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Author Topic: HSC Physics Question Thread  (Read 1030699 times)  Share 

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jamonwindeyer

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Re: Physics Question Thread
« Reply #1650 on: January 22, 2017, 10:21:48 am »
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So my parents are deciding to get two tutors for me for Physics. I have started tutoring for 1 of them which is individual tutoring. Is it bad to have two tutors, as they may teach different methods. In the upside is that I have more resources and greater understanding.

Two tutors is two more than I ever had  8)

Like, I personally think its a bit excessive to have two tutors for the same subject. Of course - More understanding, more resources, more time spent on the subject. But like, two tutors is (probably) ludicrously expensive and they are teaching you the exact same stuff. They may have different approaches/methodologies, and that could help in small ways, but the fact is that once you start the second tutoring session, you are going to be playing a session essentially on repeat. You should, if the first tutor did their job, understand things relatively well before the second tutor even says a word.

I don't see anything bad happening because of two tutors mind you. It's just like, why, you know what I mean? You'd be better off spending that money on buying a practice question booklet and doing individual practice questions, to be honest (in my opinion obviously).

PS - When you say your parents decided, do YOU want two tutors?

Aaron12038488

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Re: Physics Question Thread
« Reply #1651 on: January 22, 2017, 11:11:36 am »
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My second tutor is not individual, it is a group class. My parents believe as my tutor is a Uni student, that he will not be fully committed and also has no experience so as a precaution they will be looking for another tutor. Personally I totally agree with you, I will rather spend on practice books and past papers so I can apply my knowledge rather than learning content again. With school starting in a week, I Personally believe that my current tutor is doing a good job and with school lurking around the corner, I think I will be fine.

jamonwindeyer

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Re: Physics Question Thread
« Reply #1652 on: January 22, 2017, 11:45:49 am »
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My second tutor is not individual, it is a group class. My parents believe as my tutor is a Uni student, that he will not be fully committed and also has no experience so as a precaution they will be looking for another tutor. Personally I totally agree with you, I will rather spend on practice books and past papers so I can apply my knowledge rather than learning content again. With school starting in a week, I Personally believe that my current tutor is doing a good job and with school lurking around the corner, I think I will be fine.

Ahh right, cool cool. Well whatever works for you - Nothing bad will happen because of having two tutors. The Physics content is fairly non-interpretive and usually there is only one way to do a problem. Just be aware that two classes/tutors isn't going to give anywhere near the value for money as having just one person :)

Rathin

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Re: Physics Question Thread
« Reply #1653 on: January 22, 2017, 12:27:52 pm »
+2
Doing past paper questions helped me way more than learning the content again and again. I reckon two tutors is to excessive as learning the same content again 3 times (including school) will make it repetitive and in a way boring. Just do heaps of practice questions from textbooks like surfing physics and also school past papers from https://thsconline.github.io/s/ and you will be more than fine in the prelims. In the end it all comes down to you and how much you understand and how broad your question answering skills are. People may know their content but regurgitating it in exam time with proper exam techniques is a skill in itself.
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bsdfjnlkasn

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Re: Physics Question Thread
« Reply #1654 on: January 24, 2017, 09:48:55 am »
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Hey I have a few Space questions that I was hoping to get some help with  :)

1. Would a planet rotating quicker or slower increase the gravitational acceleration according to


2. How is an object's GPE and Work related (on Earth)? So for expressions: GPE = mgh and W = Fs
« Last Edit: January 24, 2017, 10:23:18 am by bsdfjn;lkasn »

jakesilove

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Re: Physics Question Thread
« Reply #1655 on: January 24, 2017, 10:32:01 am »
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Hey I have a few Space questions that I was hoping to get some help with  :)

1. Would a planet rotating quicker or slower increase the gravitational acceleration according to


2. How is an object's GPE and Work related (on Earth)? So for expressions: GPE = mgh and W = Fs

1. Rotation doesn't actually affect the formula, and definitely isn't something relevant to the curriculum. However, as we know that high velocities increase the mass of an object, greater spin will result in higher mass. That increases g!

2. An object doesn't have 'work', but it does have GPE. 'Work' is a force times a distance, so only occurs when work is done ON or BY the object.

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bsdfjnlkasn

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Re: Physics Question Thread
« Reply #1656 on: January 24, 2017, 10:47:27 am »
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2. An object doesn't have 'work', but it does have GPE. 'Work' is a force times a distance, so only occurs when work is done ON or BY the object.

Jake

Sorry I should have been clearer with my second question.

What I meant was: What makes work when done on an object (across a horizontal plane) comparable to GPE which acts vertically? Is there no difference? Is it something I just have to accept - that they're essentially the same just GPE accounts for gravity because it exists and therefore will affect the force?

Also, could you please check my notes for the equation GPE = mgh?

o   This equation is only really accurate when we use it to calculate the GPE of an object on Earth as g is taken as a constant and hence doesn’t account for g decreasing as altitude increases
•   It assumes that any object which has a GPE=0 on Earth can be taken as a reference point for all GPE calculations in Space
•   To rectify this, we need a universal definition of GPE that takes into account the equation for Gravitational acceleration


And some more questions: 8)

When trying to understand why the expression for potential energy is negative:

we take the theoretical case that an object is infinitely away from a gravitational field and so has a GPE=0. So when it is pushed towards this field, it begins losing Ep (as kinetic E is gained) which because it was initially 0 means a negative Ep value. Would the rate at which the Ep decreases resemble an exponential graph? I'm just thinking this because the object obviously has to accelerate (does it ever hit terminal velocity?) which stops there from being a linear relationship between the transfer of Ep to kinetic E.

Sorry i'm not being really technical here and could be completely wrong but I just thought this might be an easy question to answer (my textbook doesn't say anything about this so the question could also be completely irrelevant lol)

Thanks Jake!
« Last Edit: January 24, 2017, 11:37:29 am by bsdfjn;lkasn »

Shadowxo

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Re: Physics Question Thread
« Reply #1657 on: January 24, 2017, 12:19:09 pm »
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Hey I have a few Space questions that I was hoping to get some help with  :)

1. Would a planet rotating quicker or slower increase the gravitational acceleration according to


2. How is an object's GPE and Work related (on Earth)? So for expressions: GPE = mgh and W = Fs

Hi,
Not sure if you need to know for HSC but
1. The centripetal (rotational force) = mv2/r, so as v increases, the centripetal force increases
When a planet rotates, the centripetal force = gravitational force
So if the velocity is increased, the centripetal force increases and therefore so must the gravitational force, and as Fg = GMm/r2, r decreases.
Basically, it rotates faster when it's closer to the centre of the earth, and if it's closer then g is more.

Extra:
mv2/r = GMm/r2
v2=GM/r
GM (constant for a particular planet) = v2r
So as velocity increases, r decreases

2. Work is like a transfer of energy.
Work = Fd = ma*d.
gpe = mgh
You can do work on an object vertically to give it more gpe, as the change in work = change in gpe (as a of object= g)
So you can increase an object's gpe by doing work on it (vertically). It's like a transfer of energy, if done vertically the work is converted into gpe
Horizontally, the work is converted into ke (gpe unaffected horizontally)
Don't know if this will help but hope so :)

With your notes :
- Yes when g = 9.8 but g=GM/r2 so can be different. To accurately find gpe you'd need a graph showing the change in g for long distances where this value would change
- gpe isn't really calculated in space, just know that it's higher further away. gpe is mostly used for changes in energy, so change in (∆) gpe = mg∆h

Not sure about that last question, don't recall learning it :P
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Re: Physics Question Thread
« Reply #1658 on: January 24, 2017, 09:34:17 pm »
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Hi,
can someone help with these multiple choice questions. Thanks :)

kiwiberry

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Re: Physics Question Thread
« Reply #1659 on: January 24, 2017, 10:30:34 pm »
+1
Hi,
can someone help with these multiple choice questions. Thanks :)

The last one can be done using Kepler's Law

Remember that M refers to the mass of the central body (Saturn), not the mass of the moons orbiting. So firstly we need to find M by subbing in the given data for Titan (and using the orbital radius for r as this already includes the radius of the planet)

Now we just use this to find T for Rhea!

So the answer is B :)
« Last Edit: January 24, 2017, 10:42:06 pm by kiwiberry »
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RuiAce

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Re: Physics Question Thread
« Reply #1660 on: January 24, 2017, 11:21:22 pm »
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Hi,
can someone help with these multiple choice questions. Thanks :)
Q10 and Q9 both involve considering the principle of electromagnetic induction.

In Q10, the graph for coil X should be obvious. Once it's turned on, current starts flowing through it. Once it's turned off, it will no longer be. This limits your answer to A and C.

Recall that by the principle of electromagnetic induction, a current is induced into a conductor if there is a CHANGE in the magnetic flux around it. At what point does the change in flux occur?

At the instant the switch is turned on, coil X produces a magnetic field (you learnt this in prelim, and the direction is determined by the right-hand grip rule). This means that all in a sudden, there was a CHANGE in magnetic flux. Because the magnetic field was not there prior to the switch being turned on, and once the switch is turned on it will stay there.

So because ONLY at THAT instant, there was a change in flux, you can already see why A must be the correct answer. I'll leave you to figure out the rest of the puzzle that guarantees it.


(I don't know how to properly explain Q9 anymore. Will let someone take over.)

The last one can be done using Kepler's Law

Remember that M refers to the mass of the central body (Saturn), not the mass of the moons orbiting. So firstly we need to find M by subbing in the given data for Titan (and using the orbital radius for r as this already includes the radius of the planet)

Now we just use this to find T for Rhea!

So the answer is B :)
Nice LaTeX

davidss

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Re: Physics Question Thread
« Reply #1661 on: January 25, 2017, 06:52:55 am »
+1
Thanks guys!

bsdfjnlkasn

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Re: Physics Question Thread
« Reply #1662 on: January 25, 2017, 10:25:07 pm »
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Hey there,

I know lectures are underway and that it's a busy time so I won't mind if my questions aren't answered for a few days - they're not too urgent. (Plus, you guys deserve a break! :P)

So regarding the outcome: "Discuss the effect of the Earth’s orbital motion and its rotational motion on the launch of a rocket" I'm a little confused about some of the details that relate to Earth specifically.

My textbook first gives the definition for angular speed as

"Angular speed: How fast the angle of a line that joins the object with the centre is changing" and this is really confusing to me as I don't understand how this angle can change if an object travelling in a circle sustains tangential velocity. Doesn't this mean that the angle held at any instant by the circular-moving object and the centre is 90o ?

2. What is an object's linear speed? If it increases, will it increase the object's angular speed also?

3. How do larger linear speed reduce the effects of gravity?

4. Is Earth's elliptical shape a result of it's rotational axis?

5. What provides Earth's centripetal force which allows it to orbit? Is it the gravitational fields of other, larger celestial bodies?

Now for some questions that are separate from the quoted outcome:

1. How does the linear orbit velocity acting on a satellite cause it to move away from the Earth?

Thank you  :)
« Last Edit: January 25, 2017, 11:30:43 pm by bsdfjn;lkasn »

Spencerr

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Re: Physics Question Thread
« Reply #1663 on: January 26, 2017, 12:21:27 am »
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Hey there, I'll try answer this part of the question and see if the others can help with the rest.

So regarding the outcome: "Discuss the effect of the Earth’s orbital motion and its rotational motion on the launch of a rocket" I'm a little confused about some of the details that relate to Earth specifically.
Thank you  :)

In this question which is generally worth 3/4 marks. There are two key things you have to mention.

1) Launching from the Earth's equator in an easterly direction to utilise the Earth's rotational velocity.

2) Using the Earth's orbital velocity around the Sun

A rocket launched from Earth would be launched from the equator in an easterly direction in order to add the Earth's rotational velocity to it's own velocity. This adding of velocities is quite similar to the idea of launching an aeroplane from a moving platform. Let's say the aeroplane is launched at 5 m/s towards the right. If the platform is still and not moving, the aeroplane would only travel at 5 m/s towards the right. However if the platform was also moving towards the right at 10m/s. The aeroplane would be able to add on the platform's velocity to its own velocity and instead travel at 5 m/s. Intuitively, you can imagine a similar thing happening when javelin throwers throw their spears. The point of their run up is to increase the javelin's speed, which will allow it to travel further through the air. In the case of Earth, Earth rotates at a certain high speed around it's axis and this speed is a MAXIMUM at the equator. So if a rocket was launched at the equator, it would gain Earth's rotational velocity around its axis for FREE (without the additional use of costly fuel). This is very helpful as it can help the rocket reach the necessary ESCAPE VELOCITY and enter a stable parking orbit around the Earth before the next event occurs.
The Earth orbits around the sun with a certain velocity (that is it's tangential/linear velocity). Once the rocket enters into a parking orbit around the Earth, it can be boosted out of the orbit and into Space. However, there is a relatively tight time frame involved as the Earth's tangential velocity must align with the trajectory of the rocket. (You can imagine the tangential velocity of Earth's orbit around the Sun constantly changing as the Earth moves around the Sun). Similar situation with the moving platform here- once the rocket is boosted out of orbit, it can add to it's own velocity relative to the sun, the orbital velocity of the Earth without the use of additional fuel. This is really helpful as it reduces the amount of fuel needed for the rocket to attain such a high velocity and it increases the payload the rocket can travel, allowing interplanetary travel possible!

Hope this has helped :) I've included more information than what is needed for a 3/4 marker though!
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Mayalily

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Re: Physics Question Thread
« Reply #1664 on: January 26, 2017, 11:08:29 am »
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I have a question which i am getting close to the answe with, but not quite, and I have no idea why and I'm quite confused and frustrated.

The first part of the question requires you to find the Ep of a satellite of M=500kg, at 10,000km altitude.

i've subbed this into -G m1m2/r
hence:
-6.67*10-11* [500*(6.0*1024)]/(6371000+10,000,000)  - Where 6371000 is the radius of the earth in metres and 10,000,000 is the altitude in metres.

But I get -1.222*1010J, rather than the answer; -3.119*1010J

Am I doing something wrong that i've missed or is the wrong answer written down?
« Last Edit: January 26, 2017, 04:07:28 pm by Mayalily »