A comet travels in an elliptical orbit around the Sun. As the comet moves closer to the Sun, its orbital speed increases. Explain why the comet speeds up as it approaches the Sun, and state what happens to the direction of the comet's velocity during this part of its orbit.

Model answer (4 marks)

Examiner tips

• Use the phrase ‘gravity pulls the comet towards the Sun’ to show understanding of the force.
• Explain that the force causes acceleration via F=ma, linking to the speed increase.
• Mention that the velocity vector changes direction as the comet follows the ellipse.
• Keep the answer concise – 4 marks can be earned with two short sentences.

Common mistakes

• Confusing speed with velocity – students write ‘speed increases’ but fail to note the change in direction.
• Using vague terms like ‘the comet speeds up’ without citing gravity or acceleration.
• Omitting the fact that the velocity vector is continually changing direction along the orbit.