A2 Multiple choice orbitals

Hi, just a quick question. Would anyone be able to explain the reason for the direction of the gravitational force acting on the comet changing? I understand that A is the correct answer as Ek reduces as radius increases. I assume that the gravitational force is a vector so as the comet orbits, the vector of the gravitational force changes.

If anyone has any better way of helping me understand it that would be appreciated + if anyone has any links to revision material for this section of the AQA Unit 4 specification that would be cool too.

Cheers.

A is correct, this is the only one that is wrong here.
For the gravitational force one, think of the motion of the comet as being parabolic. This is comparable to circular motion, and so the force always acts towards the centre of the sun.
Because the force always acts towards the centre of the sun, and the comet is moving past the sun, the direction of the net force changes.
This diagram shows a satellite around earth, but can be easily likened to the comet around the sun: See how the direction of $F_{net}$ changes as the satellite moves around the Earth? same principle with the comet

As for revision, for AQA GCE this site is quite useful just scroll down abit on the left there is unit 4.

Thanks for the quick response, now I get it, that diagram really helped and thanks for the link, I'll have a look over those resources. All the best with your studies!