Twists & Turns

When a tube spins in two directions at once, what does it look like?

Have you ever watched a tea cup ride like the one in Disney World? The tea cups are rotating around two or even three axes at once. But think about what the people look like as they are whizzing by you. At some points you can clearly see their faces, its as if they were standing still, while at other times their faces are all a blur. Now think about what it feels like when you are on the ride (if you have not ridden this ride before, stop reading and go to your nearest amusement park. It counts as work, it's for science!).

At points you feel like you are spinning very fast, but at points you feel like you are sitting still even though you are still being spun in several directions at once. This spinning tube is like an ant sized tea cup ride. It’s spinning in two directions at once. One axis of rotation is down the middle of the tube and the second axis of rotation is perpendicular to the table sticking out of the very center of the tube. Once the tube is spinning in two directions because of the dual direction push from your finger, do you know how it all came together to make what you see happen? When things are moving in two ways at once, what you see is how the movement in the two directions adds up.

Think of a block with two strings tied to adjacent edges. Let say you pull on both strings at the same time. There’s force in two directions but you only see the box move in one direction, diagonally. This is because the forces add up and the movement is in the direction of the sum of these forces. The same effect happens when something spins. When the tube spins the resultant speed of the spin is found by adding the spin about both axes. Sometimes this adds up to a really fast spin and sometimes it adds up to spinning really slowly. When it adds up to a slow spin, your eye can see what is written on the tube. When you start the tube spinning by pushing on the “x,” the spinning always adds up to be slow when the “x” is on top so you can see it, but when the “o” is on top, the opposite is happening and the tube is spinning very fast so you can’t see the “o.” When the tube spins fast, you can actually see not one but 4 “x”s or “o”s. This is because it takes our eye and brain a bit of time to forget what it just saw, so the image of one “x” is still hanging out in your head when the next image appears. This keeps happening so you end up seeing all 4 “x”s at the same time.

You can try this experiment with different tubes of different lengths or diameters. It’s pretty easy to find white cardboard tubes around the house. PVC pipe works well too.

• Real world connections -
  • Using what you might have learned about torque, why are door handles usually farthest away from the door hinges? What else do you use torque to move?
• Suggestions for drawing, illustrating, presenting content in creative ways
  • Create a mobile with students and have teams compete to who can add on the most objects while keeping it balanced
  • Watch one of these videos and write, draw, or create your own video explaining torque, angular momentum, or what you learned.
• Engineering and design challenges connected to the content

Check out this video of a gyroscope in space. Think about how you could use this technology to help solve a problem.