## NSW HSC 2009 What are the answers to Qs 3&5?

A forum designed mainly for high school physics students in New South Wales, Australia.
joe
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### NSW HSC 2009 What are the answers to Qs 3&5?

I used these questions at a conference as examples to show the problems that come from reducing the physics content of a syllabus beyond a minimal level. It was suggested that I put them on this discussion site. Because of the length limitation, I'll put them on separate posts.

Joe
joe
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Location: Sydney

### Re: NSW HSC 2009 What are the answers to Qs 3&5?

Q3 A satellite is moved from a geostationary orbit to a higher orbit.
Which statement about the orbit change is correct?

(A) During the move the gravitational potential energy decreases.
(B) The change in gravitational potential energy is independent of the mass of the satellite.
(C) The work done is the difference between the gravitational potential energy of the higher orbit and that of the geostationary orbit.
(D) The work done is the energy required to move the satellite, which is in the gravitational field, from a very large distance away, to the higher orbit.

(A) has the wrong sign. (B) is not true. (C) is out by a factor of two. (D) is also wrong. The real answer is 'none of the above', but that wasn't an option. So I assume that that the examiners didn't include this question when calculating scores.
joe
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Location: Sydney

### Re: NSW HSC 2009 What are the answers to Qs 3&5?

Q5 During a lunar eclipse, Earth moves between the Sun and the Moon. (sic)
What happens to the force exerted by the Sun on the Moon?
(A) It increases.
(B) It decreases.
(C) It remains unchanged.
(D) It depends on the closeness of Earth to the Moon.

My first response to this question is that the lunar eclipse happens during a full moon, which is when the moon at its monthly maximum distance from the sun, so the force between Sun and Moon has its lowest monthly value at the time of the eclipse. So (B) looks to be the correct answer.

However, look at the (strange!) first sentence. Perhaps they meant to write "Suppose that the Sun-Moon distance remains fixed and that the Earth then moved between Sun and Moon. Would the Sun-Moon attraction suddenly decrease?" Or "During a solar eclipse, when the moon moves into Earth's light shadow, does it also move into a graviton shadow and thus have less attraction to the sun?" So presumably B and C were both accepted as being correct.

Joe
Mini
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### Re: NSW HSC 2009 What are the answers to Qs 3&5?

joe wrote:Q3 A satellite is moved from a geostationary orbit to a higher orbit.
Which statement about the orbit change is correct?
According to http://www.boardofstudies.nsw.edu.au/hs ... ide-09.pdf , the correct answer was C, which is what I would've chosen. Also, I also obtained my raw marks from the BOS which indicated that I got Q3 correct.

In HSC Physics we defined G.P.E. as -Gm1m2 / r, so wouldn't the work done be just -Gm1m2 ( 1/r2 - 1/r1) ? (r2 is the radius of the higher orbit, r1 radius of lower orbit) ... This is how we were taught at least...
joe
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Location: Sydney

### Re: NSW HSC 2009 What are the answers to Qs 3&5?

Well, I guess that answer C is only out by a factor of 2, so it is in that sense closest to being right. But it is still WRONG! Perhaps, because the right answer wasn't available, they had to mark everyone correct for that question, no matter what they chose. Only way to be fair.

You are right about the potential energy: U = -GMm/r
But a satellite also has kinetic energy, K. From the centripetal force GMm/r^2 = ma = mv^2/r, it's easy to show that K = -U/2
So E = U + K = -(1/2)GMm/r
And of course the work done on the satellite equals the increase in E. To revise, see
http://www.animations.physics.unsw.edu. ... avity.html

But the problem remains that, while the HSC syllabus has so little physics in it, it must be difficult to get people who know physics to help write or proofread the questions.

Joe
Mini
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### Re: NSW HSC 2009 What are the answers to Qs 3&5?

Ahh yes I see that now.

When I did the HSC, these types of questions were very 'standard' and I recall that textbooks didn't consider the KE of satellites in making such calculations. Probably the reason is that mechanical energy was not in the course. I only came across it during PHYS1A.

But yes they probably should've got someone knowledgable to proof read the questions.

This paper however was the most mathematical out of all the papers in the history of HSC Physics since 2001. There were no social science questions (thankfully!)