Inertia Stick: An introduction to moment of inertia for almost all ages.
This instructable shows how to build the demonstration and gives sample scripts for guiding someone through exploring the demonstration. Hopefully this will be part of a series related to rotational motion. Total cost <$10
- 1/2" pvc 2.5 to 3 feet
- 2 - 1/2" male threaded
- 2- 1/2" female threaded
- pvc primer and glue
- 1 ft 1/2" threaded rod
- paper towels
- hack saw
- concrete surface
- pvc cutter (could use the hacksaw)
Step 1: The Build
Build your inertia sticks
Cut you pvc in half.
Cut your threaded rod in half.
Wrap paper towed around threaded rod and start to slide into pvc. Make sure it is a very tight fit. Use hard surface to hammer the threaded rod into the pvc.
Glue threaded caps so that the weight is on the female side of one stick and on the male side of the other stick. (An alternate option is to attach both female ends to 1 stick and both male ends to the other.)
Congratulations you inertia sticks are complete!
Interacting with the inertia sticks
Screw the sticks together so that the weights are in the middle and ask a participant to spin the stick in the middle and rotate the stick clockwise and counter clockwise. Ask them to pay attention to how hard it is to get spinning and to stop spinning. Then unscrew the two pieces are reattach them again so the weights are at the ends. Make a big deal that you didn't add anything, and you didn't take anything away.
Ask them to try the same experiment. They should notice that it is much hard to rotate the stick. People will often say it feels heavier.
Ask several follow up questions:
Was it easier or harder the second time?
Why do you think it changed?
Ask them to explore the sticks in greater detail see if they can identify what is causing the difference.
Sample interaction: F=Facilitator, P=participant
F: Here try this hold this in the middle and spin it back and forth by rotating your wrist. Pay attention to how hard it is to spin it.
F: (unscrews sticks and screw them back together in the other orientation)
F: So I am changing something, but have I added anything?
F: Have I taken anything away?
F: I want you to try it again, see if it feels the same or different.
P: It feels heavier/harder
F: Is it heavier or just harder to turn? (might take more trials.
P: harder to turn.
F: See if you can figure out what made it harder to turn. (disassemble and hand the pieces over)
P: There are weights only on one side.
F: Will it be easier if the weights are in the middle or at the ends? (
F: try it
F: Moment of inertia is measure or how hard something is to rotate. Its made up of two components. The mass and the distance from the axis of rotation. So we add up all the masses multiplied by the distance. Which has a bigger the moment of inertia when the masses are in the center or at the ends.
F: What do we call a measure of how hard it is to get something spinning.
P: Moment of inertia
F:If the moment of inertia is big is it easier or harder to spin.
F: What is moment of inertia made up of
P: mass and distance from rotation
F: Great for our next experiment we are going to change the moment of inertia of a spinning object
Link for more and better background