HSC Physics Question Thread

[jamonwindeyer]

Hi could someone clarify how to ensure/determine the reliability, accuracy and validity of secondary sources? I think reliability is consistency in information across multiple reputable sources but I'm less sure about the difference between accuracy and validity for secondary sources.

Hey Cindy! For reliability, you are spot on.

Validity for an experiment is controlling variables, and for secondary sources, it is similar. Does this source answer the questions you were asking adequately? Is there any bias or error that could impact on its validity? Is the source credible, and current?

We assess accuracy by comparing the source to something like a scientiffic journal. We have established information that is guaranteed accurate; accurate secondary sources will match. For example, a source which says \(g=7ms^{-1}\) is not considered accurate, because it doesn't match the established information. So, ALL information in that source is therefore probably inaccurate, or at least needs to be taken with caution

[jamonwindeyer]

For this question I wrote that the rotation of the space station would provide a centripetal force that acts on the astronaut causing the astronaut to accelerate towards the centre and thus stimulates gravity. Is that enough to answer the question?
In the sample answer it mentions Newton's 3rd Law- is that necessary to mention? If so, how does the reaction force show that gravity is stimulated because I don't really understand that.
Thanks in advance

For this, you would need about 1 more sentence, maybe 2. You've started excellently.

To understand what is happening here, think of a heavy brick in a bucket. If you swing that bucket around at speed, you have a centripetal force applied by your arm which keeps the bucket in circular motion. However, provided you keep the bucket spinning, the brick stays in the bucket! Why? There is a reaction force at play here (that's where Newton's 3rd Law comes in), an equal and opposite force to the centripetal force. We colloquially call this the centrifugal force, and THIS is what keeps the brick in place. Just the same, this is what simulates gravity for your astronaut.

So you need 2 more bits of info:

- There is an equal and opposite force that acts on the astronaut due to Newton's 3rd Law
- This 'centrifugal force' is experienced as a downwards force by the astronaut, similar to gravity

Does that make sense? The analogy is the key here; but not everyone clicks with it (I guess I'm the only one swinging bricks around in buckets )

[Cindy2k16]

For this, you would need about 1 more sentence, maybe 2. You've started excellently.

To understand what is happening here, think of a heavy brick in a bucket. If you swing that bucket around at speed, you have a centripetal force applied by your arm which keeps the bucket in circular motion. However, provided you keep the bucket spinning, the brick stays in the bucket! Why? There is a reaction force at play here (that's where Newton's 3rd Law comes in), an equal and opposite force to the centripetal force. We colloquially call this the centrifugal force, and THIS is what keeps the brick in place. Just the same, this is what simulates gravity for your astronaut.

So you need 2 more bits of info:

- There is an equal and opposite force that acts on the astronaut due to Newton's 3rd Law
- This 'centrifugal force' is experienced as a downwards force by the astronaut, similar to gravity

Does that make sense? The analogy is the key here; but not everyone clicks with it (I guess I'm the only one swinging bricks around in buckets )

Hi thanks for the explanation! I'm still a bit confused though sorry. so in the sample answer it says the astronaut "reacts against the force". is this the same as "an equal and opposite force acts on the astronaut"? One sounds to me like its the astronaut providing the reaction force and the other sounds like something else is providing a reaction force-onto the astronaut, so I'm a bit confused. Also how is the astronaut/brick staying in place similar to how gravity works?
Thanks!

[jamonwindeyer]

Hi thanks for the explanation! I'm still a bit confused though sorry. so in the sample answer it says the astronaut "reacts against the force". is this the same as "an equal and opposite force acts on the astronaut"? One sounds to me like its the astronaut providing the reaction force and the other sounds like something else is providing a reaction force-onto the astronaut, so I'm a bit confused. Also how is the astronaut/brick staying in place similar to how gravity works?
Thanks!

