## HSC 2015 Q19

A forum designed mainly for high school physics students in New South Wales, Australia.
Sakage747
Posts: 1
Joined: Thu Mar 31, 2016 7:18 pm

### HSC 2015 Q19

19. An astronaut working outside a spacecraft in orbit around Earth is not attached to it. Why does the astronaut NOT drift away from the spacecraft?
(A) The force of gravity acting on the astronaut and spacecraft is negligible.
(B) The spacecraft and the astronaut are in orbit around the Sun with the Earth.
(C) The forces due to gravity acting on both the astronaut and the spacecraft are the same.
(D) The accelerations of the astronaut and the spacecraft are inversely proportional to their respective masses.

Hi

So the answer is (d) and I'm not entirely sure why that is. If anyone can explain it to me, it would be greatly appreciated. Thx in advance
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: HSC 2015 Q19

(A) is not true. One should never say 'negligible' without a phrase 'in comparison with ..."
(B) The statement itself is true. If the answer had included "They are also both in orbit around the Earth", then this would be easily the best answer.
(C) is not true.
(D) With respect to any forces between them, this is true, but it is irrelevant to the question. There is no tether between them to provide a tension. The gravitational force between them is negligible when compared with other forces acting on them. For the important force in this problem, their gravitational attraction towards the earth, their accelerations are exactly the same.

(A) and (C) are completely untrue. (D) is true in an irrelevant case but false in the relevant case. (B) is true but incomplete. My answer would be (B) because it is the closest to being correct.

I am told that, when there are bad questions, they sometimes don't include the results from such questions in the marks.