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Name: Sharon
Status: educator
Grade: 4-5
Location: NY
Country: N/A
Date: 10/16/2005

I made spin-art projects with my fifth graders using paint, a salad spinner and paper plates. What are the forces involved in creating our works of art? Why does the paint spin it centripetal? it rotational inertia?

Hi Sharon,

Good question! The answer is provided by Newton's first law of motion which says that an object will continue moving in a straight line at constant velocity unless some force is exerted on it. A special case of this law is that an object at rest remains at rest unless some force is exerted on it.

In the case of the spinner, a globule of paint spinning around is continually changing the direction of its velocity, so the plate must be exerting a force on it by Newton's first law. The force is directed toward the center of rotation and increases with the square of the speed of the globule and like the inverse of the distance of the globule from the center of rotation (so smaller force at larger distance).

When the plate's speed of rotation is increased until the force required to keep the globule rotating is more than the plate can exert on the globule, it slips and starts moving in a straight line. So the globule leaves tangentially and does NOT move directly away from the center of rotation. If you observe the paint patterns carefully, I am sure you can verify that (unless the spinner is very small in diameter).

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

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