HSC Physics Question Thread

[bsdfjnlkasn]

Hey there,

I got this question wrong and I really don't know how to reason the answer - let me know if you can help

[jamonwindeyer]

Hey there,

I got this question wrong and I really don't know how to reason the answer - let me know if you can help

Hey! Go to Kepler's Law of Periods:



By moving to the moon instead of the earth, \(M\) decreases \(r\) stays the same, so therefore, the satellite must have a longer orbital period to maintain the equality.

So compared to the satellite orbiting earth, the satellite orbiting the moon has a longer orbital period. Answer should be D

[shaner]

I've got some Gravitational Potential Energy questions.

1. A space shuttle of mass 7.5x10^4 kg is in orbit around the Moon at a height of 100 km. Calculate the gravitational potential energy of the space shuttle.
I've used the mgh formula however it's given me a pretty large value and I'm not sure whether it's right.

2. At the end of its life, our Sun will lose half its mass and shrink to the size of the Earth. Calculate the gravitational potential energy possessed by a body of mass 1.0kg on its surface.
How would I tackle a question like this?

[pikachu975]

hiii question about the pendulum prac (and all prac tho);
in the conclusion of the prac report, should we include how we can minimise errors and improve our method?

also im confused,
we always learnt that reliability is associated with repetition, so how come all answers to questions about reliability, associate with accuracy?  why do they all overlap :/

thank you

Nah conclusion ONLY answers the aim. The reducing error thing is in discussion

[jamonwindeyer]

I've got some Gravitational Potential Energy questions.

1. A space shuttle of mass 7.5x10^4 kg is in orbit around the Moon at a height of 100 km. Calculate the gravitational potential energy of the space shuttle.
I've used the mgh formula however it's given me a pretty large value and I'm not sure whether it's right.

You'd maybe want to use the other formula:



Or, if you are using \(E=mgh\), then make sure you use the value of \(g\) for the moon, either by calculation or provision. The two values would be different but both probably quite large

2. At the end of its life, our Sun will lose half its mass and shrink to the size of the Earth. Calculate the gravitational potential energy possessed by a body of mass 1.0kg on its surface.
How would I tackle a question like this?

Again, use the longer formula for gravitational potential energy:



We will take the suns mass and halve it to get \(M\), \(m=1kg\), and \(d\) would be the radius of the earth! You can calculate from there

[julia9102]

Hello  I just have a question on quanta,
How did Rutherford deduce that the electrons orbited the nucleus of an atom?

[bsdfjnlkasn]

Hello  I just have a question on quanta,
How did Rutherford deduce that the electrons orbited the nucleus of an atom?

Hey there!

He didn't! That's an assumption he made which then resulted in a lot of confusion, he literally just assumed that they took on a planetary-like orbit. And as we know, there were serious complications because of this assumption (i.e. leading the nucleus to collapse if you were to follow his reasoning). Happy to explain that last bit if you want me to?

[uusunny]

I agree with the explanation that the total momentum of the gas and rocket is conserved. Since the momentum of the gas is constant, momentum of the rocket at any time should be constant.

But the graph of momentum of the rocket vs time shows that momentum of the rocket increases? I am not really sure how these two ideas align. :/

[julia9102]

Hey there!

He didn't! That's an assumption he made which then resulted in a lot of confusion, he literally just assumed that they took on a planetary-like orbit. And as we know, there were serious complications because of this assumption (i.e. leading the nucleus to collapse if you were to follow his reasoning). Happy to explain that last bit if you want me to?

Yes please! Thank you

[bsdfjnlkasn]

Yes please! Thank you

Sure thing

So we know from Maxwell's theory of electromagnetic radiation, that an accelerating charge will emit EMR.
In Rutherford's model, the charges are orbiting around the nucleus and as the direction of their motion is constantly changing, they are considered accelerating (remembering that velocity is a vector quantity which accounts for speed AND direction of motion).