Ehh it's a little bit the same really; there is a reaction force experienced by the astronaut is what I would say. It's the least confusing and answers the question directly

If the astronaut was standing on flat ground on earth, they would expect to stay in place and not float anywhere. Why? Gravity. On the space station, the reaction/centrifugal force is what keeps the astronaut in place and keeps them from floating off. Just like gravity does on earth; and thus, the reaction force keeping them in place simulates gravity. More specifically, the reaction force that the astronaut experiences (and exerts on the space station) is kind of like the weight force that one experiences due to gravity

[aaron_solomon]

Hey could someone explain what magnetic flux is? i can seem to wrap my head around it

[jamonwindeyer]

Hey could someone explain what magnetic flux is? i can seem to wrap my head around it

Hey! Magnetic flux is magnetic field lines; it is a magnetic field! So saying that we have a changing magnetic flux, means we have a changing magnetic field!

As to what a magnetic field actually is; that's gross. It's what you get when you mix electrodynamics and special relativity; definitely not necessary to go any further than just "magnetic field lines"

[imtrying]

If I were to use calculus to solve a projectile motion question in physics, would I still be able to get full marks? Or do I need to use the physics formulas?

[RuiAce]

Hey could someone explain what magnetic flux is? i can seem to wrap my head around it

Hey! Magnetic flux is magnetic field lines; it is a magnetic field! So saying that we have a changing magnetic flux, means we have a changing magnetic field!

As to what a magnetic field actually is; that's gross. It's what you get when you mix electrodynamics and special relativity; definitely not necessary to go any further than just "magnetic field lines"

In the HSC course, they colloquially define flux as "the number of field lines passing through an imaginary area". I remember in my trials I actually got examined on phi = BA

If I were to use calculus to solve a projectile motion question in physics, would I still be able to get full marks? Or do I need to use the physics formulas?

If you want to use it to check then go for it, but please do your actual working using the physics formulae. They're designed to not need any calculus at all. That is a risk and depending on your marker you may not be awarded the marks.

[cherryred]

Hey,

I am still getting confused on lv/lo, mv/mo and tv/to. Any ways to understand whether a numerical value is given for the v or the o part would be appreciated.

Thank you

[RuiAce]

Hey,

I am still getting confused on lv/lo, mv/mo and tv/to. Any ways to understand whether a numerical value is given for the v or the o part would be appreciated.

Thank you

See a mention on posts #928 and #929

[Cindy2k16]

Hi could someone explain why the answer to this is C? Don't things with high resistance tend to heat up faster?

[Cindy2k16]

Also why is the answer for this one B? I calculated that the energy of the peak wavelength is 0.15eV and so that only overcomes the energy gap of A. Did I calculate it wrong?
TIA

[jamonwindeyer]

Hi could someone explain why the answer to this is C? Don't things with high resistance tend to heat up faster?

This was like the trickiest question ever. Remember that the rate of change of magnetic flux is a constant (presumably), and this means that the induced EMF or voltage is also a constant. Because the induced voltage is a constant, a high resistance means you get less current, and so get less heating.



Big R means little I, little I means no heating this was a weird question though, don't stress too much about it, it fooled everyone

The issue is that you want a large R, but not so large that not enough current flows

[Cindy2k16]

This was like the trickiest question ever. Remember that the rate of change of magnetic flux is a constant (presumably), and this means that the induced EMF or voltage is also a constant. Because the induced voltage is a constant, a high resistance means you get less current, and so get less heating.



Big R means little I, little I means no heating this was a weird question though, don't stress too much about it, it fooled everyone

The issue is that you want a large R, but not so large that not enough current flows

oh ok thank you

[jamonwindeyer]

Also why is the answer for this one B? I calculated that the energy of the peak wavelength is 0.15eV and so that only overcomes the energy gap of A. Did I calculate it wrong?
TIA

What did you use for your peak wavelength? If you use something just a TOUCH lower, then you will get to the value required for B. To effectively detect human radiation, we'd want an energy gap close to the energy in the typical human-emitted photon. The value you calculated is closest to B (even if it is slightly below), so we'd prefer B over A in that circumstance. A's band gap is too low; so we'd presumably pick up radiation from other stuff!