# Thread: HSC Physics Marathon 2016

1. ## Re: HSC Physics Marathon 2016

3/4?

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2. ## Re: HSC Physics Marathon 2016

Originally Posted by leehuan
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
If the object is at rest it will just obviously fall....

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3. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
If the object is at rest it will just obviously fall....

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That's all you had to say. It can't pick up momentum from the sun and instead it will fall.

Hence why I said logic. But always answer the question. Also, tbh, you could relate p=mv

4. ## HSC Physics Marathon 2016

Cannonballs P and Q are fired so that they leave their barrels from the same height. Cannonball P is fired vertically upwards while cannonball Q is fired at an acute angle.
Both cannonballs take 3 seconds to reach the same maximum height.

Explain how the resulting motion of the cannonballs supports Galileo’s analysis
of projectile motion. ( 3 marks)

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5. ## Re: HSC Physics Marathon 2016

Galileo was the first to use mathematics to analyse projectile motion. He considered that, rather than using the motion of the object as a whole, it could be split into the constituent vertical and horizontal components (note that they're independent of each other), and when added together as vectors, it produced the total velocity of the object. He realised that the only external force acting on this object is gravity, and would only influence the vertical motion of the object while the horizontal velocity of the object would sty constant. Both balls would experience the same acceleration due to gravity regardless of mass (this is assuming no resistance), and hence would reach the same height, and strike the ground at the same time (as would objects of different mass).

Didn't really understand it much, correct me if I'm wrong

6. ## Re: HSC Physics Marathon 2016

Originally Posted by RachelGreen
Galileo was the first to use mathematics to analyse projectile motion. He considered that, rather than using the motion of the object as a whole, it could be split into the constituent vertical and horizontal components (note that they're independent of each other), and when added together as vectors, it produced the total velocity of the object. He realised that the only external force acting on this object is gravity, and would only influence the vertical motion of the object while the horizontal velocity of the object would sty constant. Both balls would experience the same acceleration due to gravity regardless of mass (this is assuming no resistance), and hence would reach the same height, and strike the ground at the same time (as would objects of different mass).

Didn't really understand it much, correct me if I'm wrong
Basically yes.
- Horizontal and vertical seperate
- Only force is gravity and acceleration due to gravity was constant
- Relates the question back to these concepts

3/3

7. ## Re: HSC Physics Marathon 2016

NEXT QUESTION:
The centripetal force required to keep a satellite in orbit around a planet is provided by the gravitational attraction between the planet and the satellite.
Compare a low earth orbit satellite to a geostationary satellite, and in your own answer explain why geostationary satellites are less susceptible to orbital decay. (3 marks)

8. ## Re: HSC Physics Marathon 2016

A LEO satellite has an altitude of 300-1000km and an orbital period of approximately 90 minutes. On the other hand, a geostationary satellite has an altitude of 36,000km and an orbital period of 23hr 56min, which matches the earth's rotation. Both types of satellites require an extremely high orbital velocity, which depends on the satellites's radius and mass of the earth (insert orbital velocity formula). Due to the smaller radius between the LEO satellite and the earth, the orbital velocity is much higher at 8km/s compared to the geostationary's velocity of 3km/s.

LEO Satellites orbit at altitudes of less than 1000km, while the atmosphere extends out to 9600km where molecules of gas are still present, meaning there is some atmospheric drag leading to friction, resulting in a loss in altitude and orbital decay. Geostationary satellites don't experience this decay due to their high altitude.

9. ## Re: HSC Physics Marathon 2016

I'll mark you later, I gotta go out soon.

However, in the meantime, NEXT QUESTION:
Discuss some strategies used to minimize fuel consumption of rockets (3 marks).

10. ## 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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This is what I wanted you to realise from this question. All good.

11. ## Re: HSC Physics Marathon 2016

Originally Posted by RachelGreen
I'll mark you later, I gotta go out soon.

However, in the meantime, NEXT QUESTION:
Discuss some strategies used to minimize fuel consumption of rockets (3 marks).
1. Rockets are lunched from the equator of the earth from a westerly to easterly direction. This is done to both minimise the distance to outer orbit and take advantage of the earths rotation about its axis. (Note that the earth is an oblique spheroid making it widest at the equator)
2. The Russian scientist Konstantien had developed multistaged rockets to limit the fuel needed to launch a rocket into space. This involved dumping individual, empty fuel cells/containers when they are empty causing an acceleration.
3. A manoeuvre called the slingshot effect basically harnesses another planets orbit around the sun to enlarge its own velocity (I won't go into any detail here as it has been said earlier.).

