Final answer:
Radius OA of the circle serves as an axis of rotation and the arc length travelled along the circumference is proportional to the radius. This principle applies to rotational dynamics in physics.
Step-by-step explanation:
Radius OA of the circle acts as an axis of rotation when the circle is spun or rotated through an angle Δθ. The concept being described is related to circular motion and rotational dynamics in physics. When the circle's radius is rotated through a given angle, the corresponding arc length (Δs) described on the circumference is directly proportional to the radius of the circle.
The arc length is the distance covered along the circumference of the circle. The longer the radius (r), the greater the distance that a point on the circumference will travel for the same angle of rotation, Δθ.
This principle can be applied to objects rotating about a central point. For instance, a figure skater pulling their arms in will rotate faster because they reduce their radius of rotation, thus conserving angular momentum while reducing the moment of inertia.
In rotational motion, the radius of a circle acts as a lever arm. The lever arm is the perpendicular distance between the axis of rotation and the line along which the force acts. When a force is applied at a distance from the center of rotation, it generates a torque, which causes the circle to rotate. So, the radius OA of the circle shown below acts as a lever arm when an external force is applied.