Demos: 1Q-11 Change in Rotational Inertia


This demonstration uses the same equipment as in 1Q-10, except that the ring(s) is replaced by the bar containing sliding weights from Demo 1Q-22. The two weights are held apart by a string which is burned after the system has begun rotating. The weights transfer quickly to the center resulting in a significant increase in the rotational acceleration, demonstrating that a is inversely proportional to I.

Directions: This is best done as a qualitative demonstration, since the timing the two separate motions is quite difficult. With the weights stretched out, let the hanging weight fall. Then after a turn or two when the angular acceleration is evident, burn the string. The increase in a will be evident visually.

Suggestions for Presentation: If Demo 1Q - 10 has been performed (either qualitatively or quantitatively), ask what will happen to the angular acceleration if the rotational inertia is changed internally without any outside influence. Focus on the fact that the spring force pulling the two masses together is an internal action-reaction pair.

Note: This demonstration is also illustrating conservation of angular momentum instantaneously as in the Demo 1Q - 21, but the difference here is that an outside torque continues to act on the system.

Applications: Basically the same as in 1Q - 10.

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Last Updated: May 9, 2016 11:44 AM