Demos: 1D-09 Centripetal Force

Two heavy carts are constrained to move along a single direction. The carts are attached to a central spring, as illustrated. When the entire is apparatus is set into rotation, the carts move outward along the track, causing the spring to stretch, indicating a larger required central force.

Directions: With the carts positioned near the center, as shown, start the rotation by means of the crank located on the side of the apparatus. Eventually the carts will move to their extreme position and the spring will be stretched considerably.

Suggestions for Presentation: Before starting the rotation, show how the spring mechanism works by pulling the carts toward the end of their track and noting the spring stretch. Then ask what should happen to the carts as the apparatus is set into rapid rotation. Most students will answer with something like “the carts will be slung outward.” Then ask “What keeps them moving in a circle?” After you have demonstrated what happens, ask the students what force caused the carts to move outward as long the track. Many will suggest a “centrifugal force.” But then ask “What is its origin?” Discussion should lead to the fact that it is only in the frame of reference of the cart that the cart moves outward away from the center. In the Newtonian (room) frame, the carts are merely attempting to travel in a straight line along the direction of the tangent and a central force is required to keep them moving in a circle.

Applications: When riding a merry-go-round, a central force keeps one moving in a circle instead of going off on a tangent.

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