HSC Physics Question Thread

[jakesilove]

Hey MysteryMarker,
For Q14, the train is turning right, (option C as specified). Try to visualise the situation (this might be hard for some people!)

In the reference frame of the ground, as the train begins to turn right, the ball travels in a straight line. Newton III predicts that the ball will tend to retain its straight path in the inertial frame of the ground. This is achieved more or less, assuming friction between ball and table isn't massive.

In the non-inertial frame of the turning train carriage, the ball appears to bend as shown.

I'm having severe difficulties with explaining this one . Best if you can visualise it by putting yourself 'in' the carriage and doing it intuitively.

Q19.
This is rather weird, because from theory we know that their accelerations are the same. However, this comes to mind...
\( a = \dfrac{F_g}{m}\), so we see that acceleration is inversely proportional to mass. (considering only gravity here)

But \(F_g \propto m\) as well, so if we combine this with a, we find that m cancels and acceleration is independent of mass.

Not a well-phrased question in my opinion.

Thanks for the great response Steve! I totally agree with all of the above. I'll just add that Q19 is a tricky bugger, and I seem to remember getting it wrong myself. Basically, if you go through each answer, you'll find that all but D are wholly incorrect. D is also true so, even if you don't FULLY understand the answer (as the question is badly worded, I agree), you can get the answer correct

[MysteryMarker]

Hey MysteryMarker,
For Q14, the train is turning right, (option C as specified). Try to visualise the situation (this might be hard for some people!)

In the reference frame of the ground, as the train begins to turn right, the ball travels in a straight line. Newton III predicts that the ball will tend to retain its straight path in the inertial frame of the ground. This is achieved more or less, assuming friction between ball and table isn't massive.

In the non-inertial frame of the turning train carriage, the ball appears to bend as shown.

I'm having severe difficulties with explaining this one . Best if you can visualise it by putting yourself 'in' the carriage and doing it intuitively.

Q19.
This is rather weird, because from theory we know that their accelerations are the same. However, this comes to mind...
\( a = \dfrac{F_g}{m}\), so we see that acceleration is inversely proportional to mass. (considering only gravity here)

But \(F_g \propto m\) as well, so if we combine this with a, we find that m cancels and acceleration is independent of mass.

Not a well-phrased question in my opinion.

Wouldn't the train travelling right or left have the same effect on the motion of the ball as viewed from the sideview?

Sorry, just confused with Q14.

[Spencerr]

For q 14 I'm going to use a different scenario to take a look at forces.

Imaging that theres a ball hanging on a string attached to to the top of the train and the train accelerates straight forward. Hopefully you can easily see in your mind's eye that the ball will hang at an angle away from the direction of motion. So if the train is moving to the left, the string will look like this: \ with the ball attached on the end. This is because there is a phantom force acting on the ball along with the force of gravity. The phantom force is produced by the acceleration of the train and acts in the opposite direction.

You can apply this scenario to the billiard ball question. The force of gravity can be ignored. you see that the ball has moved to the left, therefore it has experienced a force to the left (doesnt matter if its centripetal or not). Since there is a force to the left acting on the ball, and using the first scenario, we can deduce that there must be a force acting on the train towards the right that produces an acceleration i.e. its turning towards the right.

or you could just visualise it.

[MysteryMarker]

For q 14 I'm going to use a different scenario to take a look at forces.

Imaging that theres a ball hanging on a string attached to to the top of the train and the train accelerates straight forward. Hopefully you can easily see in your mind's eye that the ball will hang at an angle away from the direction of motion. So if the train is moving to the left, the string will look like this: \ with the ball attached on the end. This is because there is a phantom force acting on the ball along with the force of gravity. The phantom force is produced by the acceleration of the train and acts in the opposite direction.

You can apply this scenario to the billiard ball question. The force of gravity can be ignored. you see that the ball has moved to the left, therefore it has experienced a force to the left (doesnt matter if its centripetal or not). Since there is a force to the left acting on the ball, and using the first scenario, we can deduce that there must be a force acting on the train towards the right that produces an acceleration i.e. its turning towards the right.

or you could just visualise it.

When you say that the ball has moved to the left, how is this so? isn't the ball going to the back of the carriage, and so from our perspective going to the 'right'?

Sorry for all these posts, i still don't really understand how we can deduce the train to be turning left/right. I understand the pendulum analogy you used but other than that i can't really see how the train is moving to the right.

[conic curve]

Hey

So I got a group physics presentation coming up and I need someone to give me feedback on the assessment (once the assignment is completed)

Where can I post my physics assignment for feedback?

Thanks 

[RuiAce]

Hey

So I got a group physics presentation coming up and I need someone to give me feedback on the assessment (once the assignment is completed)

Where can I post my physics assignment for feedback?

Thanks 

Watch out for what you post. We can give you tiny feedback but we cannot do your assignment for you or we are breaking the rules of all my own work.

Feel free to post it here.

[jamonwindeyer]

When you say that the ball has moved to the left, how is this so? isn't the ball going to the back of the carriage, and so from our perspective going to the 'right'?

Sorry for all these posts, i still don't really understand how we can deduce the train to be turning left/right. I understand the pendulum analogy you used but other than that i can't really see how the train is moving to the right.

Let me have a go at explaining, this is definitely tough 

Consider the train moving forward and us looking down on it, as per the question.

Now an important concept to understand here is inertia, the tendency for objects to remain in their set path. It's the reason why the tissue box at the back of your car flies forward when you brake suddenly: Because the tissue box wants to keep moving forward, and there is nothing forcing it to stop with the car.

In this example, the train makes a movement halfway through the balls path (when it starts to curve). Now, moving forward or backward would never cause the ball to curve, like braking a car on a straight road would never cause the tissue box to hit the side window. Doesn't make sense. But why is the train turning right not left?

