma) since, the question claims F is the same, while mass is different, their accelerations must be different, and well, if their accelerations are  different, how on earth can they travel at the same speed? So C is incorrect, leaving only D as the answer (if you would like me to explain why, i'll be happy to!)

Most people do pick B for that reason, they don't really understand C or D, so go with B, because it is technically correct but it doesn't actually explain anything relevant to the question

I hope this helps!

[jasn9776]

Hi, I was just working on a past trial question about why the acceleration of a rocket increases as time increases.
why is this complete rubbish (I found it in the marking criteria)?

Many students wrote that the momentum of the rocket must be constant, and since p = mv and
mass is decreasing as fuel is burnt, velocity must increase to keep the momentum of the rocket
constant. This is COMPLETE RUBBISH!. The rocket accelerates (increases its velocity) because there
is a non-zero net force acting on the rocket in the forward direction! The rate of acceleration
increases for the reasons outlined above.

looks pretty similar to the sample answer from the 2016 physics q28 guidelines:
Since the mass of the rocket is decreasing while the force acting on it due to the gases is
constant, the acceleration of the rocket increases as it ascends.

So i guess the mathematical explanation ignores the physics even though it is technically correct but doesn't actually explain it? is momentum is not constant!? Why is that a flawed explanation? because it didn't use F=ma? OF COURSE IT IS NOT CONSTANT! It is constantly changing. it is change in momentum that is equal between gasses and rocket

Momentum of the rocket:
Δp rocket = -Δp fuel (Newton’s 3rd Law) and the rocket is continually gaining momentum as Δp rocket is positive
(upwards).

[Fizzycyst]

Hi guys i have a couple HSC questions where I have trouble wrapping my head around:

https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-physics.pdf (Q19)
B was my answer because if the astronaut was in orbit, it wouldnt "fall" down, it would just go around and around the planet.

Thanks so much

One of the worst (if not THE worst) MCQ in HSC history.

I remember presenting at HSC Meet The Markers for that years HSC Physics paper. That MCQ caused a massive uproar amongst the 100+ Physics teachers, academics and university professors in both sessions and myself and the other 2 presenters really could not defend that question.
The audience was legit fuming, until I started mentioning the question which I marked for the HSC that year (the semiconducting modelling question) and how my favourite response was a drawing of an elephant. LoL.

From what I have heard it was a last minute replacement question which replaced another question (could it have been worse?) and it didn’t go through the regular checks that the questions usually do.

It really does not have a good, correct answer. The given response, (d), as it reads implies that they would certainly experience different accelerations due to their vastly different masses.

gg HSC.

[Aaron12038488]

not sure if it is just me or for everyone else, but I cant seem to access the old physics stage 6 syllabus . It keeps redirecting me to the NESA website, and all I cn find is the new syllabus instead of the old...

[sadfd45678]

Hello!!

This is MUCH easier if we rule out the incorrect ones, For A, the force of gravity is NOT negligible, as that's what keeps objects in orbit! so it is wrong. While B is true, this is not answering the question, it is just a statement.

We now have it to C or D, for C, IF the forces were to be the same, (ie F = ma) since, the question claims F is the same, while mass is different, their accelerations must be different, and well, if their accelerations are  different, how on earth can they travel at the same speed? So C is incorrect, leaving only D as the answer (if you would like me to explain why, i'll be happy to!)

Most people do pick B for that reason, they don't really understand C or D, so go with B, because it is technically correct but it doesn't actually explain anything relevant to the question

I hope this helps!

Hi Can ou explain why it is D then. Also, is my understanding correct: Anything in orbit is constantly freefalling due to the same acceleration?

[Fizzycyst]

not sure if it is just me or for everyone else, but I cant seem to access the old physics stage 6 syllabus . It keeps redirecting me to the NESA website, and all I cn find is the new syllabus instead of the old...

https://educationstandards.nsw.edu.au/wps/wcm/connect/0f85cdff-baa0-402e-b066-57b684cea56a/physics-st6-syl+Physics.pdf?MOD=AJPERES&CVID=

[jamonwindeyer]

Hi Can ou explain why it is D then. Also, is my understanding correct: Anything in orbit is constantly freefalling due to the same acceleration?

