HSC Physics Question Thread

[jakesilove]

hi jake
i understand that its negative
let me rephrase
wouldnt change in y be
<-120m becos the projectile travels above the building
then to the ground?

sorry if this isn't clear

Ah I see! Remember that the time I used was the 6.4 seconds. The equation asks for the total displacement, at a particular time. If I throw a projectile up, it lands on the ground, the delta y value when it hits the ground is zero! That's just want delta y means; final y minus initial y

[Swagadaktal]

Hey guys, tbh only posting here coz i ceebs going to vce and this squad is bomb af soo

With a motor if it is at an angle (say 30 degrees to the vertical) - do you use the formula flux = B*A*cos(thetre) or B*A*sin(thetre) --- if this makes any sense?

Another Q: I'll try explain the scenario if it doesnt work I'll scour the interwebs for a diagram to illustrate my question.
Say there's a generator, with a coil situated within 2 magnets, with the N to the left and S to the right (so b field going to the right), and the coil is currently in its horizontal position, with the 4 corners (where bottom left, top left, top right to bottom right corners are labelled A, B , C and D respectively). And the question asks which way must the coil turn if you want the current to travel through C to D,

how would you answer this question in terms of magnetic flux? And is it appropriate to say that Lenz law informs us that a current will be induced to oppose the motion?

Soz i know this is really convoluted - if you manage to answer this ty xoxo <3 but if you need any further clarification im more than happy to give u guys a better question

[jamonwindeyer]

Hey guys, tbh only posting here coz i ceebs going to vce and this squad is bomb af soo

With a motor if it is at an angle (say 30 degrees to the vertical) - do you use the formula flux = B*A*cos(thetre) or B*A*sin(thetre) --- if this makes any sense?

Another Q: I'll try explain the scenario if it doesnt work I'll scour the interwebs for a diagram to illustrate my question.
Say there's a generator, with a coil situated within 2 magnets, with the N to the left and S to the right (so b field going to the right), and the coil is currently in its horizontal position, with the 4 corners (where bottom left, top left, top right to bottom right corners are labelled A, B , C and D respectively). And the question asks which way must the coil turn if you want the current to travel through C to D,

how would you answer this question in terms of magnetic flux? And is it appropriate to say that Lenz law informs us that a current will be induced to oppose the motion?

Soz i know this is really convoluted - if you manage to answer this ty xoxo <3 but if you need any further clarification im more than happy to give u guys a better question

Hey Swag!

For the first one, I believe you'd use the cosine version, in the HSC we calculate the torque for a motor as \(\tau=BAIn\cos{\theta}\), so I think you're extracting terms from there to calculate the flux in the coil? That would make the most sense to me

For your second one, you are spot on the money about Lenz's Law. So it's best to rejig the question a bit, and hopefully it makes sense given Lenz's Law why we'd do this:

If we pretend we are dealing with a motor and send a current through C to D, which way would the coil turn? The generator would need to be turned the OPPOSITE way to that.

The reason for this is (roughly) that when you turn the coil for the generator, the current is induced to oppose it. That current acts to oppose the motion, and so generates a torque in the opposite direction to your turn. So, figure out the direction of the OPPOSING TORQUE like you would a motor, then reverse it

Incidentally, doing this (if I picture your diagram correctly), the answer should be clockwise? C to D would rotate the coil anticlockwise, so we must have turned the coil clockwise to generate that

[Swagadaktal]

Hey Swag!

For the first one, I believe you'd use the cosine version, in the HSC we calculate the torque for a motor as \(\tau=BAIn\cos{\theta}\), so I think you're extracting terms from there to calculate the flux in the coil? That would make the most sense to me

For your second one, you are spot on the money about Lenz's Law. So it's best to rejig the question a bit, and hopefully it makes sense given Lenz's Law why we'd do this:

If we pretend we are dealing with a motor and send a current through C to D, which way would the coil turn? The generator would need to be turned the OPPOSITE way to that.

The reason for this is (roughly) that when you turn the coil for the generator, the current is induced to oppose it. That current acts to oppose the motion, and so generates a torque in the opposite direction to your turn. So, figure out the direction of the OPPOSING TORQUE like you would a motor, then reverse it

Incidentally, doing this (if I picture your diagram correctly), the answer should be clockwise? C to D would rotate the coil anticlockwise, so we must have turned the coil clockwise to generate that

OOH yeah I understood the reasoning but was having trouble putting it into words properly. Thanks Jamon <3
- Can't say that it acts like a reverse motor in my exam but I'll include that it must oppose the motion, and using RH slap rule we would want the force being down so generator must be moving up or w/e

- and with my first question, I was asking about which angle we used in the formula. Do we use the angle to the horizontal or angle to the vertical? ty xo

