Spinning Blackboard

Marking-pen version:

1. Cut a hole in the center of a piece of paper,
2. Mount it on the turntable. (If the turntable has grooves in it, cover it first with a sheet of cardboard.)

Sand version (be sure to use a junk turntable if you do this version; sand and precision equipment don't mix):

1. Fit a large piece of butcher paper between the turntable and the body of the record player to protect the mechanism from the sand. (Even if the turntable is junk, you’ll want it to survive long enough so you can repeat this Snack another time!)
2. Cover the turntable with a cardboard disk.
3. Spread a thin layer of sand evenly on the cardboard disk.

Start the turntable rotating. Move a marking pen at a constant speed in a straight line from the center of the turntable to the outside edge. (On the sand-covered turntable, trace the straight line with your finger.) Notice the spiraling curve that appears on the turntable. This curve is called a spiral of Archimedes. Move your pen or finger out from the center at different speeds and notice how the spiral changes.

Try drawing other straight lines. For example, start at the edge of the turntable and draw a line toward the center, or start at the edge and draw a line making a 45-degree angle with the edge. Draw straight lines with different constant speeds to make new curves.

Draw many straight lines radiating out from a point halfway between the center and the edge of the turntable. Try to draw a triangle or a square on the rotating turntable.

When you draw a straight line from the center of the spinning turntable toward the center of your body, the turntable rotates beneath your finger as you draw the line. Your finger traces a curve on the turntable. Because record players rotate clockwise, the line appears to curve to the left, when viewed from its starting point, which was at the center. The spiral made by your finger also appears to curve to the left. The pattern on the turntable shows the motion of your finger from the perspective, or frame of reference, of a speck of sand on the spinning turntable. (Physicists would say that the speck of sand is in a rotating frame of reference.)

Objects move in a straight line at a constant speed when there are no net forces on them. The person drawing the straight line can see no net forces on the pen or fingertip, but an ant rotating with the turntable sees the pen or fingertip curve in an arc and so believes that there must be a force pushing it into this curved path. In the rotating frame of reference, observers make up forces named centrifugal and Coriolis to explain the curvature of the line.

Like a speck of sand or an ant on the turntable, a person on the surface of the earth is in a rotating frame of reference. You can picture the earth as a giant turntable. If you are in the Southern Hemisphere looking north toward the equator, the earth is rotating clockwise. If a jetliner or a wind or an ocean current were traveling in a straight line from the South Pole toward the equator, you’d see them curve to the left.

From the Northern Hemisphere, the earth appears to rotate counterclockwise. Objects moving from the North Pole toward the equator appear to curve to the right of their direction of motion. In fact, objects moving in any direction appear to curve to the right. This explains why air flowing into the low-pressure center of a hurricane in the Northern Hemisphere bends to the right, and so flows around the hurricane in a counterclockwise direction.