HSC Physics Question Thread

[jamonwindeyer]

At one time it was proposed to launch satellites directly into a circular orbit at a constant distance from the Earth using a giant WW2 cannon. Assuming that the cannon would give the satellite a largely sufficient velocity, explain and use calculation to show why this proposal would not work ?
I just need a guide on what i need to do i was thinking something to do with newtons thought expirements but not sure about the calculation part.

Hey there Ahsun, welcome to the forums! 

Hmm, this question could be heading a few ways, but I think what it is looking at is the accelerations involved! Basically, your response to this question will revolve around the fact that the acceleration involved will create G-forces that cannot be tolerated by a satellite (or, well, anything). Let's do some calculations. The orbital velocity of the satellite:



Let's say we want the satellite in a low earth orbit, so put the radius equal to 300 000 metres. Thus, the velocity could approximated to be:



Now, a WW2 cannon would accelerate a satellite from rest, to this velocity, instantly. Let's be generous and say it takes 0.1 seconds, and let's say we have a 1000kg satellite. So we can say:




So, this is close to the gravitational force between the earth and the moon, experienced by something the size of a car. No chance in hell, torn to pieces.

The calculation might be a tad confusing (if so, let me know), and it's just guesswork for the most part  Importantly though, we recognise that using a cannon creates too much acceleration, and thus too much G-force for any satellite to withstand. Current launch techniques are designed to spread the acceleration over as much time as possible.

Just as an addition, the sort of forces above would turn a human into a smoothie.

This would be my interpretation!! Feel free to play around with the values in the calculation, I hope this helps! 

[jamonwindeyer]

hey can someone link me the hsc physics study design (or equivalent thing?)

Physics here seems so interesting in comparison to vce physics.
if i didnt have an amazing teacher i dont think id be enjoying physics as much as i do atm but based off the questions in the hsc physics you guys seem like you're having a blast. some really fascinating ideas

Hey!! The Prelim/HSC syllabus is here. Scroll down a bit through all the fluff  it's a super interesting course!! It definitely doesn't prepare you for tertiary physics very well, but in terms of generating interest and getting a broad understanding of how our world works, it's fantastic 

[Ahsun]

Hey there Ahsun, welcome to the forums! 

Hmm, this question could be heading a few ways, but I think what it is looking at is the accelerations involved! Basically, your response to this question will revolve around the fact that the acceleration involved will create G-forces that cannot be tolerated by a satellite (or, well, anything). Let's do some calculations. The orbital velocity of the satellite:



Let's say we want the satellite in a low earth orbit, so put the radius equal to 300 000 metres. Thus, the velocity could approximated to be:



Now, a WW2 cannon would accelerate a satellite from rest, to this velocity, instantly. Let's be generous and say it takes 0.1 seconds, and let's say we have a 1000kg satellite. So we can say:




So, this is close to the gravitational force between the earth and the moon, experienced by something the size of a car. No chance in hell, torn to pieces.

The calculation might be a tad confusing (if so, let me know), and it's just guesswork for the most part  Importantly though, we recognise that using a cannon creates too much acceleration, and thus too much G-force for any satellite to withstand. Current launch techniques are designed to spread the acceleration over as much time as possible.

Just as an addition, the sort of forces above would turn a human into a smoothie.

This would be my interpretation!! Feel free to play around with the values in the calculation, I hope this helps! 

Thanks so much this really helped me 

[jamonwindeyer]

Thanks so much this really helped me 

Anytime! I hope it all made sense, feel free to let me know if you needed any help finding anything around the forums 

[soldmychildforyeezys]

Hi

For these questions http://m.imgur.com/a/5WrVp

For Q55) I just want to check if my answer is correct, since the book states that Lenz's law determined the direction of the induced current which is given by the induced magnetic field. Would my answe be fine.

For Q56&57) I seem to have the opposites of the book's easier for both. How would the the induced magnetic field be to the right in 56, and anticlockwise for 57?

Cheers

[Ahsun]

Mathematically analyse, using diagrams, a rocket burning in terms of momentum conservation and the rate of fuel consumption R and derive the acceleration for a rocket during its launch stage.
Not sure exactly what I need to do any help would be appreciated
Thanks

[jamonwindeyer]

Hi

For these questions http://m.imgur.com/a/5WrVp

For Q55) I just want to check if my answer is correct, since the book states that Lenz's law determined the direction of the induced current which is given by the induced magnetic field. Would my answe be fine.

For Q56&57) I seem to have the opposites of the book's easier for both. How would the the induced magnetic field be to the right in 56, and anticlockwise for 57?

Cheers

Hey!!

For Q55, It's spot on in principle, though I think you might want to explain what Lenz' Law is a little bit better. I'd be doing something like:

Lenz's Law is the principle that any change must oppose the change that created it (due to the conservation of energy). In this experiment...

Just to make sure I got the 2 marks on offer 

For both 56 and 57, I agree with the book. Let me quickly explain them both using the Right Hand Grip Rule, have you heard of it?

If not, basically, hold your right hand in a thumbs up position. Looking at your hand, if your thumb points in the direction of the north pole of the induced field, then your fingers wrap in the direction of induced current.

Let's start with 57 because it's a tad easier. The south pole is moving away from the loop, so, the loop will want to set up a North pole facing towards the magnet to bring it back in (Lenz's Law). So, your thumb is pointing to the left of the page in this case. Thus, your current is wrapping anti-clockwise as viewed by the magnet (sort of hard to explain, but the opposite direction to what you drew).

