Motion in the Universe
4 marks · EasyHalley's Comet has an orbit that brings it close to the Sun, then takes it far out into the solar system. Meanwhile, Earth’s Moon orbits Earth at nearly constant speed.
Describe the main difference in the orbit of a comet compared with that of a moon.
State what happens to the speed of the comet as it moves closer to the Sun.
Explain your answer to part b using ideas about forces in the solar system.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) A comet follows a highly elongated (elliptical) orbit around the Sun, while a moon usually moves in a nearly circular orbit around a planet. · must show Comet has elongated/elliptical orbit, Moon has nearly circular orbit around planet
(b) The speed of the comet increases as it gets closer to the Sun. · must show States 'speed increases'
(c) The gravitational force from the Sun is stronger when the comet is closer, pulling it in more and making it move faster. · must show Gravitational force increases near Sun, Stronger force causes greater speed
Common mistakes: Saying speed decreases near Sun
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states that comet’s orbit is very elongated/elliptical.
B2 (a) · 1 mark · States moon’s orbit is nearly circular around a planet.
B3 (b) · 1 mark · States speed increases as comet approaches Sun.
B4 (c) · 1 mark · Explains higher speed is due to stronger Sun’s gravity when closer.
Worked solution
a) A comet has a highly elongated orbit around the Sun, while a moon has a nearly circular orbit around a planet.
b) The speed of the comet increases as it approaches the Sun.
c) When the comet is nearer to the Sun, the gravitational force from the Sun is much stronger, pulling the comet faster and increasing its speed.