Orbital speeds of planets and satellites
3 marks · EasyA moon orbits its planet at a radius of 450,000 km. It takes 40 days to complete one orbit.
Convert the orbital period into seconds.
Calculate the average orbital speed of the moon in km/s. Give your answer to 2 significant figures.
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(a) 3,456,000 s
(b) 0.82 km/s
Common mistakes: Not converting days to seconds; Using incorrect formula
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct calculation: s.
M2 (b) · 1 mark · Substitutes correct values into orbital speed formula. (follow-through)
A1 (b) · 1 mark · Obtains correct value of 0.82 km/s (2 significant figures).
Worked solution
a) 40 days × 86,400 s/day = 3,456,000 s b) Speed = (2 × π × 450,000 km) ÷ 3,456,000 s = ≈ 0.82 km/s