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I Explanation of how spin art machine works

  1. Jan 25, 2017 #1
    I've got to explain how spin art works to some 5th graders tomorrow.

    Basic setup: Paper is attached to center of a horizontal disk. Paint droplets are placed at different places on the paper. Disk is spun which causes paint to produce a pattern flowing away from the center of the disk/paper.

    Basic explanation: As paper is spinning, a centripetal force is exerted on the paint (towards center of rotation) initially keeping it in place. As speed of rotating disk increases, the force required to keep the paint from wanting to follow a tangential path is more than the paper can exert on the paint, therefore paint droplet slides tangentially to the curved path of the paint. My thought is that the droplet moves in a straight path, but that it's actually not directly away from the center of rotation, but actually at an angle. Is that correct? It LOOKS like it's directly away from the center, but it doesn't seem like that's actually true...

  2. jcsd
  3. Jan 25, 2017 #2
    I would start with centrifugal force ....any mass making a circular motion will have a centrifugal force pushing it away from the center of rotation .... if the rotation is very slow the ink drop stays put , because the the adhesion with the paper is strong enough to resist the outward push ... just the same as when a water drop is on a window ... there, gravity want's to slide the drop down , but often the adhesion is enough to stop this ...

    With the drop on the paper , as the rotation speed increases the outward force will increase , and finally overcome the adhesion , so it flies away from the center of rotation .... If the rotational speed is increased slowly the drop will always trace a line that points to the center of rotation... If the rotational speed is increased extremely rapidly the drop should trace a spiral ( I think)..
  4. Jan 25, 2017 #3
    I don't think the science teachers are going to appreciate talking about a "centrifugal force". that's only in the frame of reference of the spinner. in our frame of reference there's no such thing...
  5. Jan 25, 2017 #4

    Stephen Tashi

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    Get the kids started out right in life, explain that the centrifugal force is just due to the inertia of the paint droplets as they try to move in a straight line.
  6. Jan 26, 2017 #5

    Andy Resnick

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    This is an awesome demo- I wish I had thought of it first!

    My suggestion- forget starting with an 'explanation'. Instead, start by asking the kids what they think will happen- take some time, ask them questions ('why do you think that?'), etc.

    Then run the setup a few times, maybe incorporate some of the discussion: does it matter where the drop starts out? Does it matter how large the drop is? What happens if you drop the paint on the paper while it is already spinning? Whatever the kids come up with. And definitely (within reason) make a mess!

    Finally, compare the results with their predictions. When there is a discrepancy, see if you can get the kids to come up with alternative ideas. Try to avoid labels, stick to the observations. Ask them to imagine they are looking down on the paper and rotating with the paper, for example.

    5th grade is a little young to get anything from this, but you may find the video "Frames of Reference" helpful (IMO, it's the best science education video ever made):

    My reasoning is that simply telling them what is going to happen and then showing them is, frankly, boring. It would be unfortunate starting with something that is highly visual and entertaining (the ever-present possibility of a giant mess!) and making it dull by calling it 'science'.
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