# Thread: HSC Physics Marathon 2017

1. ## HSC Physics Marathon 2017

Welcome to the HSC physics marathon This thread can be used to ask for help or test others!
Just some simple rules:
- Answer the question asked previously, before posting a new question.
- After you have answered a question, post another question to keep the thread alive.

I will start off:
A cannon is fired at a velocity of 400.0ms^-1, 30.0° above horizontal. Determine the vertical and horizontal components of this initial velocity.  Reply With Quote

2. ## Re: HSC Physics Marathon 2017

I'm not gonna do the question, just asking how is projectile motion different in physics to 3U? I've heard it's different, but don't know how.

Edit: might as well add something to the question. Assuming I am at the same height as the cannon, how far away from me must you place the cannon if you want to hit me?  Reply With Quote

3. ## Re: HSC Physics Marathon 2017 Originally Posted by calamebe Edit: might as well add something to the question. Assuming I am at the same height as the cannon, how far away from me must you place the cannon if you want to hit me?  Reply With Quote

4. ## Re: HSC Physics Marathon 2017 Originally Posted by calamebe I'm not gonna do the question, just asking how is projectile motion different in physics to 3U? I've heard it's different, but don't know how.

Edit: might as well add something to the question. Assuming I am at the same height as the cannon, how far away from me must you place the cannon if you want to hit me?
There are several differences between HSC Physics projectile motion and HSC 3U. In HSC Physics, you are allowed to quote formulas like equations of motion and other relevant equations (at least ones on the Formula Sheet), whereas for 3U you need to derive them generally. For 3U, projectile motion Q's are generally going to require calculus and thus can have a much broader range of types of things that can be asked, whereas in HSC Physics there's no calculus and the Q's are mainly (or at least often) substituting values into formulas.  Reply With Quote

5. ## Re: HSC Physics Marathon 2017 Originally Posted by jathu123 Yep that's the correct answer!  Reply With Quote

6. ## Re: HSC Physics Marathon 2017 Originally Posted by InteGrand There are several differences between HSC Physics projectile motion and HSC 3U. In HSC Physics, you are allowed to quote formulas like equations of motion and other relevant equations (at least ones on the Formula Sheet), whereas for 3U you need to derive them generally. For 3U, projectile motion Q's are generally going to require calculus and thus can have a much broader range of types of things that can be asked, whereas in HSC Physics there's no calculus and the Q's are mainly (or at least often) substituting values into formulas.
Oh ok, that's a little disappointing. Thanks for the reply though!  Reply With Quote

7. ## Re: HSC Physics Marathon 2017

Calculate the GPE of a 2000 kg satellite which orbits the Earth at an altitude of 35 000 km. The radius of Earth is 6378km.  Reply With Quote

8. ## Re: HSC Physics Marathon 2017 Originally Posted by eyeseeyou Calculate the GPE of a 2000 kg satellite which orbits the Earth at an altitude of 35 000 km. The radius of Earth is 6378km.
GPE=-(Gm1m2)/r
=-(6.67x10^-11*2000*6x10^24)/(6378x10^3+35000x10^3)
=-1.93^10 Joules  Reply With Quote

9. ## Re: HSC Physics Marathon 2017

Calculate Work done when an object with mass 5x10^2kg is moved from surface of the Earth to the altitude 300km. The radius of the Earth is 6370km.  Reply With Quote

10. ## Re: HSC Physics Marathon 2017

7.5*10^-22

is that right?  Reply With Quote

11. ## Re: HSC Physics Marathon 2017 Originally Posted by JackPatel 7.5*10^-22

is that right?
Nope, show me your working out.  Reply With Quote

12. ## Re: HSC Physics Marathon 2017

I am not sure that this is right but let me try. Please don't laugh if I did this completely wrong

I think the formula for gravitational potential energy is
W= mgy

and since you would need the same amount of energy to potential energy to move the object upwards(assuming constant velocity)

then it would be

5.0*10^2*9.8*300*10^3 = 1.47*10^9 J  Reply With Quote

13. ## Re: HSC Physics Marathon 2017 Originally Posted by jiujiu1123 I am not sure that this is right but let me try. Please don't laugh if I did this completely wrong

I think the formula for gravitational potential energy is
W= mgy

and since you would need the same amount of energy to potential energy to move the object upwards(assuming constant velocity)

then it would be

5.0*10^2*9.8*300*10^3 = 1.47*10^9 J
By using E(p)=mgh we assume that g is constant at any altitude which is incorrect and this formula only works for low altitudes as we make the surface the point where E(p)=0. So a more accurate way of measuring E(P) is by -(Gm1m2)/r as it takes in consideration that g is now always constant do to many factors such as variation in the thickness of the Earth's crust and lithosphere, earth not being a perfect sphere so having flattened poles thus the value of 'g' is greater near the poles, and the spin of Earth creates a centrifugal effect which affects the 'g' near the equator. However by making E(p) a point a very large distance away thus getting the formula E(p)= -(Gm1m2)/r it assumes that GPE is the only thing changing, but rockets can loose fuel or gain/loose velocity which increases E(k). However you would still get 1 out of 2 marks for it but the correct way to do it is using E(p)= -(Gm1m2)/r.  Reply With Quote

14. ## Re: HSC Physics Marathon 2017

Work done = Gm1m2 (1/r,initial - 1/r,final)
= (6.67x10^-11)(5.972x10^24)(5x10^2)(1/(6370x1000) - 1/(6370x1000+300x1000))
= 1.4 x 10^9 Joules

To get the work done formula:
WD = Ep,final - Ep, initial
= -Gm1m2/r,final - (-Gm1m2/r,initial)
= Gm1m2/r,initial - Gm1m1/r,final
= Gm1m2 (1/r,initial - 1/r,final)  Reply With Quote

15. ## Re: HSC Physics Marathon 2017

BTW what options are you all doing?

Quanta, Astro or med?  Reply With Quote

16. ## Re: HSC Physics Marathon 2017 Originally Posted by Rathin By using E(p)=mgh we assume that g is constant at any altitude which is incorrect and this formula only works for low altitudes as we make the surface the point where E(p)=0. So a more accurate way of measuring E(P) is by -(Gm1m2)/r as it takes in consideration that g is now always constant do to many factors such as variation in the thickness of the Earth's crust and lithosphere, earth not being a perfect sphere so having flattened poles thus the value of 'g' is greater near the poles, and the spin of Earth creates a centrifugal effect which affects the 'g' near the equator. However by making E(p) being a point a very large distance away thus getting the formula E(p)= -(Gm1m2)/r it assumes that GPE is the only thing changing, but rockets can loose fuel or gain/loose velocity which increases EPE. However you would still get 1 out of 2 marks for it but the correct way to do it is using E(p)= -(Gm1m2)/r.
You are such a nerd! I'm really looking forward to working with you and Jathu!!   Reply With Quote

17. ## Re: HSC Physics Marathon 2017

Gosh, Thank you so much pikachu975 and Rathin.  Reply With Quote

18. ## Re: HSC Physics Marathon 2017 Originally Posted by Rathin By using E(p)=mgh we assume that g is constant at any altitude which is incorrect and this formula only works for low altitudes as we make the surface the point where E(p)=0. So a more accurate way of measuring E(P) is by -(Gm1m2)/r as it takes in consideration that g is now always constant do to many factors such as variation in the thickness of the Earth's crust and lithosphere, earth not being a perfect sphere so having flattened poles thus the value of 'g' is greater near the poles, and the spin of Earth creates a centrifugal effect which affects the 'g' near the equator. However by making E(p) being a point a very large distance away thus getting the formula E(p)= -(Gm1m2)/r it assumes that GPE is the only thing changing, but rockets can loose fuel or gain/loose velocity which increases EPE. However you would still get 1 out of 2 marks for it but the correct way to do it is using E(p)= -(Gm1m2)/r.
Did you get this info from page 22 of the booklet xD  Reply With Quote

19. ## Re: HSC Physics Marathon 2017

just out of curiosity, are you guys currently in year 11 or year 12? I do not remember learning this in the prelim course but you guys seem to be highly proficient at it.  Reply With Quote

20. ## Re: HSC Physics Marathon 2017 Originally Posted by jiujiu1123 just out of curiosity, are you guys currently in year 11 or year 12? I do not remember learning this in the prelim course but you guys seem to be highly proficient at it.
Starting year 12 now, but already learnt some content.  Reply With Quote

21. ## Re: HSC Physics Marathon 2017 Originally Posted by eyeseeyou BTW what options are you all doing?

Quanta, Astro or med?
Most likely Med  Reply With Quote

22. ## Re: HSC Physics Marathon 2017

How come there are no question left for me to answer!? Btw I like the idea of this thread and it would have been great if some people could have moved their discussions somewhere else as you are kind of spamming.

Heres my question and this should be really easy. So like calculate the period of a satellite orbiting the earth at an altitude of say 700,000 km. Okay so the radius of the earth is 6.38 × 10^6 m and the earth’s mass is 5.97 × 102^4 kg. Work it out people and show your full working.

I will look at the solutions tomorrow morning maybe.  Reply With Quote

23. ## Re: HSC Physics Marathon 2017 Originally Posted by Bluee How come there are no question left for me to answer!? Btw I like the idea of this thread and it would have been great if some people could have moved their discussions somewhere else as you are kind of spamming.

Heres my question and this should be really easy. So like calculate the period of a satellite orbiting the earth at an altitude of say 700,000 km. Okay so the radius of the earth is 6.38 × 106 m and the earth’s mass is 5.97 × 1024 kg. Work it out people and show your full working.

I will look at the solutions tomorrow morning maybe.
I think you forgot to put the ^ symbols for the radius and mass of the earth, but I'll try solve it now.

Edit:
Is orbital period one of Kepler's laws?  Reply With Quote

24. ## Re: HSC Physics Marathon 2017 Originally Posted by pikachu975 I think you forgot to put the ^ symbols for the radius and mass of the earth, but I'll try solve it now.
Oops thats so stupid of me how could I have not realised that!  Reply With Quote

25. ## Re: HSC Physics Marathon 2017 Originally Posted by pikachu975 Edit:
Is orbital period one of Kepler's laws?
Just apply Keplers 3rd Law.

Yes it is.  Reply With Quote

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