Step 56 is similar. South Pole is coming in, so the loop will set up a South pole to push it back away. Magnetic field lines go towards the South pole. Thus, the magnetic field lines will be from left to right (assuming we only want the field lines of the induced field, if we considered the field of the magnet too we'd have some interactions to draw, which is complicated) 

I hope this helps!! If it doesn't seriously let me know and I'll break it down a little bit further 

[soldmychildforyeezys]

Thanks man!

Would my method by wrong entirely? Since I considered the direction of the bar magnet's magnetic field lines when looping back in to its South Pole, hence just said the induced magnetic field would oppose this..

Hm, I might've confused myself since when I learnt how to solve these questions initially, I might've just considered the direction of the magnetic field lines coming straight out of the North Pole. I think this is pretty much the way you did it.

Thanks again!

[jamonwindeyer]

Mathematically analyse, using diagrams, a rocket burning in terms of momentum conservation and the rate of fuel consumption R and derive the acceleration for a rocket during its launch stage.
Not sure exactly what I need to do any help would be appreciated
Thanks

Hey Ahsun! Ouch, that's a nasty question. I really like these though, they force you to be really analytical. Okay, well, heres a few ideas I have and I'll leave you to tie them together in a way that makes sense to you! 

For the diagram, I'd draw a simple picture of a rocket moving in one direction and the fuel exhaust moving in the other direction. Label it with any of the information I mention here that you think suits.

The big principle here is the Conservation of Momentum. The rocket and its fuel start from rest (that is, with zero momentum, since momentum is a product of velocity and mass. Therefore, the total momentum of the rocket and its fuel must remain zero, by the conservation of momentum. Of course, the rocket does move, so the solution is this: The momentum of the rocket must be equal and opposite to the momentum of the fuel being ejected in the other direction.



We can get a few expressions here in terms of R. The mass of the rocket will decrease by R every second, since it is ejecting R kilograms of fuel per second. So, if we let M be the total mass at the start, and t be time:



By the same logic, the mass of the fuel:



The momentum formula would now be:



Note that Rt < M since Rt was included in M.

So theres a few ideas. Let's derive that acceleration. The force upwards can be given by:



To be honest, I was a little unsure on linking the thrust force to the rate of fuel consumption, which is the last bit of the puzzle. But remember a formula from Prelim Physics linking momentum to the product of force and time, and the answer comes out:



Since R is just the mass of fuel ejected per unit time, or, m/t.



This is subject to a little inaccuracy, I've never done this before! 

I hope this gives you some ideas. I've done some pretty wacky math, so go away, try and follow what I've done, and if after some thought it doesn't quite make sense please let me know and I'll slow down a little! I kind of got away from myself ahaha 

[jamonwindeyer]

Thanks man!

Would my method by wrong entirely? Since I considered the direction of the bar magnet's magnetic field lines when looping back in to its South Pole, hence just said the induced magnetic field would oppose this..

Hm, I might've confused myself since when I learnt how to solve these questions initially, I might've just considered the direction of the magnetic field lines coming straight out of the North Pole. I think this is pretty much the way you did it.

Thanks again!

I think it is okay in principle, but for your initial answer, consider the magnetic field lines of the magnet and the loop right next to the South pole. If the induced field is going to the left, and the magnetic field at the South pole goes into the magnet, it is also going to the left!! They match, so you are actually assisting it! 

[soldmychildforyeezys]

I think it is okay in principle, but for your initial answer, consider the magnetic field lines of the magnet and the loop right next to the South pole. If the induced field is going to the left, and the magnetic field at the South pole goes into the magnet, it is also going to the left!! They match, so you are actually assisting it! 

Thanks, I actually meant the direction of the magnetic field lines when they are parallel to the bar magnet. So when it's in the middle of the process of coming back in to the South Pole.

I'll just shut up now and use your method since its obviously much clearer 

[Swagadaktal]

I think it is okay in principle, but for your initial answer, consider the magnetic field lines of the magnet and the loop right next to the South pole. If the induced field is going to the left, and the magnetic field at the South pole goes into the magnet, it is also going to the left!! They match, so you are actually assisting it! 

Hey - do you reckon you could explain Lenz law a bit more for me?

As the south pole is approaching the loop - the magnetic field goes from N to S so the magnetic field is going right (-->) . Can you tell me the flaw in my logic. Soz i havent really read this part of the book yet

OH WAIT right next to the south pole? So at the S part the lines are going into the S so it's going from right to left?   
ohhhh you're a legend

[jamonwindeyer]

Thanks, I actually meant the direction of the magnetic field lines when they are parallel to the bar magnet. So when it's in the middle of the process of coming back in to the South Pole.

I'll just shut up now and use your method since its obviously much clearer 

Ohhhhh gotcha! Hm, I'm not sure why that doesn't work TBH, probably because the south pole is closest to the loop and so the effects there are the most pronounced? Not sure 

Definitely don't shut up ahaha, my methods are just what works for me, by all means not perfect  I do like the right hand grip rule though, it just made sense for me 

[jamonwindeyer]

...

OH WAIT right next to the south pole? So at the S part the lines are going into the S so it's going from right to left?   
ohhhh you're a legend

Yep that's right! 

[zsteve]

Hey - do you reckon you could explain Lenz law a bit more for me?

As the south pole is approaching the loop - the magnetic field goes from N to S so the magnetic field is going right (-->) . Can you tell me the flaw in my logic. Soz i havent really read this part of the book yet

OH WAIT right next to the south pole? So at the S part the lines are going into the S so it's going from right to left?   
ohhhh you're a legend

Yo Swagdaktal you might find this interesting. Assume the opposite of Lenz's law holds, then imagine what would happen if there was a flux change - you would get infinite current ~ infinite energy.
So clearly, you have proved that the opposite of Lenz's law is impossible. Hence Lenz's law must hold.