HSC Physics Question Thread

[jamonwindeyer]

Hey everyone,

Got a physics depth study due soon on motors in particular DC ones.

Was just after a few ideas of an inquiry question to base my depth study on.

Also if anyone has any tips and tricks on writing writing a band 6 depth study, would be greatly appreciated if you can send it my way

Cheers

Hey there!

There are lots of inquiry questions you could ask about DC motors. Some that spring to mind:

- Something on efficiency (how does changing X, Y and Z effect the efficiency of the motor)
- Modifications to the motor structure (different materials) and how they effect the motor/its operation
- How the speed of the motor can be controlled

All of these go a bit beyond the syllabus, which is the whole point!

As for tips on a B6 depth study, that's tougher because no one has ever gotten a B6 for a Physics Depth study yet (no one's done one ) I think it's about demonstrating that you've gone beyond the syllabus, primarily! If you do the bare minimum, you've defeated the purpose of a depth study. Show that you've done your research. Additionally, make sure any experimental reports are done well with proper discussion of error, accuracy, reliability, all that usual stuff

[david.wang28]

Hey David! How did you end up going with this? If you are struggling with an entire worksheet of questions on a topic it is probably worth going back to the notes/textbook on the topic, watching some YouTube videos, etc. It will be better for you than just having the answers served to you by someone else!! I'd also wager that wanting 10 questions worth of answers is probably why you didn't get much of a response to this post - You'll get help much faster if you work with us a bit and show us what understanding you do have and where your confusions lie

I ended up doing fine. Thanks for asking

[david.wang28]

Hello,
I am stuck on two question, Q 2.2 b) and Q 2.5 a) in the attachments below. Can anyone please help me out? Thanks

[DrDusk]

Hello,
I am stuck on two question, Q 2.2 b) and Q 2.5 a) in the attachments below. Can anyone please help me out? Thanks

First one:
Consider any hand on the clock, the minute or second hand. Now consider a point on the end of the minute hand. This point traces a circular path where the distance of this path is l = 2*pi*r where r is the length of the minute hand(Note this is using the arc length formula l = r* theta).

Now if you derive an expression for the period of rotation T, it will be T = (2*pi*r)/v which is just Time = distance/speed. Now if you make v the subject you get v = 2*pi*r/T - Eqn 1
Now remember the bolt in the clock always supplies the same centripetal force on the minute hand where F = m*v^2/r, but according to out equation 1 above, the time is dilating which means the period T will be different, but if our centripetal force is always constant how do we account for this difference?? Well simple length contraction!!!, the length of the minute hand r will decrease.
 
EDIT:
Note: The mass M will also increase so to keep F constant its a tug of war between M, v^2 and r. Since v^2 decreases much faster than m can increase, we need r to decrease which will end up balancing v^2 with m

P.S please do not write this in an exam, its way too long haha. I just wrote it so you can understand

[jamonwindeyer]

Hello,
I am stuck on two question, Q 2.2 b) and Q 2.5 a) in the attachments below. Can anyone please help me out? Thanks

I really like DrDusk's explanation above! I've never seen it before, might steal that as another way to explain it in the future

2.5(a) is a trick question - Or at least I think it is! The concept of mass dilation only really makes sense for an object moving relative to the observer. If the observer is in the same frame of reference, they'll measure the mass the same as if the object is at rest - That's the whole point (and this confusing point is why we normally talk about relativistic momentum, not mass).

Neglecting this, however, the way the question is set up I'm thinking it is just a weird wording - The answer is that the rest mass is much, much smaller than the relativistic mass

[david.wang28]

First one:
Consider any hand on the clock, the minute or second hand. Now consider a point on the end of the minute hand. This point traces a circular path where the distance of this path is l = 2*pi*r where r is the length of the minute hand(Note this is using the arc length formula l = r* theta).

Now if you derive an expression for the period of rotation T, it will be T = (2*pi*r)/v which is just Time = distance/speed. Now if you make v the subject you get v = 2*pi*r/T - Eqn 1
Now remember the bolt in the clock always supplies the same centripetal force on the minute hand where F = m*v^2/r, but according to out equation 1 above, the time is dilating which means the period T will be different, but if our centripetal force is always constant how do we account for this difference?? Well simple length contraction!!!, the length of the minute hand r will decrease.

P.S please do not write this in an exam, its way too long haha. I just wrote it so you can understand

This is quite a creative way of explaining this question; I like it! Thanks a lot!

