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If an object undergoes uniform circular motion, the acceleration vector and the sum of the forces always point towards the center of the circle. In the radial direction, Newton’s Second Law gives \begin{aligned} \sum \vec F = ma_R = m\frac{v^2}{R}\end{aligned} If an object’s speed is changing as it moves around a circle the acceleration vector will have a component that is towards the center of the circle (the radial component) and a component that is tangential to the circle. The tangential component is responsible for the change in speed, whereas the radial component is responsible for the change in direction of the velocity.