Question:
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 out....is it
centripetal?...is it rotational inertia?
Replies:
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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