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12. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
1. Rockets are lunched from the equator of the earth from a westerly to easterly direction. This is done to both minimise the distance to outer orbit and take advantage of the earths rotation about its axis. (Note that the earth is an oblique spheroid making it widest at the equator)
2. The Russian scientist Konstantien had developed multistaged rockets to limit the fuel needed to launch a rocket into space. This involved dumping individual, empty fuel cells/containers when they are empty causing an acceleration.
3. A manoeuvre called the slingshot effect basically harnesses another planets orbit around the sun to enlarge its own velocity (I won't go into any detail here as it has been said earlier.).

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1)Not necessary. Since it orbits about around its equator that means the centrifugal force is max at the equator not because of the egg like shapes of the world.
2) Seems a bit lousy...

13. ## Re: HSC Physics Marathon 2016

1. Isn't the orbital velocity significantly greatest about the equator ?
2. Yeh sorry :/ right concept more detail?

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14. ## Re: HSC Physics Marathon 2016

Originally Posted by Glyde
1. Isn't the orbital velocity significantly greatest about the equator ?
2. Yeh sorry :/ right concept more detail?

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What does orbital velocity have to do with launching lol?

15. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
What does orbital velocity have to do with launching lol?
Tea leaves

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16. ## Re: HSC Physics Marathon 2016

More orbital velocity is gained launching from the Equator contrasted to a launch at the poles?

17. ## Re: HSC Physics Marathon 2016

Originally Posted by leehuan
More orbital velocity is gained launching from the Equator contrasted to a launch at the poles?
Orbital velocity - The speed at which an object orbits another object.

How does this affect launch ?

Even if the world was a perfect sphere you would still launch from the equator.

18. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
Orbital velocity - The speed at which an object orbits another object.

How does this affect launch ?

Even if the world was a perfect sphere you would still launch from the equator.
If I think about it now, it's maybe just wrong terminology. Yeah, for some reason I was thinking centrifugal force as well and then came back to the mistake of v.

19. ## Re: HSC Physics Marathon 2016

Originally Posted by leehuan
If I think about it now, it's maybe just wrong terminology. Yeah, for some reason I was thinking centrifugal force as well and then came back to the mistake of v.
Common misconception. Its because the Earth revolves about the equator and therefore it has the max centrifugal force While the poles basically experience 0 since they're not on the axis of rotation.

20. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
Common misconception. Its because the Earth revolves about the equator and therefore it has the max centrifugal force While the poles basically experience 0 since they're not on the axis of rotation.
iForgot.

21. ## Re: HSC Physics Marathon 2016

That's fine, you just gotta talk about the direction in which the rocket is launched (to the east) which gives the rocket a velocity boost due to earths rotational motion and this velocity is maximised from the location the rocket is launched from (equator), lower 'g' due to larger 'r' is a perfectly good reaso, but sure you can talk about the centrifuge effect reducing the effective value of 'g', and that effect is more noticeable along the equator since rotational motion is great (you can relate to v=2(pi)r/t if you want)

You can also talk about the alignment of the Earth relative to the Sun to achieve orbital velocity to go beyond the solar system, slingshot effect etc.

22. ## Re: HSC Physics Marathon 2016

Originally Posted by Drsoccerball
What does orbital velocity have to do with launching lol?
I think he meant the rotational motion that is greatest along the equator

23. ## Re: HSC Physics Marathon 2016

Originally Posted by RachelGreen
That's fine, you just gotta talk about the direction in which the rocket is launched (to the east) which gives the rocket a velocity boost due to earths rotational motion and this velocity is maximised from the location the rocket is launched from (equator), lower 'g' due to larger 'r' is a perfectly good reaso, but sure you can talk about the centrifuge effect reducing the effective value of 'g', and that effect is more noticeable along the equator since rotational motion is great (you can relate to v=2(pi)r/t if you want)

You can also talk about the alignment of the Earth relative to the Sun to achieve orbital velocity to go beyond the solar system, slingshot effect etc.
Its very weak if you just say the radius is bigger at the equator. That slightly decreases the level of g.but not as much as centifugal force...

24. ## Re: HSC Physics Marathon 2016

Meh it says that in most textbooks

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25. ## Re: HSC Physics Marathon 2016

Concept of centrifugal force always relates to rotational velocity as RachelGreen said. That's why I got confused and had a brief memory lapse.

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