A small moon moves in a circular orbit around a planet. At one instant, the moon is directly east of the planet and is moving north at 6 km/s.
After one quarter of an orbit, the moon is directly north of the planet.
Which statement explains the moon's velocity at this new position?
The velocity now points towards the planet; gravity therefore makes the moon move south at 6 km/s6\ \mathrm{km/s}6 km/s.
The velocity remains north at 6 km/s6\ \mathrm{km/s}6 km/s because a constant orbital speed means that the velocity is constant.
The moon gains speed as gravity pulls it around the orbit, so it moves west at more than 6 km/s6\ \mathrm{km/s}6 km/s.
The tangent at the new position points west; gravity has rotated the velocity while its magnitude remains 6 km/s6\ \mathrm{km/s}6 km/s.
80 exam-style questions on Edexcel GCSE Physics 7.1 The Solar System and gravity, covering 7.1.1 Weight and g on different bodies in space, 7.1.2 The Solar System, 7.1.3 Changing ideas about the Solar System, 7.1.4 Orbits of moons, planets, comets and satellites, and 7.1.5 Gravity and orbital motion. Each one has a worked solution and a mark scheme showing where the marks go.