[david.wang28]

I really like DrDusk's explanation above! I've never seen it before, might steal that as another way to explain it in the future

2.5(a) is a trick question - Or at least I think it is! The concept of mass dilation only really makes sense for an object moving relative to the observer. If the observer is in the same frame of reference, they'll measure the mass the same as if the object is at rest - That's the whole point (and this confusing point is why we normally talk about relativistic momentum, not mass).

Neglecting this, however, the way the question is set up I'm thinking it is just a weird wording - The answer is that the rest mass is much, much smaller than the relativistic mass

I mean, rest mass is smaller than the relativistic mass, according to my calculations for the next part (not shown). Thanks Jamon!

[_Himani_]

Hi everyone,
Just a quick question from the charged particles, Conductors and ELectric and Magnetic Fields Topic Test from the ATAR Notes book.
Question 3: There are two parallel plates set up half a metre apart. The voltage can be used to accelerate particles from rest, from one plate to another. a) AN electron is used in this setup with a voltage of 10,000V. Calculate the speed of the electron when it strikes the top plate.

Okay, so I calculated my electric field (20,000)  and the acceleration of the particle (3.52 x 10^15). Then I went on to use the formula v^2=u^2+2as which would be v^2=o+2(3.52 x 10^15)(0.5). The worked solution uses v^2=o+2(3.52 x 10^15) with, I'm assuming s=1. Why is this? The diagram with the question has also marked the distance between the plates as one metre and not half a metre. Is this just a typo or am I missing something in the calculation?

BTW: This book is a total lifesaver! Thanks!

[DrDusk]

Yep typo in that case, just go with the diagram in that case taking s = 1.

Just building on that you can also use Work done to calculate its velocity where W = (m*v^2)/2 = F * s.

[jamonwindeyer]

Just confirming that’s a typo! Distinctly remember changing the image and answer for some reason, but must have forgotten to change the question - Thanks for the pick up, so glad it’s proving useful!!

[sidzeman]

A 3.0kg cat jumps from a branch situated 2.0m above a point where a 1.8kg skateboard will pass moving at 4.2m/s. The cat must land on the skateboard.
    a) How long will the cat be in the air before she lands on the board?
    b) At what distance must the skateboard be when the cat steps off the branch?
    c) What is the combined horizontal speed of the skateboard and the cat after the cat lands on the board?
 
Please see below for my working out for part A and B - I was stuck on part C
Thanks  

[louisaaa01]

Can someone please help with this question from the 2006 HSC Physics paper? Thanks!

[DrDusk]

Can someone please help with this question from the 2006 HSC Physics paper? Thanks!

Well firstly you have relative motion between the disk and a magnet, since the disk is conductive it can harbor the production of a current. At the bottom near Y the disk is moving into the page and the magnetic field is to the right.

Now we know that since the disk is moving into the page, the produced current must produce a force that opposes this motion. Hence a force must be generated pointing out of the page. So using right hand palm rule, your palm points out of the page, fingers to the right, and you thumb then must point downwards as your thumb must be perpendicular to your fingers.
The direction of your thumb is the direction of your current and since the thumb points downwards from Y, we can say the current goes from Y to X. Thus B.

Oh and its not alternating because the disk only ever moves one direction relative to the magnetic field. 

[jamonwindeyer]

A 3.0kg cat jumps from a branch situated 2.0m above a point where a 1.8kg skateboard will pass moving at 4.2m/s. The cat must land on the skateboard.
    a) How long will the cat be in the air before she lands on the board?
    b) At what distance must the skateboard be when the cat steps off the branch?
    c) What is the combined horizontal speed of the skateboard and the cat after the cat lands on the board?
 
Please see below for my working out for part A and B - I was stuck on part C
Thanks  

Hey! Nice work on Parts A-B! So for Part C, we need to use the conservation of momentum in the horizontal direction. The skateboard initially possesses a horizontal momentum of \(\rho_i=1.8\times4.2=7.56\text{kgms}^{-1}\). This momentum must be the same after the mass of the cat is added (the cat doesn't contribute any extra momentum in the collision because it possesses only vertical momentum after the jump, which we assume is transferred to the ground and lost). So we are solving for:

[Dillan]

Hey guys,

Just after a basic run-down on the pros and cons of an AC and DC motor, and what would be ideal for an appliance like a vacuum cleaner. (i.e. Brushless or induction motors)

Cheers in advance