HSC Physics Question Thread

[MarkThor]

Could someone please explain to me how to get the answer for this question?

[Cindy2k16]

Could someone please explain to me how to get the answer for this question?

You estimate the peak weavelength ~7 micrometres from the graph and calculate the energy with E=hf. You'll find the value you get is closest to the band gap of B.

[znaser]

A mathematical explanation for the shorter path and lower max. height would just be formulated by manipulating the equations as the acceleration due to gravity will be proportional or inversely proportional to ... But I was wondering in terms of a more scientific approach, would it be because there is more resistance to the movement of the projectile upwards due to a stronger gravitational force acting downwards and as a result, a lower maximum height. and since horizontal velocity is not affected by acceleration, a shorter range would result due to a lower time in the air. or is it something else? Thanks

[jakesilove]

A mathematical explanation for the shorter path and lower max. height would just be formulated by manipulating the equations as the acceleration due to gravity will be proportional or inversely proportional to ... But I was wondering in terms of a more scientific approach, would it be because there is more resistance to the movement of the projectile upwards due to a stronger gravitational force acting downwards and as a result, a lower maximum height. and since horizontal velocity is not affected by acceleration, a shorter range would result due to a lower time in the air. or is it something else? Thanks

Yep, you're right on the money! I probably wouldn't use the word 'resistance', as that implies air resistance etc; it's about a greater downwards acceleration/force due to gravity! But yeah, you've answered perfectly

[Cindy2k16]

Hi for this question how do you work out what the answer is? I can see why each option would affect the amount of current but not which one would affect it the least
TIA

[Cindy2k16]

Another question: for mass dilation, is the mass only observed to increase from an observer in another frame of reference to the object travelling at relativistic speeds? Or does the object travelling at relativistic speeds experience the increase in mass?
TIA

When a spacecraft in orbit fires its rockets to leave its orbit, is this just an application of just newton's third law (conservation of momentum) or does it also apply Newton's second law- does the firing of the rockets apply a force on the rocket causing it to accelerate and therefore leave its orbit?
TIA (sorry for all these questions!)

Moderator Edit: Posts merged. If you have a few in a row, use the 'Modify' button to add them to your initial question!

[jamonwindeyer]

Hi for this question how do you work out what the answer is? I can see why each option would affect the amount of current but not which one would affect it the least
TIA

Hey! So let's consider each option; the trick here is remembering Faraday's Law for Induced EMF (and thus current):



So we can change the current by increasing how much of a change in flux we expose the wire to.

Making it longer does this, because the length of the wire is cutting through more field lines. Changing your orientation with respect to the earths magnetic field DEFINITELY does this as well. Ditto for the speed of the wire.

Changing the thickness doesn't do much, because it specifies that the wire has a very low resistance. Making something with a low resistivity thicker isn't going to alter things as much as the others. Therefore, the answer (imo) would be B

[RuiAce]

Another question: for mass dilation, is the mass only observed to increase from an observer in another frame of reference to the object travelling at relativistic speeds? Or does the object travelling at relativistic speeds experience the increase in mass?
TIA

It's observed to increase. Note that the guy travelling at relativistic speeds actually thinks everyone else is travelling at relativistic speeds (because all inertial frames of reference are relative), so he thinks his mass is fine and observes everyone else getting heavier

[Albertenouttaten]

Hi for this question how do you work out what the answer is? I can see why each option would affect the amount of current but not which one would affect it the least
TIA

Hi is the answer C?

[imtrying]

You estimate the peak weavelength ~7 micrometres from the graph and calculate the energy with E=hf. You'll find the value you get is closest to the band gap of B.

Hey I was just doing this question as well, and I'm probably missing something obvious, but why would you calculate it from the peak of the graph?

[jamonwindeyer]

Hi is the answer C?

Not quite! See my response above

Hey I was just doing this question as well, and I'm probably missing something obvious, but why would you calculate it from the peak of the graph?

No no this is actually pretty subtle. If we want to detect human radiation, we would want a work function roughly equal to the peak of our human black body curve. Lower than this, and we would get stuff from other sources and cause interference. Higher, and we'd not detect our radiation entirely thus, we use the peak (the answer won't be exact to any of the options, just close)

[jamonwindeyer]

When a spacecraft in orbit fires its rockets to leave its orbit, is this just an application of just newton's third law (conservation of momentum) or does it also apply Newton's second law- does the firing of the rockets apply a force on the rocket causing it to accelerate and therefore leave its orbit?
TIA (sorry for all these questions!)

That's okay! Rui answered your other one; but for this, it's all of the above! Conservation of momentum plays a role, Newton's 2nd Law also plays a role; they are just different ways of analysing the same scenario (and indeed, you should be able to apply both to a rocket launch)

[imtrying]

No no this is actually pretty subtle. If we want to detect human radiation, we would want a work function roughly equal to the peak of our human black body curve. Lower than this, and we would get stuff from other sources and cause interference. Higher, and we'd not detect our radiation entirely thus, we use the peak (the answer won't be exact to any of the options, just close)

Thanks! 

[Mei2016]

So Pauli predicted the existence of the neutrino after studying beta decays and suggested the existence of them to satisfy the laws of conservation of momentum and energy. However, beta decay (ie beta minus) is due to emission of neutrons, where the neutrons decay into a proton, an electron and an anti-neutrino. So an anti-neutrino is produced by beta decay, so why did Pauli suggest the  neutrino?

-When Pauli suggested the neutrino, this covers the three types right? (the electron neutrion, tau and muon neutrinos?)

-Also, is it advised to remember the nuclear equation for what was actually happening in Fermi's Uranium Problem(the neutron + 235 U --> 2 daughter products and 3 neutrons)?

-In 2006 paper, in Q9, the answer is B and do they mean clockwise or anticlockwise?
(is it anticlockwise because charges would move clockwise Y to X, however, it's an induced current so due to Lenz's law it would flow the opposite direction, so anticlockwise?)

And for 2013, Q16, why is the answer D and not C?, because isn't it in metals that Valence Band and Conduction Band are both full at the same time. The question doesn't specify what band it is 'electrons in a fully filled band' so I wasn't quite sure on this one.

-Also, for 2011 Q34 c) for forces in the atomic nucleus, is electrostatic only for protons to protons or electrons to electrons? (so it's direction is always 'repulsive'?) or could it be between protons and electrons (then the direction would be 'attractive')
-and isn't the direction for SNF attractive at certain distances and then replusive if the distances are less than 1x10^-15m? ( the answers for the table in Q34c) only had 'attractive' as the direction...)

[Cindy2k16]

Hey! So let's consider each option; the trick here is remembering Faraday's Law for Induced EMF (and thus current):



So we can change the current by increasing how much of a change in flux we expose the wire to.

Making it longer does this, because the length of the wire is cutting through more field lines. Changing your orientation with respect to the earths magnetic field DEFINITELY does this as well. Ditto for the speed of the wire.

Changing the thickness doesn't do much, because it specifies that the wire has a very low resistance. Making something with a low resistivity thicker isn't going to alter things as much as the others. Therefore, the answer (imo) would be B

That's okay! Rui answered your other one; but for this, it's all of the above! Conservation of momentum plays a role, Newton's 2nd Law also plays a role; they are just different ways of analysing the same scenario (and indeed, you should be able to apply both to a rocket launch)

It's observed to increase. Note that the guy travelling at relativistic speeds actually thinks everyone else is travelling at relativistic speeds (because all inertial frames of reference are relative), so he thinks his mass is fine and observes everyone else getting heavier

Thank you!