Anything in Orbit is constantly freefalling due to the same acceleration: I'd say this is fair! I'd say it's a tad better to say that an object is in constant freefall because the curvature of its flight path matches the curvature of the earth (or whatever planet). Only because I associate those two things a bit more closely. But you're right - Anything in orbit is undergoing (roughly) uniform circular motion, which means a constant magnitude of acceleration

[cthulu]

Hey, just joined this forums a few weeks ago and have been lurking the entire time, got my half-yearlies next week and I am unable to ask my teacher for help so I came here! I found this question recently in a Girraween past paper and I am unable to solve it, would be appreciated if anyone can help! Thanks!

[Fizzycyst]

Hey, just joined this forums a few weeks ago and have been lurking the entire time, got my half-yearlies next week and I am unable to ask my teacher for help so I came here! I found this question recently in a Girraween past paper and I am unable to solve it, would be appreciated if anyone can help! Thanks!

(Image removed from quote.)
(Image removed from quote.)

Using RHG rule, it is noted that in region II, both currents give rise to a magnetic field in the same direction  (into the page), therefore this is where the field in strongest as the fields combine.

Part (d), you have field lines into the page, proton moving to the right — so you can use the RHP Rule to determine the direction of force on the proton being up the page or towards the wire carrying 10A right. Therefore you could say the proton accelerates or deflects upwards.

I hope that helps!

[itssona]

how does back emf mean current loss in regards to AC Generators?

"There is constant back emf induced in transmission wires as eddy currents are produced in wires, resulting in current loss."

thank you!

[dermite]

Hi, I would like some help with this question.

Thanks in advance,

[clovvy]

A projectile has time of flight of 7.5s and a range of 1200 m.  Calculate:
Its maximum height and the velocity it is projected

[blasonduo]

how does back emf mean current loss in regards to AC Generators?

"There is constant back emf induced in transmission wires as eddy currents are produced in wires, resulting in current loss."

thank you!

Hey! Back EMF is only produced where there is a change in magnetic flux, so theoretically, if anything is near-by that is able to conduct electricity, eddy currents will be induced (even if its minuscule!). These eddy currents do not actually get transferred with the rest of the current, so there will be a current loss.

Hope this answers your question, if not, please ask away!!

Hi, I would like some help with this question.

Thanks in advance,

Hey! I'll try to guide you through it!

This whole question relates to the production of back emf. As we know, back emf opposes the flow of the supply current, so it decreases the net flow of current. The slower the motor, the less back emf and thus more supply current. This increase in current can destroy the wires due to heat. Hopefully this will be able to guide you through the question! If not, please ask!!

A projectile has time of flight of 7.5s and a range of 1200 m.  Calculate:
Its maximum height and the velocity it is projected

Although it does not state where it was launched from the ground, nor landed on ground level, I am going to assume this.

viy = initial velocity (vertical)
vfy = final velocity (vertical)
others should be self-explanatory.

Finding initial horizontal velocity;

Pretty easy....
Finding initial vertical velocity;





For height

[Dragomistress]

For the syllabus point, "Describe the main features of an AC electric motor".
It says that we need one AC electric motor design so, can I use the motor which is highly similar to the DC motor or do I also have to do the AC induction motor?

[Fizzycyst]

For the syllabus point, "Describe the main features of an AC electric motor".
It says that we need one AC electric motor design so, can I use the motor which is highly similar to the DC motor or do I also have to do the AC induction motor?

Tbh, I would go with both. Questions are always asked about both.

If you have time also look over the universal motor and synchronous motors, just in case.

The universal motor did come up in the 2016 HSC (MC Q20), synchronous motors have never been asked anywhere, but doesn't mean it would happen, it is a type of AC motor and it is in most textbooks, which is something the exam committee takes into account. They are quite similar in construction to an AC induction, so its not learning that much extra.

Do also keep in mind that 'simple' AC motors aren't really around -- as they would need to be rotating at the same frequency as the AC input