Whirl a ball on a string and it traces a circle. Its speed can stay perfectly steady, yet it is accelerating the whole time, because its direction keeps changing. The secret is a force that points sideways, always square to the motion.
An object moving in a circle at constant speed is acted on by a resultant force directed toward the centre. This force is perpendicular to the motion, so it changes the direction of the velocity without changing its size. Remove the force and the object flies off in a straight line.
Watch the ball travel around the circle. The blue arrow is its velocity, always pointing along the path (tangent). The red arrow is the resultant force, always pointing to the centre. Alter the parameters and watch how the required force responds.
Because the object goes round at a steady speed, it is tempting to say there is no resultant force. But velocity includes direction, and the direction changes constantly, so the object is accelerating and a resultant force is needed. That force acts toward the centre, perpendicular to the motion, not along it. Cut it, and the object continues in a straight line.