NYU Physics Demos

Centripetal Force Paradox

Topics: Mechanics - Forces- Centripetal, Centrifugal, Buoyant; Rotational Motion

Equipment: “Centripetal Force Paradox” apparatus and instruction sheet, water.

Setup: Fill the clear plastic jars almost completely with water. Screw the jars onto the lids that are attached to the apparatus (as shown). Spin the entire apparatus, slowly at first. Refer to the instruction sheet for more details/experiments.

As the apparatus spins, the balls dangling freely will start to hang outward, as students might expect. What they may not expect, however, is that the floating balls inside the jars will begin to move towards the central axis of rotation. The water is resisting the force of the jar pushing it towards the center (which is the centripetal force), so it is the water that is moving away from the center (this is the centrifugal force, directed away from the center). The floating ball would normally also be moving away from the center, but because the water is more dense, it has more inertia, and pushes the ball in the opposite direction.

Location: Shelf B-g3


Keywords: Force, Hang, Float, Spin, Rotate, Revolve, Jar, Bobber

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