An object moving in a circle at constant speed is still accelerating, because its direction is changing. That acceleration points inward, toward the centre, and something has to supply the force that produces it.
There is no centrifugal force
The outward push you feel on a roundabout is not a force acting on you. It is your inertia — your body trying to continue in a straight line while the seat pushes you inward. In an inertial frame, the only real force is the centripetal one, supplied by friction, tension, gravity or a normal force depending on the situation.
Radius matters more than you expect
Acceleration goes as the square of speed but only inversely with radius. Halving the radius of a turn at the same speed doubles the force required. This is why a tight corner is far harder on tyres than a sweeping one at the same speed, and why a centrifuge is short rather than long — it is easier to reach high acceleration by spinning a small radius quickly.
Where the force comes from
- A car turning: sideways friction between tyre and road, which has a hard limit.
- A satellite: gravity, which is why orbital speed depends only on altitude.
- A ball on a string: tension, which is why the string snaps above a certain speed.
- A banked track: the horizontal component of the normal force, which is why banking lets a corner be taken faster than friction alone would allow.