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AP Physics Students Study Circular Motion.

By creating a tension force in the string with a hanging mass from the bottom, the tension can become the resultant Centripetal Force acting on the object which causes it to turn. By timing how long it takes to complete one period, and knowing the radius of the circle in which the object is moving, students can calculate the centripetal acceleration. Using Newton’s 2nd law, F = ma, students can then determine the mass of the object that is moving in a circle.
The heavier the hanging mass (thus additional gravitational force, thus greater tension force, thus greater centripetal force), the faster the mass has to move in its circular path.