The apparatus and set up used demonstrates the magnitude and direction of centripetal forces.
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An object moving in a circle at a constant speed is said to be undergoing "uniform circular motion". Though the speed of the object is not changing, the direction of motion is which implies the object is experiencing acceleration. To cause this acceleration, there must be a force on the object. This force is directed toward the center of the circle, so it is called a "centripetal force".
The machine used in this demonstration (Fig.1) can be used to show some properties of centripetal force. It consists of a mass (M) hanging by a string from a bar on a rotating post (A). The vertical post (B) and the mass are joined by a spring (S), which is responsible for exerting the centripetal force that keeps the mass moving in a circle. Watching the video, one can see not only that the centripetal force is directed toward the center of the circle, but also that it is possible to calculate the magnitude of the centripetal force using this set-up. As the mass rotates faster and faster, it stretches the spring and moves in a circle with a larger and larger radius. Once the circle is large enough, the mass strikes a flexible post (P). Since the spring is exerting the centripetal force, the magnitude of the spring force is equal to the centripetal force. Calculating the spring force at this radius will therefore give the magnitude of the centripetal force.
Attach a string to the side of the mass facing the pulley and place it over the pulley.
To calculate the magnitude of the centripetal force at the radius of the post: