HSC Physics Question Thread

[harry.braithwaite]

Mani.s just about your conical pendulum, I feel your apprehensions about swinging it by hand are pretty unnecessary as you can get it to be both very accurate and valid, you just have to be smart about how you go about it. Depth studies aren't about making things complex, they are just about explaining physics well, so best to keep it simple.

I would suggest changing radius, which is probably the easiest way to go about doing the experiment. For a specific radius (can then change radius to use as independent variable) I would recommend hollowing a pen and passing a string through, which gives you a nice handle to hold. Start spinning above your head (making the plane of revolution horizontal) at such a speed that you keep that marked length at the tip of the pen and get someone to film you as you keep that orbital radius constant (If you fail to keep it constant for a run you can just discard that value), the experiment can be made very accurate if you keep it at the one radius for 10-20 revolutions, so that when looking back through the footage you can find the average orbital period for each revolution (which should be the same anyway if your radius is kept constant).

If you are keeping your radius constant, then the mass on the string isn't moving up and down and isn't accelerating which enables you to equate centripetal (mv^2/r) and weight (mg) and them from their determine your relationship between force and orbital radius.

As for validity, the only thing that isn't valid about this method would be the friction of the tip of the pen hole and the string and if you are spinning ensuring that you keep it flat. Also just with depth studies, it isn't necessarily about making the most accurate or most valid experiment anyway, as long as you can identify the sources of error and their impact. Just another thing, never try and make your depth study too complex, much better to do something simple very well than fail to comment on everything for example I wrote 4500 words on launching a marble at different angles, and someone in the previous year got full marks for dropping masses at 1m, 2m and 3m heights which goes to show you can tease words out of anything so do something simple so that you can really get across the actual physics.

[louisaaa01]

Hi,

I was doing this multiple choice question:

An object's gravitational potential energy is tripled. Assuming all other variables are constant, this gain in energy is accompanied by a change in radius equivalent to:

a. Tripling the radius
b. Reducing the radius by a third.
c. Doubling the radius.
d. Halving the radius.

I was just a bit confused, for, ordinarily I would have said a (which is given as the correct answer) since there is an increase in gravitational potential energy with radius. However, given the equation U = -GMm/r, this suggests an inversely proportional relationship between the magnitude of U and radius (tripling U would actually make it more negative, thus decreasing it) - which suggests b. I was just wondering what the correct approach is?

[theolast_]

I was doing this multiple choice question:

An object's gravitational potential energy is tripled. Assuming all other variables are constant, this gain in energy is accompanied by a change in radius equivalent to:

a. Tripling the radius
b. Reducing the radius by a third.
c. Doubling the radius.
d. Halving the radius.

I was just a bit confused, for, ordinarily I would have said a (which is given as the correct answer) since there is an increase in gravitational potential energy with radius. However, given the equation U = -GMm/r, this suggests an inversely proportional relationship between the magnitude of U and radius (tripling U would actually make it more negative, thus decreasing it) - which suggests b. I was just wondering what the correct approach is?

Hi. If you consider the equation U=-Gmm/r, reducing r by one third doesn't actually work either because that takes r down to 2r/3.  With 2r/3, the GPE only increases (in magnitude) by 1.5. So you can rule out option b.

Because it says that the GPE is tripled and that this was a gain in GPE, you can assume that they are using the equation for GPE in a uniform field, U=mgh. This implies U is in linear proportion to the radius, which makes sense for option A.

Overall, the question is quite vague.

[david.wang28]

Hello,
I ran into this question on advanced mechanics; my working out was very contrasting compared to the answers. I wanted to clarify which was the right way of doing the question; can anyone please help me out? Thanks

[DrDusk]

Hello,
I ran into this question on advanced mechanics; my working out was very contrasting compared to the answers. I wanted to clarify which was the right way of doing the question; can anyone please help me out? Thanks

I would encourage you to do it the way in the book. The reason is sometimes they might ask a specific sig fig or decimal place, and for your answer your already substituting values in before the final answer which means your decimals or sig figs have the chance of being a bit off. This CAN lead to marks being taken off depending on how lenient they are. It's always best to be safe than sorry imo! =)

[mani.s_]

Hi, whats a good inquiry question for advanced mechanics??? I had one in mind "what causes objects to maintain their circular motion, but the teacher told me that's a research question, not an inquiry question. What would be a good inquiry question for Advanced Mechanics??

[louisaaa01]

Hi, I was just wondering, how do you define torque direction? For instance, if an object is moving anticlockwise, is torque just anticlockwise? Because I just read that the Right Hand Grip Rule is used to determine torque direction, but I had never come across this before. Thank you!

[DrDusk]

Hi, I was just wondering, how do you define torque direction? For instance, if an object is moving anticlockwise, is torque just anticlockwise? Because I just read that the Right Hand Grip Rule is used to determine torque direction, but I had never come across this before. Thank you!

You've answered your question yourself. Use Right Hand Grip Rule. =)

[classof2019]

How do you do this question? I have come across conflicting sources - some say that the motor force is the centripetal force (which I thought it was since it acts perpendicular to current / the magnetic field) while others say it isn't as motor force is actually parallel to velocity (and rather centripetal force is sum of magnetic force, drag, etc).

Cheers

[DrDusk]

How do you do this question? I have come across conflicting sources - some say that the motor force is the centripetal force (which I thought it was since it acts perpendicular to current / the magnetic field) while others say it isn't as motor force is actually parallel to velocity (and rather centripetal force is sum of magnetic force, drag, etc).

Cheers

The latter is correct. Motor force is not what results in the circular motion, but rather the fact that the magnetic field of the wire interacts with the one of the Magnet known as Magnetic force. It doesn't matter whether the motor force is perpendicular to the current/magnetic field. The definition of Centripetal force is that it must be parallel to velocity.

I remember getting this question in an exam and I proved my teachers answer wrong lol

[rorygolledge]

Need help with the syllabus dotpoint "analyse the experimental evidence that supported the models of light that were proposed by Newton and Huygens"

Everywhere I look seems to give vague answers on what the actual experimental evidences were for both theories and I would like something clear if anyone is able to explain.

[louisaaa01]

Need help with the syllabus dotpoint "analyse the experimental evidence that supported the models of light that were proposed by Newton and Huygens"

Everywhere I look seems to give vague answers on what the actual experimental evidences were for both theories and I would like something clear if anyone is able to explain.

Hi!

Foucault's rotating mirror experiment is a good starting point. Essentially, Newton's corpuscular model relied on the idea that light travelled faster in denser media, while Huygens' wave model revolved around light slowing down in denser media. Foucault's experiment proved the latter, thus providing support for Huygens' model (and invalidating Newton's model - which predicted the opposite).

You could also look at Young's Double Slit experiment - phenomena such as diffraction and interference could only be explained sufficiently by the wave model. Polarisation experiments / Malus' law could also only be satisfactorily explained by a wave model (though Huygens' model inevitably required some modification). Newton attempted to explain both phenomena, though his model proved insufficient.

[louisaaa01]

Hi,

I just had a question in relation to the attached sample q from NESA.

Why is the initial induced emf negative? I've applied the Right Hand Grip Rule and I've concluded that since the top of the solenoid must induce a South pole, current flows from the positive terminal of the voltage probe, down through the solenoid, to the negative terminal - why is this regarded a negative rather than positive voltage?

Thanks.

[DrDusk]

Hi,

I just had a question in relation to the attached sample q from NESA.

Why is the initial induced emf negative? I've applied the Right Hand Grip Rule and I've concluded that since the top of the solenoid must induce a South pole, current flows from the positive terminal of the voltage probe, down through the solenoid, to the negative terminal - why is this regarded a negative rather than positive voltage?

Thanks.

I suggest you look up conventional current and Non-Conventional current. When we use right hand grip rule, our fingers curl in the direction of the current flow. This would be true IF current was the flow of positive charge, however current is actually the flow of NEGATIVE charge. Hence you really need to flip the direction because Non-Conventional current is the real direction a current flows i.e. it's the flow of electrons NOT positive charge.

[006896]

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