HSC Physics Question Thread

[RuiAce]

Could someone please explain the answer to these multiple choice questions? Would be much appreciated! Thanks
(Answers in order of attachments: A, D, C)

___________________________

[jamonwindeyer]

Could someone please explain the answer to these multiple choice questions? Would be much appreciated! Thanks
(Answers in order of attachments: A, D, C)

That last one is nasty! Okay, so basically, the golf ball is travelling at 0.1c, and if we didn't take special relativity into account, we would say 0.7c is the answer, since the ship is travelling at 0.6c. However, we must remember that:
 
- The length of the ship as measured from earth (also the distance travelled by the golf ball as measured from earth) is shorter due to length contraction
- The time taken for the ball to reach the other side of the ship (as measured from earth) is longer due to time dilation.

If we look at the equation for speed:



Then clearly, the speed of the golf ball as viewed from earth will be less than what is measured by the astronauts. However, it will still add to the 0.6c, just not as much as we would expect.

This gives the answer of C let me know if this makes sense!

[jakesilove]

That last one is nasty! Okay, so basically, the golf ball is travelling at 0.1c, and if we didn't take special relativity into account, we would say 0.7c is the answer, since the ship is travelling at 0.6c. However, we must remember that:
 
- The length of the ship as measured from earth (also the distance travelled by the golf ball as measured from earth) is shorter due to length contraction
- The time taken for the ball to reach the other side of the ship (as measured from earth) is longer due to time dilation.

If we look at the equation for speed:



Then clearly, the speed of the golf ball as viewed from earth will be less than what is measured by the astronauts. However, it will still add to the 0.6c, just not as much as we would expect.

This gives the answer of C let me know if this makes sense!

Just going to add a bit to this answer, because I've been asked about it a couple to times during the Lectures and private messages.
Jamon is totally right, and his method is how you get to the correct solution. However, I think what throws most students is that this LOOKS like a straight up maths question; they should be able to sub numbers into a formula somewhere, and a number should pop out. However, this isn't the case. There IS a formula for exactly this scenario, but it isn't part of the syllabus (except in Western Australia for some reason!).
So, if you ever get stuck and genuinely can't figure out how to approach a maths question, you can generally logic it out. I would have done this as Jamon did, or by elimination; we can easily eliminate the other three multiple choice answers, as they don't make any logical sense. Either way, just remember that this is a legit method of answering what seems to be a Maths question!

[Skidous]

Just a question about content required. Do I need to know Maxwell's equation for the exam, or is it alright just to know that Hertz verified Maxwell's equation?

[RuiAce]

Just a question about content required. Do I need to know Maxwell's equation for the exam, or is it alright just to know that Hertz verified Maxwell's equation?

Just that Hertz verified them.

Maxwell's equations have gone way outside of the syllabus

[Skidous]

Just that Hertz verified them.

Maxwell's equations have gone way outside of the syllabus

Ok just verified, thanks Rui

[Skidous]

Another question, when talking about Maglev trains, do we need a specific example of one currently in use?
Also is it good to have an example of developments into improvements of maglev trains?

[jakesilove]

Another question, when talking about Maglev trains, do we need a specific example of one currently in use?
Also is it good to have an example of developments into improvements of maglev trains?

I don't believe so; just understand how they 'technically' work, and that's all you need. From memory, I contrasted two models of Maglev (German and Japanese I think?) which essentially just switched where the superconductors/electromagnets were in the construction.

[Skidous]

Ah ok, but would it be ok to add them in an answer to help get marks?

[jakesilove]

Ah ok, but would it be ok to add them in an answer to help get marks?

Oh absolutely! If you already have the knowledge, definitely use it. For instance, in a standard "Discuss uses of Superconductors", an exemplar answer would discuss MagLev, issues with it, and developments in the field. If you have the knowledge, use it whereever you can! I was just trying to say that you don't need to learn it, necessarily.

[Skidous]

Ok, so not for the syllabus but for band 5-6 answers will have that knowledge?

[jakesilove]

Ok, so not for the syllabus but for band 5-6 answers will have that knowledge?

It will basically secure you full marks if you're bordering on a 5-6 out of six

[FallonXay]

Hi!
Could someone please explain how to answer these questions?

(The Answer is A for Q9; and B for Q8; and C for Q14)

Thanks.

[jakesilove]

Hi!
Could someone please explain how to answer these questions?

(The Answer is A for Q9; and B for Q8; and C for Q14)

Thanks.

Hey!

For Q9, the way I understand it, the Earth's magnetic field will be strongest right at the pole, and then become weaker (in, like, concentric circles) as the distance from the pole increases. So, the original loop contains all of the strongest magnetic field lines. However, the final loop will not. It may still contain the very strongest point (the pole), but it no longer contains field lines radiating from areas just outside the pole. Thus, the answer is A. I hope this makes sense, just imagine lines coming out of the earth, that get weaker as they move away from the pole. The first loop contains more lines!

For Q8, I think of it like this. We know that Lenz's law states that a current will be induced that resists whatever change has been made. In this case, MORE field lines have gone through the loop, into the page. Therefore, Lenz's law will 'want' LESS field lines to go through the loop into the page, by CREATING field lines pointing out the the page. By the right hand rule, this will create an anti-clockwise current, thus making the answer B.

[FallonXay]

Hey!

For Q9, the way I understand it, the Earth's magnetic field will be strongest right at the pole, and then become weaker (in, like, concentric circles) as the distance from the pole increases. So, the original loop contains all of the strongest magnetic field lines. However, the final loop will not. It may still contain the very strongest point (the pole), but it no longer contains field lines radiating from areas just outside the pole. Thus, the answer is A. I hope this makes sense, just imagine lines coming out of the earth, that get weaker as they move away from the pole. The first loop contains more lines!

For Q8, I think of it like this. We know that Lenz's law states that a current will be induced that resists whatever change has been made. In this case, MORE field lines have gone through the loop, into the page. Therefore, Lenz's law will 'want' LESS field lines to go through the loop into the page, by CREATING field lines pointing out the the page. By the right hand rule, this will create an anti-clockwise current, thus making the answer B.

Thanks, Q8 makes sense. But for Q9 wouldn't there be an EMF induced due to the changing magnetic flux on the wire to vectorially add magnetic field lines? So how come it's detecting a decrease?