# Rotational Physics Demonstration Device

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The great thing about these tops is that you can make thousands of them inexpensively. The cans are basically free, the rivets cost \$0.02, and the center spiral is just a paper printout. Each student in your physics or science class can have one to use in class and to take home to contemplate. You can use them to do physics demonstrations and experiments in class, and/or you can demonstrate certain rotational optical illusions. They're mesmerizing and thought-provoking.

### Supplies:

• Soda can
• Rivet (1/8th diameter rivet body; preferably large-flanged)
• Pushpin
• Nail (size 6D)
• Hammer
• Scissors
• Steak knife
• Green PVC wall anchor (optional)
• Plastic top of spice bottle (optional)

## Step 1: Step 1: Remove the Bottom of the Can

Rinse the inside of the can. Then poke a starter hole in the can with a pushpin. The hole should be positioned a few millimeters down from the top edge of the outer wall of the can. Then insert the tip of a steak knife into the hole (parallel to the bottom) and sink it about a centimeter or two deep.

Then slice a tiny strip away from the circumference of the can with the knife using its flat edge (not the sharp cutting edge, and not the flat side). You'll be holding the knife with your fingertips pressed against the flat of the blade, so they are safely away from the cutting edge of the blade. Make sure that you're bracing the top part of the can with the hand you're using to grip the body of the can -- you don't want the top wobbling around. When the top is mostly all the way off, scissor-off the remaining bit, since the last few millimeters of the cut are difficult.

Make sure the flat of the blade stays parallel to the bottom surface as you cut. You'll find that there's a sweet spot where the can goes from stiff to pliable about three millimeters or so from the top edge; that's the position where you'll need to maintain the knife as you cut off the top. If you drift too high, cutting will become more difficult (but not impossible). If you drift too low, the material may collapse in a bit -- again, making it more difficult.

Once the top is off, you'll scissor down the side until you're a few millimeters from the bottom outermost edge. Then you'll cut around the circumference so that a lip about five millimeters high remains on the bottom (like a short cookie cutter). You'll feel where the metal goes from flexible to stiff, and you want to avoid the stiff zone.

Once the sides are off, you have a nice sheet of smooth metal you can use for other projects.

Finally, trim off the lip you left with scissors. Make sure you're removing the side, not the bottom, or else you'll distort the shape of the top. This metal is thicker toward the bottom and will be a bit tougher to scissor-off than the middle. Deburr the edge by running the back edge of your scissors along it to dull and scrape off the sharp bits.

## Step 2: Step 2: Poke Your Center Hole

Use your pushpin to poke a small hole in the very center of the dome of the can. Locating the precise center is tricky but aesthetically important. I laser-cut a piece of acrylic with a center hole that I could use to find the center of the can, but that's overkill. You could instead printout a circle of the appropriate diameter (66 mm; file below) with the center marked by a black dot, cut out the circle from the sheet, place it flat over the dome with the paper's edges lined up to the can's edges, and then poke through the dot.

The lazy way to do it, though, is to place the tip of a pushpin over the spot where you think the center is, then rotate the can bottom. If the tip rolls in a circle, you've missed the center. Reposition and repeat until the tip appears to stay still when the bottom is rotated. The tip is now at the center. Now that you've located it, you'll need to poke with some force, but the pushpin should work. Small kids may need help with this step, as they can lack the finger and arm strength. A gentle tap with the hammer should work nicely, though, if you can hold the pushpin in place without hitting your fingers.

Another very accurate way to find the center is to start with a pushpin that has a flat head. You rest that pushpin pin-up on the table, then balance the disc on the tip so that the disc is parallel to the table, then press gently down on the disc with increasing pressure until you mark the center with an indentation. Then you punch it with the pushpin the easy way -- by holding the disc in one hand by its rim-groove and punching out the center hole with the pushpin with your other hand.

Once the pushpin hole has been poked, roll the can belly-down and place it on the upside-down spice cap. The rim of the spice cap should rest exactly in the groove on the can. Now enlarge the center hole in the can bottom by hammering-in a size 6D nail. The hole should now be nearly 1/8th inch in diameter, which is nearly the width of the rivet body. If you don't have a spice cap, you can still hammer in a nail -- but you'll need to take steps to protect your table and lift up the can bottom so it has a bit of clearance for the nail to punch through. Before I discovered the spice cap solution, I would place the can-bottom-hole over the gap between two books of identical height, so the nail could sink through the gap.

## Step 3: Step 3: Ram the Rivet

Insert the mandrel of the rivet inside the green wall anchor. Position it over the hole. Hammer on the green wall anchor until the rivet body sinks partway through the hole. Then turn it over and gently push on the flanges until the rivet body is fully sunk into the hole, with the flanges resting against the can bottom. The spice cap will just pop off as you do this.

You can buy large flange rivets in a box of 500 rivets for about \$.02 each. See Large Flange Blind Rivet from Graingers. You can buy the green expanding wall anchors (1.5" length for .25" hole) at Home Depot for about \$0.13 each. See Green Anchor.

For the spice cap, I found a garlic-powder-plastic-bottle-cap that worked nicely and had a deep well and a 47id/50od diameter that fit the groove in the can like they were made for each other. As an alternative, you could position the pinhole over a gap between two identical books; or you could drill a large hole into very thick catalog.

You may be wondering, "Why aren't I hammering on the mandrel of the rivet like it was a nail?" It's because the mandrel would be knocked free of the rivet body and separate/slide out.

## Step 4: Step 4: Add the Spiral

Poke a hole in the center of your spiral with the pushpin and mount it on the mandrel. Invariably, it will be off-center, so pull the edge of the paper so the center tears slightly so that all the edges of the paper now touch the edge of the "well." The pink spiral and other center images are in the pdf below.

## Step 5: Step 5: Play

There's a bit of a knack to spinning this thing. You need to roll it between your fingers, as opposed to just snapping your fingers. In other words, the shaft of the mandrel needs to take as many spins as it can possibly take while being pressed between your fingers before you release it. If you want the spiral to go the other way, just spin it with your nondominant hand or print a reversed image of the spiral. Other center images could be a roulette-style decision wheel, or a color wheel with discrete color sections that will blend those colors and shapes as it spins, and so on.

For more info and other designs, see http://aluminoids.com/spinner.html at: Turn cans into fun with the Aluminator!.

So how can you use this in the classroom? Easy.

1. Demonstrate the conservation of angular momentum by spinning it.
2. Demonstrate gyroscopic precession by spinning it at an angle.
3. Add an eccentric weight to the center to see how it affects centering. (I like to use the tab of a can dropped over the mandrel/axis).
4. Cut the rim in multiple places around its circumference with a heavy-duty wire cutter and then fold-in or fatigue-off the sections to demonstrate how the redistribution of weight towards or away from the center affects rotational speed, thereby conserving angular momentum.
5. Demonstrate rotational inertia. You can spin the top in a level fashion on a flat surface so that there is no precession, then tilt the surface, and note how the axis remains in a consistent orientation, like a gyroscope.
6. Explain how, as energy is lost to friction, the energy of the system is conserved by the center of gravity lowering, which causes precession and collapse.
7. Demonstrate how friction of the surface affects the spinning (lubricated vs. unlubricated table).
8. Use the spinner itself as a prize. If you finish the lesson, you get to take it home to keep.
9. Demonstrate spinning disc illusions, such as (i) how the spiral appears to constrict or expand based on the direction of its spin, (ii) how persistence of vision makes colors regions bleed together or change under rotation (e.g., a Newton's Disc), (iii) how imaginary colors leap out of a black & white Benham Disc. Or use a smartphone camera to demonstrate the Wagon Wheel Effect.

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