Q20.

(a) In 2002 a large asteroid was discovered orbiting the Sun. It was named Quaoar. The diagram below shows Quaoar in four positions in its orbit.

(i) In which of the four positions, A, B, C or D, is the effect of the Sun's gravity on Quaoar the greatest? Explain your answer.

C as it is the closest to the Sun.

Accept 'it is closer or close'

Both the letter and the correct explanation are required for the mark

1 mark

(ii) On the diagram above, draw arrows to show the direction of the Sun's gravity on Quaoar in each of the positions A, B, C and D.

An arrow from each of the points A, B, C and D towards the Sun - all four arrows are required for the mark. See diagram

The force of gravity acts along a straight line in between the centres of mass of the two objects concerned. In an examination it is a good idea to draw a dashed construction line between these centres of mass and then construct your arrow along that path.

1 mark

(iii) At which position, A, B, C or D, is Quaoar travelling most slowly? Explain your answer.

A as it is furthest from the Sun, therefore the effect of the Sun's gravity is the weakest at this point.

Accept 'it is further or far away'; accept 'gravity is low or lower'

Both the letter and the correct explanation are required for the mark.

1 mark

(b) The table below gives information about three of the planets in our solar system.

planet	average distance from Sun (millions of km)	time for one orbit (Earth years)	Average surface temperature of planet (ºC)
Saturn	1427	30	–180
Uranus	2870	84	–210
Pluto*	5900	248	–230

* Pluto has been demoted and is no longer a planet - but when this question was written it was classed as a planet.

(i) The time for one orbit of the planet Neptune is 165 Earth years. Use the table to estimate the average distance of Neptune from the Sun.

Any number greater than 2870 and smaller than 5900 gets a mark.

1 mark

(ii) How does the surface temperature of these planets vary with distance from the Sun? Use information in the table to help you.

The further away the planet's surface is from the Sun, the lower the temperature will be.

Accept the converse; accept 'the further away the colder it is'

1 mark

(iii) Explain why the temperature varies with distance from the Sun in this way.

The average surface temperature decreases as you get further from the Sun because planets that are further away receive less energy/heat/sunlight on each square metre of their surface. This is because the energy from the Sun spreads out (becomes less concentrated) as you get further from the source/Sun.

2 marks

(maximum 7 marks)