[jamonwindeyer]

OOH yeah I understood the reasoning but was having trouble putting it into words properly. Thanks Jamon <3
- Can't say that it acts like a reverse motor in my exam but I'll include that it must oppose the motion, and using RH grip we would want the force being down so generator must be moving up or w/e

- and with my first question, I was asking about which angle we used in the formula. Do we use the angle to the horizontal or angle to the vertical? ty xo

Ohh right, angle to the horizontal! Or more specifically, the angle that is formed by the plane of the coil and the field lines

[katnisschung]

i got it wrong again 

4) a canon ball is fired at 50m/s from the top of a 200m high cliff so that maximum
range is achieved (presumably delta x... don't know what they mean)

what is the initial vertical velocity

[jakesilove]

i got it wrong again 

4) a canon ball is fired at 50m/s from the top of a 200m high cliff so that maximum
range is achieved (presumably delta x... don't know what they mean)

what is the initial vertical velocity

For this, you just need to know that the maximum range will occur when theta equals 45 degrees. Then, construct a triangle, and find the vertical velocity!

[jamonwindeyer]

For this, you just need to know that the maximum range will occur when theta equals 45 degrees. Then, construct a triangle, and find the vertical velocity!

I did this, but I think the fact that it is on a cliff changes that fact! It will definitely travel further at shallower angles; I'm not sure if I can find a way to tackle this in a way that isn't mathematically rigorous? Seems very MX1 style, where's the question from Katniss?

*someone else feel free to have a shot, but I got a few lines in and just thought that it seemed a bit too mathematically rigorous for a HSC Physics problem...

[RuiAce]

I did this, but I think the fact that it is on a cliff changes that fact! It will definitely travel further at shallower angles; I'm not sure if I can find a way to tackle this in a way that isn't mathematically rigorous? Seems very MX1 style, where's the question from Katniss?

*someone else feel free to have a shot, but I got a few lines in and just thought that it seemed a bit too mathematically rigorous for a HSC Physics problem...

Hmm. There are ways of translating an MX1 approach into a physics approach (but I'm a bit too lazy to do the question though so only if you want to put it down aha)

[jamonwindeyer]

Hmm. There are ways of translating an MX1 approach into a physics approach (but I'm a bit too lazy to do the question though so only if you want to put it down aha)

Oh I definitely agree, but it gets to a point where it's like "Nah, this doesn't seem right."

Maybe the question is just really bloody tough aha but much more likely is I'm missing some approach, because I can't see how to do it without Calculus

[katnisschung]

Thanks Jamon and Jake
the question is from surfing (our teacher likes to bombard us with a lot of q
from it problem is the answers are quite frequently wrong)

the answers says that theta is 45 degrees so maybe they ignored the fact it was on a cliff.

question jake said "the maximum range will occur when theta equals 45 degrees."
im a little confused is this absolute for all types of questions?

[jamonwindeyer]

Thanks Jamon and Jake
the question is from surfing (our teacher likes to bombard us with a lot of q
from it problem is the answers are quite frequently wrong)

the answers says that theta is 45 degrees so maybe they ignored the fact it was on a cliff.

question jake said "the maximum range will occur when theta equals 45 degrees."
im a little confused is this absolute for all types of questions?

Hey Katniss! Okay, that's interesting! If it is on a cliff, then it's not an absolute; you can show that you'll have longer ranges for lower angles. You can use this to visualise the scenario! So I think that's another wrong answer to add to the list

If you are on flat ground, then yes! Maximum range will always occur (for a given velocity) if you launch the projectile at 45 degrees to the horizontal!

[katnisschung]

i need help again
sighs projectiles....
a canon ball is fired at 40 degrees to the horizontal from the top of a 218.7m
cliff and hits a target 300m from the base of the cliff

a) the initial velocity of the projectile
b) its initial horizontal velocity
c) its initial vertical velocity
d) its range
e) its maximum height
f) the time it takes to reach max height
g) its time of flight

[jakesilove]

i need help again
sighs projectiles....
a canon ball is fired at 40 degrees to the horizontal from the top of a 218.7m
cliff and hits a target 300m from the base of the cliff

a) the initial velocity of the projectile
b) its initial horizontal velocity
c) its initial vertical velocity
d) its range
e) its maximum height
f) the time it takes to reach max height
g) its time of flight

Hey! Check out the working I supplied here, and try taking a similar approach! If you're still struggling, show us your working out, and we can talk you through it

[katnisschung]

Hey! Check out the working I supplied here, and try taking a similar approach! If you're still struggling, show us your working out, and we can talk you through it

hi Jake

yeah i did do it just need confirmation for the time.
seems the answers may be wrong again.
my teacher is giving us the old version of surfing which has quite a few mistakes