An example of an alternative conception
It is common for learners to think that an object in orbit (the Moon around the Earth; a planet around the Sun) is subjecct to balanced forces.
Sometimes this is understood in terms of the centripetal force attracting the satellite being balanced by a centrifugal force pusing the satellite away form the central body. Actually, orbital motion is accelerated motion: as the direction, and so velcocity, is constantly changing – as the satellite turns towards the central body (compared with moving in a striaght lines). The centripetal force towards the centre of the orbit acts on the satellite to accelerate it so it does not leave orbit by moving off in a straight line (as Newton's first law would require if there was no net force acting.)
Read: Conceptions of planetary orbits
Read: Scaffolding learning of circular motion
Read about centrifugal force as an alternative conception