Consider the ball as a separate object, because it is. When the train turns, the ball is going to want to continue to go straight on the initial path. Consider the train turning left, that is, moving down the page if we are looking at the diagram, and the ball staying where it is. Except this isn't the right way to think of it. We want to stick with the train. For this to make sense, as we move down the page with the train, the ball stays where it is, and so moves upwards on the diagram. Not quite what we need.

Consider the train turning right. The train would move up the page, and so as we go up the page with the train, the ball must stay where it is and so move down the page in our reference frame. This corresponds to the turn in the diagram.

I tried to take out all the jargon, does this help at all? I feel like an animation would be the best way to explain this, hopefully this helps a little

[conic curve]

Watch out for what you post. We can give you tiny feedback but we cannot do your assignment for you or we are breaking the rules of all my own work.

Feel free to post it here.

Yes I know, don't worry. If I make you guys do it for me, then obviously I'm not going to learn anything from it. It's like making you guys write an essay/creative for me, if you write it for me, then I will not benefit from anything hence I'll struggle to write an essay/creative on the day of the exam (I hope I did not confuse anyone here)

Anyways, I was asking for feedback. How much feedback is allowed when it comes to physics assignments here? (i.e. is it extensive like the free essay marking)

[jamonwindeyer]

Yes I know, don't worry. If I make you guys do it for me, then obviously I'm not going to learn anything from it. It's like making you guys write an essay/creative for me, if you write it for me, then I will not benefit from anything hence I'll struggle to write an essay/creative on the day of the exam (I hope I did not confuse anyone here)

Anyways, I was asking for feedback. How much feedback is allowed when it comes to physics assignments here? (i.e. is it extensive like the free essay marking)

It will depend on what sort of task it is, but it will probably just be some general recommendations made at the end, it would be hard to give specific feedback throughout a presentation.

You should also post it in a new thread in the Physics section, keep this section for Q+A and physics chat 

[conic curve]

It will depend on what sort of task it is, but it will probably just be some general recommendations made at the end, it would be hard to give specific feedback throughout a presentation.

You should also post it in a new thread in the Physics section, keep this section for Q+A and physics chat 

It's a presentation and speech (in pairs). I will show you the marking criteria and will also be telling you of our teacher's feedback. I think the task is on researching the GPS/internet or doing research on the application of waves, I don't know what it exactly it to be honest

Our teacher was talking about it today and we haven't gotten a copy of the assessment notification yet

[MysteryMarker]

Let me have a go at explaining, this is definitely tough 

Consider the train moving forward and us looking down on it, as per the question.

Now an important concept to understand here is inertia, the tendency for objects to remain in their set path. It's the reason why the tissue box at the back of your car flies forward when you brake suddenly: Because the tissue box wants to keep moving forward, and there is nothing forcing it to stop with the car.

In this example, the train makes a movement halfway through the balls path (when it starts to curve). Now, moving forward or backward would never cause the ball to curve, like braking a car on a straight road would never cause the tissue box to hit the side window. Doesn't make sense. But why is the train turning right not left?

Consider the ball as a separate object, because it is. When the train turns, the ball is going to want to continue to go straight on the initial path. Consider the train turning left, that is, moving down the page if we are looking at the diagram, and the ball staying where it is. Except this isn't the right way to think of it. We want to stick with the train. For this to make sense, as we move down the page with the train, the ball stays where it is, and so moves upwards on the diagram. Not quite what we need.

Consider the train turning right. The train would move up the page, and so as we go up the page with the train, the ball must stay where it is and so move down the page in our reference frame. This corresponds to the turn in the diagram.

I tried to take out all the jargon, does this help at all? I feel like an animation would be the best way to explain this, hopefully this helps a little

Holy crap, i just realised i've been looking at the diagram the bloody wrong way. This whole time i thought we were looking at a side view and not an aerial view! Damn, thanks Jamon that explanation was freakin mint. Totally gonna ace this concept if it comes in any of my future exams. 

[jamonwindeyer]

Holy crap, i just realised i've been looking at the diagram the bloody wrong way. This whole time i thought we were looking at a side view and not an aerial view! Damn, thanks Jamon that explanation was freakin mint. Totally gonna ace this concept if it comes in any of my future exams. 

Nice!! Aha love that click moment  awesome work! 

[wyzard]

It's a presentation and speech (in pairs). I will show you the marking criteria and will also be telling you of our teacher's feedback. I think the task is on researching the GPS/internet or doing research on the application of waves, I don't know what it exactly it to be honest

Our teacher was talking about it today and we haven't gotten a copy of the assessment notification yet

Ah the GPS You might wanna look into how relativistic correction to time measurement is needed due to the gravitational time dilation. This serves a very important application to Einstein's relativity. I think that will be a really cool presentation topic.

[MysteryMarker]

Quanta to Quarks Questions: (That's alotta Q's)

'Describe Planck's contribution to the development of the atomic model and explain how his work was critical in allowing the development of the Bohr model' (4 marks)

The marking criteria states that TWO of Bohr's postulates must be linked with Planck's concept of quantisation. I understand how to link Bohr's 2nd postulate to planck, but how would i link any of his others?

[jamonwindeyer]

Quanta to Quarks Questions: (That's alotta Q's)

'Describe Planck's contribution to the development of the atomic model and explain how his work was critical in allowing the development of the Bohr model' (4 marks)

The marking criteria states that TWO of Bohr's postulates must be linked with Planck's concept of quantisation. I understand how to link Bohr's 2nd postulate to planck, but how would i link any of his others?

I'm hoping a helpful student reading this will lend a hand with this question, neither Jake, Rui or myself studied Quanta to Quarks, help out a fellow peer! It will help you too!