My understanding is that so-called 'centrifugal force' is a fictitious force, i.e. from the reference frame of an outside (inertial) observer, there is only an inward, centripetal force on a body being accelerated in uniform circular motion, but from the reference frame of that body, objects would seem to move outwards (e.g. pendulums would swing outwards, a marble held in your hand would fly off outwards, etc.) So you have to construct a force in that reference frame, the centrifugal force, to explain that apparent outward acceleration. However, my understanding is also that said outward motion is due to inertia (the same reason a pendulum suspended from the roof of an accelerating car flies backwards, or a person wearing roller skates on a bus, and therefore not coupled by friction to the bus floor, will roll backwards when the bus accelerates forwards. You'd have to invent fictitious forces from their reference frames to explain their apparent acceleration with respect to the car/bus too, but there is no acceleration in an inertial reference frame.) But wouldn't a body's inertia give it tangential motion, at right angles to the direction of spin rather than straight out? So why is centrifugal force (apparently) directed outwards from the centre of the circle instead of at right angles to the centre and in the direction of motion of the spinning body?