HSC Physics Question Thread

[znaser]

I think that's a completely sufficient answer! Go into a bit of depth with each of the points you've outlined, but I wouldn't recommend more

Thanks Jake!

[bethjomay]

No, not quite; when the car 'speeds up', the apparatus will act as a motor. Electrical energy will be converted into kinetic energy, propelling the car forward. However, when the car needs to 'slow down', the apparatus begins to work as a generator. It will draw kinetic energy away, converting it back to electrical energy! So, whether it acts as a generator or a motor will depend on what actually needs to happen/what sort of outcome we want

Ah ok, that makes sense! Thank you!

[wyzard]

Hi!

How would you do the calculations for the graph in part b?

Thanks~

Basically break it down into horizontal and vertical components and examine them separately.

For the horizontal bit, the speed is uniform, so for every second the x-coordinate increases by the same amount.

For the vertical bit it's a more challenging as there's acceleration due to gravity to account for. Fortunately since both the cannon balls have the same vertical speed, they will be aligned at every second and you can use that. For the remaining time however, you can use the following equation:

[RuiAce]

Note that, on the HSC formula sheet, the above equation is

[FallonXay]

Basically break it down into horizontal and vertical components and examine them separately.

For the horizontal bit, the speed is uniform, so for every second the x-coordinate increases by the same amount.

For the vertical bit it's a more challenging as there's acceleration due to gravity to account for. Fortunately since both the cannon balls have the same vertical speed, they will be aligned at every second and you can use that. For the remaining time however, you can use the following equation:

Note that, on the HSC formula sheet, the above equation is

Hey! Such a weird question, hey? It was from my paper, so I know it well You just need to make sure that the horizontal displacement of Ball Q stays consistent (ie 3 blocks). Then, you know that acceleration due to gravity adds a consistent amount of velocity per second. It starts by 'moving down' 3 blocks, and then 9 blocks, which means each second ADDS 6 blocks worth of down. So, the next vertical position should be 9+6=15 blocks further down! Absolutely stupid question, but it does make you think about the actual physics behind projectile motion You can also check the 'actual' maths on the BOSTES answer page, here

Got it - Thanks 

[Cindy2k16]

Hi so I understand that they needed the aether since they thought light needed a medium to propagate (is this right?) and the MM experiment was designed to test the speed of the earth through the aether. But I don't understand how Einstein's special theory of relativity meant the aether was no longer needed- why did light being constant in all frames of reference mean that light didn't need a medium to travel in? Could someone explain? Thanks in advance

[RuiAce]

Hi so I understand that they needed the aether since they thought light needed a medium to propagate (is this right?) and the MM experiment was designed to test the speed of the earth through the aether. But I don't understand how Einstein's special theory of relativity meant the aether was no longer needed- why did light being constant in all frames of reference mean that light didn't need a medium to travel in? Could someone explain? Thanks in advance

The fact that the speed of light was constant in all frames of reference, and that all inertial frames of reference are relative to each other (these are the two statements of special relativity), meant that there was no longer a need for some:

a) absolute frame of reference
b) medium of propagation of light

as if the above were true, then technically the speed of light would be affected. If the aether existed, then the existence of an aether wind would've naturally affected the speed of light. But of course, MM showed that the aether may as well not exist, so the model can be discarded as unnecessary.

[Cindy2k16]

The fact that the speed of light was constant in all frames of reference, and that all inertial frames of reference are relative to each other (these are the two statements of special relativity), meant that there was no longer a need for some:

a) absolute frame of reference
b) medium of propagation of light

as if the above were true, then technically the speed of light would be affected. If the aether existed, then the existence of an aether wind would've naturally affected the speed of light. But of course, MM showed that the aether may as well not exist, so the model can be discarded as unnecessary.

Hi thanks for the reply I understand now
Also is the speed of light constant in all frames of reference- both inertial and non-inertial- or is it all inertial frames of reference?
TIA

[RuiAce]

Hi thanks for the reply I understand now
Also is the speed of light constant in all frames of reference- both inertial and non-inertial- or is it all inertial frames of reference?
TIA

Whoops forgot to say that above. But I believe inertial only.

Weird things happen in non-inertial frames of reference.

[Cindy2k16]

Whoops forgot to say that above. But I believe inertial only.

Weird things happen in non-inertial frames of reference.

ok cool thanks. much appreciated!

[jamonwindeyer]

Whoops forgot to say that above. But I believe inertial only.

Weird things happen in non-inertial frames of reference.

Confirm inertial only. If we are in a non-inertial reference frame we can use the altered behaviour of light to calculate our acceleration

[znaser]

I'm not exactly sure as to why a step-down transformer is used to transfer energy from a regional to local sub-station? and for the laptop or pretty much any household appliance, I always thought a step-up transformer is used so there isn't too much current flowing through the wires of the appliance that would cause it to burn. But according to the answers (c), a step-down transformer is also actually used. Why is this the case? Thanks

[RuiAce]

I'm not exactly sure as to why a step-down transformer is used to transfer energy from a regional to local sub-station? and for the laptop or pretty much any household appliance, I always thought a step-up transformer is used so there isn't too much current flowing through the wires of the appliance that would cause it to burn. But according to the answers (c), a step-down transformer is also actually used. Why is this the case? Thanks

To bring it to the regional substation we step it up so that we minimise the current flow. This minimises power losses.

But once it reaches the regional substation, we need to bring it to houses. At the substation, the voltages have reached things such as 1M V. If we keep stepping it up, it will probably reach 10M V. Obviously, things in the household do NOT need so much voltage; a powerpoint is only about 240V. So we need to step it down or else there will be too much voltage for things in the house.

Whereas something like a TV requires high voltages, we need the opposite for a laptop. Once we are down at 240V, there is no longer a direct risk of wires burning, or else we wouldn't have electricity in our homes. Laptops and etc. only need about 10V to power, otherwise the higher voltages are what damage the delicate appliances.

[znaser]

To bring it to the regional substation we step it up so that we minimise the current flow. This minimises power losses.

But once it reaches the regional substation, we need to bring it to houses. At the substation, the voltages have reached things such as 1M V. If we keep stepping it up, it will probably reach 10M V. Obviously, things in the household do NOT need so much voltage; a powerpoint is only about 240V. So we need to step it down or else there will be too much voltage for things in the house.

Whereas something like a TV requires high voltages, we need the opposite for a laptop. Once we are down at 240V, there is no longer a direct risk of wires burning, or else we wouldn't have electricity in our homes. Laptops and etc. only need about 10V to power, otherwise the higher voltages are what damage the delicate appliances.

That makes much more sense now. Thanks Rui!

[MysteryMarker]

Don't quite understand how to go about this question.

Recently the weapons unit of the UN discovered the components for a 100m long cannon barrel in an unspecified country. The country's defence agencies have denied that this is an offensive weapon to be used against neighbouring countries. They have explained that is is to be used to launch communication satellites into orbit. Explain why this is not considered to be a plausible explanation. (3 marks)