Since the electrons (charges) are accelerating, they would be emitting EMR. As a result, they would be losing energy (because it's being radiated out). This energy loss will slow their motion (remembering the equation for kinetic energy which is proportional to the velocity). This means they will slowly spiral into the nucleus, causing the atom to collapse which we know doesn't happen (just look at the world around us )

In summary :

Electrons orbit nucleus --> electrons change direction --> electrons said to be accelerating --> electrons emit EMR --> electrons lose energy --> electrons move at a slower speed --> altitude drops even more --> electron slowly spirals into nucleus --> atom collapses --> but wait, we're still here...

Let me know if you want anything clarified!

[bsdfjnlkasn]

Hey there!

I have some questions

I'm confused with the first one, because I thought apparent weight = mg + ma. Would this be a correct explanation?
If we're moving in the same direction as the gravitational field, there is no net force acting on the body since it's not doing work against the field to move upwards?

It seems that i've just tried to join 3 different concepts in that explanation and I don't really understand..

And for Q19, I thought it was d?

[bsdfjnlkasn]

I wasn't able to attach any more photos so sorry for spamming!
I thought the answer for Q16 was B? Especially because of the way the coils are orientated?

[jamonwindeyer]

I agree with the explanation that the total momentum of the gas and rocket is conserved. Since the momentum of the gas is constant, momentum of the rocket at any time should be constant.

But the graph of momentum of the rocket vs time shows that momentum of the rocket increases? I am not really sure how these two ideas align. :/

Hey! I'm guessing that graph doesn't assume that the thrust is constant, which would mean the momentum of the rocket does change. It's important to understand that the way we look at it in HSC Physics is a huge simplification.

So I suppose the answer is, the graph and the explanation don't match. The explanation is an idealisation, whereas the graph probably isn't, if that makes sense?

[jamonwindeyer]

Hey there!

I have some questions

I'm confused with the first one, because I thought apparent weight = mg + ma. Would this be a correct explanation?
If we're moving in the same direction as the gravitational field, there is no net force acting on the body since it's not doing work against the field to move upwards?

It seems that i've just tried to join 3 different concepts in that explanation and I don't really understand..

And for Q19, I thought it was d?

So for the first one, go back to intuition. I'm assuming the answer is B? If that's right, it is because when you move the bag downwards, in that motion you could consider it sort of 'weightless,' like in free-fall. Moving it upwards quickly, you'd feel the bag pull back down on you - The apparent weight has increased. Moving it downwards, the apparent weight has decreased... I think?

For Q19, you can't explain the levitation with eddy currents because the magnet was resting on the superconductor to begin with. Therefore, no change in flux, therefore, no induced emf, therefore, no eddy currents. So every explanation with eddy currents is immediately incorrect (and a good time to remind, the Meisner Effect cannot be fully explained using eddy currents.

That leaves C - You are looking for the words 'exclusion' or 'repulsion' of the magnetic field, because this is what characterises the Meisner Effect (and the word diamagnetic is another one)

I wasn't able to attach any more photos so sorry for spamming!
I thought the answer for Q16 was B? Especially because of the way the coils are orientated?

The current carrying coils act as electromagnet that induce a field going up and down the page (consider each end of the coil as a north/south pole). Use your right hand grip rule and you'll see the induced force on the moving electron due to the magnetic field is actually horizontal

[uusunny]

Hey! I'm guessing that graph doesn't assume that the thrust is constant, which would mean the momentum of the rocket does change. It's important to understand that the way we look at it in HSC Physics is a huge simplification.

So I suppose the answer is, the graph and the explanation don't match. The explanation is an idealisation, whereas the graph probably isn't, if that makes sense?

Thank you for your reply! So in realistic situations, momentum isn't entirely conserved?

How would you answer this multiple choice??

The rocket travelling out of earth's atmosphere turns its engines off over a period of 5 seconds, and then immediately ejects the first stage fuel tanks. How does the momentum of the rocket change during this procedure?

a) the momentum of the rocket remains constant

b) the momentum of the rocket remains constant then drops

c) the momentum of the rocket decreases

d) the momentum of the rocket increases and drops