HSC Physics Question Thread

[kiwiberry]

Hey, could I get some help with the following:

If a 10kg mass were to be released from 1000km above the surface of Earth, initially being stationary, determine its velocity just before it hits the surface of the Earth.

I'm not sure, but I think you can use \(a=\frac{GM}{r^{2}}\) to find acceleration, and then sub into \(v^{2}=u^{2}+2as\) to find v!

[jamonwindeyer]

I'm not sure, but I think you can use \(a=\frac{GM}{r^{2}}\) to find acceleration, and then sub into \(v^{2}=u^{2}+2as\) to find v!

I think that's the general idea! But, the acceleration at 1000km above the surface is different to the acceleration at the surface. Indeed, it changes slightly the whole way down. To do it super accurately, you'd actually need a more complete formula that took that into account those changes (I think there'd be Calculus involved)

In this case, we can use the reasonable estimate that the acceleration is just a uniform \(g=9.8ms^{-2}\) the whole way through the drop. At an altitude of 1000km, that's a reasonable estimate. Then you just pop that value for acceleration into \(v^2=u^2+2as\)

As a side note, we didn't get earths radius, so that's a clue we can't use \(a=\frac{GM}{r^2}\), even though it is really tempting!

[beau77bro]

Why does special relativity apply to inertial frames of reference? What does acceleration do to affect the theory? Speed of light is constant I assume still holds? But does it ahhahaha? Please feel free to drop bombs on me but I don't think I could hack like a Uni level paper explaining hahaha basics I'm dumb lmao

[jakesilove]

Why does special relativity apply to inertial frames of reference? What does acceleration do to affect the theory? Speed of light is constant I assume still holds? But does it ahhahaha? Please feel free to drop bombs on me but I don't think I could hack like a Uni level paper explaining hahaha basics I'm dumb lmao

Hey Beau! Don't stress too much about accelerating frames etc; it's beyond the curriculum, and beyond Special relativity. Essentially, the whole idea of special relativity stems from two propositions; that the speed of light is constant, and that the laws of physics hold true in all inertial frames of reference. From there, using trains as analogies etc etc, you can show that time must dilate etc when inertial frames move in relation to each other. So, why must the frames be inertial (ie. non-accelerating)? Well, that's basically part of the definition of the principle of relativity!

Bit of a circular answer, but hope that makes sense.

[RuiAce]

I suppose you could say that special relativity was built on inertial frames of reference. Of course, something must apply to what it was built on.

But yeah, not much to add onto Jake's answer. "Acceleration warps space-time" but you don't know what space-time even is in the HSC course.

[beau77bro]

I suppose you could say that special relativity was built on inertial frames of reference. Of course, something must apply to what it was built on.

But yeah, not much to add onto Jake's answer. "Acceleration warps space-time" but you don't know what space-time even is in the HSC course.

"Warps space time" hahaha ok that's all I needa know, thankyou both of you. Cheers Jake and Rui. Absolute gods

[beau77bro]

Stroboscopic experiments--> we didn't really do them and I keep seeing questions on it and I have no idea how to do them? Are we supposed to be able to do them? I feel my class doesn't do the required pracs, opinions? Thankyouuuu

[jakesilove]

Stroboscopic experiments--> we didn't really do them and I keep seeing questions on it and I have no idea how to do them? Are we supposed to be able to do them? I feel my class doesn't do the required pracs, opinions? Thankyouuuu

Stroboscopic measurement is definitely not in the syllabus. Where are you finding the questions you're talking about? Don't worry about it at all, it isn't relevant!

[beau77bro]

Stroboscopic measurement is definitely not in the syllabus. Where are you finding the questions you're talking about? Don't worry about it at all, it isn't relevant!

really? omg praise the lord. its in surfing, dot point and like this prac book. also a physics topic test i ordered hasnt arrived?

[jakesilove]

really? omg praise the lord. its in surfing, dot point and like this prac book. also a physics topic test i ordered hasnt arrived?

Hey Beau! Very weird; would you like to private message me your name and when you ordered the topic tests, and I'll look into that for you?

[Iminschool]

For this question, the answer is B however i wanted to know how the current was reversed for it not to be A, is it because of the initial direction of current?

[jamonwindeyer]

For this question, the answer is B however i wanted to know how the current was reversed for it not to be A, is it because of the initial direction of current?

Hey! Sorry for the late reply - I really don't even think that question is 'answerable,' because it doesn't define the direction of current properly. Like, the flow of current can only take a positive/negative value if you DEFINE it that way, by saying, "Okay, if it flows this way, it is positive." Usually we set this definition with voltages or whatever, but we don't get that here. So, I don't think it is appropriate in that regard

The only thing I can think of is, using the mathematical version of Faraday's Law:



That negative symbol represents Lenz's Law - Perhaps they think it most appropriate to put the current negative to start with, since the induced current is opposing the motion of the generator? That seems a bit of a stretch though, in my opinion

[RuiAce]

Hey! Sorry for the late reply - I really don't even think that question is 'answerable,' because it doesn't define the direction of current properly. Like, the flow of current can only take a positive/negative value if you DEFINE it that way, by saying, "Okay, if it flows this way, it is positive." Usually we set this definition with voltages or whatever, but we don't get that here. So, I don't think it is appropriate in that regard

The only thing I can think of is, using the mathematical version of Faraday's Law:



That negative symbol represents Lenz's Law - Perhaps they think it most appropriate to put the current negative to start with, since the induced current is opposing the motion of the generator? That seems a bit of a stretch though, in my opinion

Yep that was what I thought - Lenz's law. But I wasn't certain so I didn't want to post.

[jamonwindeyer]

Yep that was what I thought - Lenz's law. But I wasn't certain so I didn't want to post.

Yeah neither am I... Don't stress about it too much Iminschool, HSC would never be that vague

[Iminschool]

Yeah neither am I... Don't stress about it too much Iminschool, HSC would never be that vague

Haha, awesome