Additional Info

• ID Code: D3-03
• Purpose: Illustrates conservation of angular momentum
• Description:
  A subject, holding the weights with their arms extended, is started into rotation. When the weights are pulled inward to the chest of the subject, the moment of inertia of the system is decreased, leading to significant increase in the angular speed of the rotating chair.

  Please take care when using this device, especially when accelerating. You can gain a significant increase in rotational speed, so hold on! And it is best not to have a person push the chair around very much, as it is very easy to hit them with a weight by accident.

  Engagement Suggestions

  • Consider inviting a participant from the class.
  • Encourage students to predict what will happen before performing the demonstration.
  • Once the demonstration has been performed, discuss the activity both in terms of angular momentum and its conservation, and in terms of kinetic energy.
  • For extended discussion, introduce the idea of friction. How does friction work in this system? How does it affect the angular momentum? Where does the kinetic energy go?
  

  Background

  This device illustrates the conservation of angular momentum. When the heavy weights are moved closer to or farther from the axis of rotation, the distribution of mass and thus the rotational inertia (or moment of inertia) changes.

  To show this in a different way, a single user with a single weight can move themself in a circle by swinging their arm wide holding the weight from front to back, then drawing it inwards before extending their arm forwards again and repeating the motion. This is essentially a rotational analogue of pumping a swing.

• Availability: Available
• References: REFERENCES: (PIRA 1Q40.10)
• Loc codes: FS0
• Videos-1: Clockwise Rotation
• Videos-2: Counter-Clockwise Rotation
• Videos-3: Demonstration video on YouTube