# Thread: HSC Physics Marathon 2016

1. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
Ohhhh so if we are constantly burning fuel from the same rank the whole way , the ship would be experiencing too many g's causing a black out. Hence dumping them will keep the acceleration at a constant level instead and that constant acceleration would be under say 6 g's to stop people's dying

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No ahaha. We don't even know if we could lift off with such low acceleration. We use it so that its as if you have tiny rocket launches throughout the journey. From A to B we have one rocket launch and when the fuel stops being used it stops accelerating and a reduces drastically reducing the g forces experienced. From B to C we have another rocket launch which starts from basically 0 acceleration... Lets say that the acceleration of A to B is X and B to C is also X. The max g force experienced would be = $1+ \frac{X}{g}$ However if this was done in one journey it woud be $1+ \frac{2X}{g}$...

2. ## Re: HSC Physics Marathon 2016

Omg I get it now!!!!!!!! Thanks sooooo much !!! I had no idea that the rockets stopped in mid air and then turned on there next rocket

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3. ## Re: HSC Physics Marathon 2016

Sorry it took me so long

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4. ## Re: HSC Physics Marathon 2016

Originally Posted by leehuan
1+a/g confused the crap out of me. I used sigma(a)/g where the sum of all forces could only be from gravity and thrust (i.e. no 'normal reaction'). So on Earth's surface I treated sigma(a) as 9.8 instead.
$G force = 1 \pm \frac{a}{g}$

$If in free fall without a normal force g force is less than one AKA 1-\frac{a}{g}$

$If there is a force then g force is greater than one AKA 1+\frac{a}{g}$

$Since 1 g force is what we normally feel$

5. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
$G force = 1 \pm \frac{a}{g}$

$If in free fall without a normal force g force is less than one AKA 1-\frac{a}{g}$

$If there is a force then g force is greater than one AKA 1+\frac{a}{g}$

$Since 1 g force is what we normally feel$
To add to this, the reason why it is sometimes plus or minus is because the acceleration (a) is a vector. If can be either pointing downwards which is negative and why it is negative in your first example, or it can be pointing upwards which is why it is positive in your second example.

6. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
$G force = 1 \pm \frac{a}{g}$

$If in free fall without a normal force g force is less than one AKA 1-\frac{a}{g}$

$If there is a force then g force is greater than one AKA 1+\frac{a}{g}$

$Since 1 g force is what we normally feel$
Although that's the most accurate interpretation, when I think about that I get confused. I'm just saying instead I actually imagine we are accelerating at 9.8ms^-2 right now instead.

7. ## Re: HSC Physics Marathon 2016

Bump
Originally Posted by RachelGreen
I saw this question from Syd Grammar's old trial paper and thought was a great question:
"The Earth exerts the exact same gravitational force on the Sun as the Sun does on the Earth due to Newtons 3rd Law". Assess the above statement (4 marks)

8. ## Re: HSC Physics Marathon 2016

According to Newtons 3rd Law, 'Every reaction will have an equal and opposite reaction' we are aware that when two objects interact with each other that there will be a force between them that is the same in magnitude however opposite in direction. Hence it is logical to assume that the Earth and Sun will follow this rule. That is the gravitational pull of the sun on the earth will be the same magnitude as the gravitational pull of the earth on the sun. We can also show that the Sun and Earth are attracted towards each other using the formulae 'F=(Gm1m2)/r^2' where the mass (m) are representative of the earth and sun.

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9. ## Re: HSC Physics Marathon 2016

I wouldn't be surprised if I am completely wrong

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10. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
According to Newtons 3rd Law, 'Every reaction will have an equal and opposite reaction' we are aware that when two objects interact with each other that there will be a force between them that is the same in magnitude however opposite in direction. Hence it is logical to assume that the Earth and Sun will follow this rule. That is the gravitational pull of the sun on the earth will be the same magnitude as the gravitational pull of the earth on the sun. We can also show that the Sun and Earth are attracted towards each other using the formulae 'F=(Gm1m2)/r^2' where the mass (m) are representative of the earth and sun.

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We call the formula Newton's Law of Universal Gravitation. Also, it's every action will have an equal and opposite reaction, so be careful of this. I'll let someone else put on a mark.

11. ## Re: HSC Physics Marathon 2016

Okay, yes I new that I just wasn't thinking about what I was writing haha thank you

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12. ## Re: HSC Physics Marathon 2016

New question : Explain why the object used for slingshot motion (planet, star, etc...) needs to be moving.

13. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
New question : Explain why the object used for slingshot motion (planet, star, etc...) needs to be moving.
LOL.

Sorry, the question is genuine but that seemed like logic to me.

14. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
According to Newtons 3rd Law, 'Every reaction will have an equal and opposite reaction' we are aware that when two objects interact with each other that there will be a force between them that is the same in magnitude however opposite in direction. Hence it is logical to assume that the Earth and Sun will follow this rule. That is the gravitational pull of the sun on the earth will be the same magnitude as the gravitational pull of the earth on the sun. We can also show that the Sun and Earth are attracted towards each other using the formulae 'F=(Gm1m2)/r^2' where the mass (m) are representative of the earth and sun.

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3/4! Marking criteria says to don't forget to make a judgement of the statement at the end

15. ## Re: HSC Physics Marathon 2016

Could you please explain what you mean by this or provide an example

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16. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
Could you please explain what you mean by this or provide an example

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To assess is to simply provide your stance, i.e. do you agree or disagree, to what extent can an aforementioned statement be true

17. ## HSC Physics Marathon 2016

Ohhh so I could say the this statement is genuinely accurate however the vectors of which these two planets attract are in opposite directions?

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18. ## HSC Physics Marathon 2016

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19. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
New question : Explain why the object used for slingshot motion (planet, star, etc...) needs to be moving.
How many marks would this question be. Also is there any rule to follow in relation to marks per question ? Should I approach questions differently in relation to there given marks ?

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20. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
Could you please explain what you mean by this or provide an example

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Once you've written all of your points to back up your opinion. Simply write a judgement like for say "This statement is invalid". I don't think you have to explain why it's invalid since you have already written a lot of points on why it's invalid above. This is just an example

21. ## Re: HSC Physics Marathon 2016

Originally Posted by RachelGreen
Once you've written all of your points to back up your opinion. Simply write a judgement like for say "This statement is invalid". I don't think you have to explain why it's invalid since you have already written a lot of points on why it's invalid above. This is just an example
Wow that's worth a whole mark ?

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22. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
Wow that's worth a whole mark ?

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Yeah, since the Key Word is "assess", which means you need to make a judgement.
Assess: "Make a judgement of value, quality, outcomes, results or size"

( The BOSTES definitions of key words is found here, and you should be familiar with them http://www.boardofstudies.nsw.edu.au..._keywords.html )

23. ## Re: HSC Physics Marathon 2016

You would be surprised how many candidates lose the mark for making a judgement every year

24. ## HSC Physics Marathon 2016

Originally Posted by Drsoccerball
New question : Explain why the object used for slingshot motion (planet, star, etc...) needs to be moving.
When an object such as a space probe approaches a planet, it is accelerated by the larger mass' gravitational pull, this statement is often misinterpreted as the source of the slingshot effect. However when the space probe continues back out of the planets atmosphere it will again be effected by the planets gravitational field but in the opposite direction. It will begin to decelerate back down to its initial velocity. The thing that actually causes the slingshot effect is the fact that the planet is orbiting something else, such as the sun. Hence when the space probe enters the planets atmosphere or gravitational pull it is accelerated to the planets orbitory speed around the sun. This causes the vectors of the space probes initial velocity and the planets orbitory velocity around the sun to add, consequently resulting in an angle change and velocity change. It is also important to note that the original misconception does not cause a slingshot effect due to the conservation of momentum. The space probe received an additional speed boost causing the planet to slow down. This is not noticeable because the planets mass is usually a lot bigger then the space probes mass hence having little effect of the planets speed. Similarly the space probe is slowed down to its original speed and the planet is sped up to its original speed.

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25. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
When an object such as a space probe approaches a planet, it is accelerated by the larger mass' gravitational pull, this statement is often misinterpreted as the source of the slingshot effect. However when the space probe continues back out of the planets atmosphere it will again be effected by the planets gravitational field but in the opposite direction. It will begin to decelerate back down to its initial velocity. The thing that actually causes the slingshot effect is the fact that the planet is orbiting something else, such as the sun. Hence when the space probe enters the planets atmosphere or gravitational pull it is accelerated to the planets orbitory speed around the sun. This causes the vectors of the space probes initial velocity and the planets orbitory velocity around the sun to add, consequently resulting in an angle change and velocity change. It is also important to note that the original misconception does not cause a slingshot effect due to the conservation of momentum. The space probe received an additional speed boost causing the planet to slow down. This is not noticeable because the planets mass is usually a lot bigger then the space probes mass hence having little effect of the planets speed. Similarly the space probe is slowed down to its original speed and the planet is sped up to its original speed.

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Question not fully answered though; since he's asking for why it must be moving you must also explain what the problem will be when the space probe is somehow at rest.

The bolded part must be emphasised, but the misconception that you gave can be made very brief

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