# Oxford PAT 2015, Question 19

## 11 thoughts on “Oxford PAT 2015, Question 19”

1. George says:

Hey for the last part: can you talk about the effects of air resistance which will reduce the K.E of satellite which will slow down the satellite which may cause the satellite further away from the centre of earth?

1. No, I don’t think you can, because it says “ignore the atmosphere”.

2. Tim Forrer says:

Hi, thanks so much for your solutions.
Why do you not need to include the radius of the Earth for the first part of the question? (i.e. v=[GM/(R+r)^2].) Is it because the circular orbit of radius r includes that already?

Thanks again!

1. Yes exactly right, I am assuming that the radius of an orbit is given from the centre of the object that we are orbiting around.

Maybe that’s interpreting the question’s wording wrongly, but if the examiners meant differently they should have said an orbit of ‘altitude’ r.

If you used the other interpretation (that r is the altitude) then I’m sure it’s right as long as you stick to that consistently.

1. Tim says:

Ok that makes sense. Thanks very much!

3. c says:

Hi. I understand all of the solution however I think it might be missing one part. At the start you write that your expression for [Force]=[Acceleration] however since F=ma shoudl it not be that [Force]=M[Acceeration]?
Thanks for all the solutions

1. Yes sorry, my writing was careless, the second line that says ‘Acceleration’ should actually say ‘Mass x Acceleration’

4. Sam says:

For part B can you not simply talk about the required speed being crazy high etc?

1. Yes I guess so. It’s obviously a bit of a silly question, and I wasn’t sure exactly what to say.

5. ian says:

Hi, sorry but can you write more clearly the last 2 statements after the two bullet points? thank you!

1. I’ve written ‘Earth’s radius is not constant/uniform’ and ‘Earth’s density is not uniform so the orbital radius will vary’.